pcre2/doc/pcre2.txt

-----------------------------------------------------------------------------
This file contains a concatenation of the PCRE2 man pages, converted to plain
text format for ease of searching with a text editor, or for use on systems
that do not have a man page processor. The small individual files that give
synopses of each function in the library have not been included. Neither has
the pcre2demo program. There are separate text files for the pcre2grep and
pcre2test commands.
-----------------------------------------------------------------------------


PCRE2(3)                   Library Functions Manual                   PCRE2(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

INTRODUCTION

       PCRE2 is the name used for a revised API for the PCRE library, which is
       a set of functions, written in C,  that  implement  regular  expression
       pattern matching using the same syntax and semantics as Perl, with just
       a few differences. After nearly two decades,  the  limitations  of  the
       original  API  were  making development increasingly difficult. The new
       API is more extensible, and it was simplified by abolishing  the  sepa-
       rate  "study" optimizing function; in PCRE2, patterns are automatically
       optimized where possible. Since forking from PCRE1, the code  has  been
       extensively refactored and new features introduced.

       As  well  as Perl-style regular expression patterns, some features that
       appeared in Python and the original PCRE before they appeared  in  Perl
       are  available  using the Python syntax. There is also some support for
       one or two .NET and Oniguruma syntax items, and there are  options  for
       requesting   some  minor  changes  that  give  better  ECMAScript  (aka
       JavaScript) compatibility.

       The source code for PCRE2 can be compiled to support 8-bit, 16-bit,  or
       32-bit  code units, which means that up to three separate libraries may
       be installed.  The original work to extend PCRE to  16-bit  and  32-bit
       code  units  was  done  by Zoltan Herczeg and Christian Persch, respec-
       tively. In all three cases, strings can be interpreted  either  as  one
       character  per  code  unit, or as UTF-encoded Unicode, with support for
       Unicode general category properties. Unicode  support  is  optional  at
       build  time  (but  is  the default). However, processing strings as UTF
       code units must be enabled explicitly at run time. The version of  Uni-
       code in use can be discovered by running

         pcre2test -C

       The  three  libraries  contain  identical sets of functions, with names
       ending in _8,  _16,  or  _32,  respectively  (for  example,  pcre2_com-
       pile_8()).  However,  by defining PCRE2_CODE_UNIT_WIDTH to be 8, 16, or
       32, a program that uses just one code unit width can be  written  using
       generic names such as pcre2_compile(), and the documentation is written
       assuming that this is the case.

       In addition to the Perl-compatible matching function, PCRE2 contains an
       alternative  function that matches the same compiled patterns in a dif-
       ferent way. In certain circumstances, the alternative function has some
       advantages.   For  a discussion of the two matching algorithms, see the
       pcre2matching page.

       Details of exactly which Perl regular expression features are  and  are
       not  supported  by  PCRE2  are  given  in  separate  documents. See the
       pcre2pattern and pcre2compat pages. There is a syntax  summary  in  the
       pcre2syntax page.

       Some  features  of PCRE2 can be included, excluded, or changed when the
       library is built. The pcre2_config() function makes it possible  for  a
       client  to  discover  which  features are available. The features them-
       selves are described in the pcre2build page. Documentation about build-
       ing  PCRE2 for various operating systems can be found in the README and
       NON-AUTOTOOLS_BUILD files in the source distribution.

       The libraries contains a number of undocumented internal functions  and
       data  tables  that  are  used by more than one of the exported external
       functions, but which are not intended  for  use  by  external  callers.
       Their  names  all begin with "_pcre2", which hopefully will not provoke
       any name clashes. In some environments, it is possible to control which
       external  symbols  are  exported when a shared library is built, and in
       these cases the undocumented symbols are not exported.


SECURITY CONSIDERATIONS

       If you are using PCRE2 in a non-UTF application that permits  users  to
       supply  arbitrary  patterns  for  compilation, you should be aware of a
       feature that allows users to turn on UTF support from within a pattern.
       For  example, an 8-bit pattern that begins with "(*UTF)" turns on UTF-8
       mode, which interprets patterns and subjects as strings of  UTF-8  code
       units instead of individual 8-bit characters. This causes both the pat-
       tern and any data against which it is matched to be checked  for  UTF-8
       validity.  If the data string is very long, such a check might use suf-
       ficiently many resources as to cause your application to  lose  perfor-
       mance.

       One  way  of guarding against this possibility is to use the pcre2_pat-
       tern_info() function  to  check  the  compiled  pattern's  options  for
       PCRE2_UTF.  Alternatively,  you can set the PCRE2_NEVER_UTF option when
       calling pcre2_compile(). This causes a compile time error if  the  pat-
       tern contains a UTF-setting sequence.

       The  use  of Unicode properties for character types such as \d can also
       be enabled from within the pattern, by specifying "(*UCP)".  This  fea-
       ture can be disallowed by setting the PCRE2_NEVER_UCP option.

       If  your  application  is one that supports UTF, be aware that validity
       checking can take time. If the same data string is to be  matched  many
       times,  you  can  use  the PCRE2_NO_UTF_CHECK option for the second and
       subsequent matches to avoid running redundant checks.

       The use of the \C escape sequence in a UTF-8 or UTF-16 pattern can lead
       to  problems,  because  it  may leave the current matching point in the
       middle of  a  multi-code-unit  character.  The  PCRE2_NEVER_BACKSLASH_C
       option can be used by an application to lock out the use of \C, causing
       a compile-time error if it is encountered. It is also possible to build
       PCRE2 with the use of \C permanently disabled.

       Another  way  that  performance can be hit is by running a pattern that
       has a very large search tree against a string that  will  never  match.
       Nested  unlimited repeats in a pattern are a common example. PCRE2 pro-
       vides some protection against  this:  see  the  pcre2_set_match_limit()
       function  in  the  pcre2api  page.  There  is a similar function called
       pcre2_set_depth_limit() that can be used to restrict the amount of mem-
       ory that is used.


USER DOCUMENTATION

       The  user  documentation for PCRE2 comprises a number of different sec-
       tions. In the "man" format, each of these is a separate "man page".  In
       the  HTML  format, each is a separate page, linked from the index page.
       In the plain  text  format,  the  descriptions  of  the  pcre2grep  and
       pcre2test programs are in files called pcre2grep.txt and pcre2test.txt,
       respectively. The remaining sections, except for the pcre2demo  section
       (which  is a program listing), and the short pages for individual func-
       tions, are concatenated in pcre2.txt, for ease of searching.  The  sec-
       tions are as follows:

         pcre2              this document
         pcre2-config       show PCRE2 installation configuration information
         pcre2api           details of PCRE2's native C API
         pcre2build         building PCRE2
         pcre2callout       details of the pattern callout feature
         pcre2compat        discussion of Perl compatibility
         pcre2convert       details of pattern conversion functions
         pcre2demo          a demonstration C program that uses PCRE2
         pcre2grep          description of the pcre2grep command (8-bit only)
         pcre2jit           discussion of just-in-time optimization support
         pcre2limits        details of size and other limits
         pcre2matching      discussion of the two matching algorithms
         pcre2partial       details of the partial matching facility
         pcre2pattern       syntax and semantics of supported regular
                              expression patterns
         pcre2perform       discussion of performance issues
         pcre2posix         the POSIX-compatible C API for the 8-bit library
         pcre2sample        discussion of the pcre2demo program
         pcre2serialize     details of pattern serialization
         pcre2syntax        quick syntax reference
         pcre2test          description of the pcre2test command
         pcre2unicode       discussion of Unicode and UTF support

       In  the  "man"  and HTML formats, there is also a short page for each C
       library function, listing its arguments and results.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

       Putting an actual email address here is a spam magnet. If you  want  to
       email  me,  use  my two initials, followed by the two digits 10, at the
       domain cam.ac.uk.


REVISION

       Last updated: 17 September 2018
       Copyright (c) 1997-2018 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2API(3)                Library Functions Manual                PCRE2API(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

       #include <pcre2.h>

       PCRE2  is  a  new API for PCRE, starting at release 10.0. This document
       contains a description of all its native functions. See the pcre2 docu-
       ment for an overview of all the PCRE2 documentation.


PCRE2 NATIVE API BASIC FUNCTIONS

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       void pcre2_match_data_free(pcre2_match_data *match_data);


PCRE2 NATIVE API AUXILIARY MATCH FUNCTIONS

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);


PCRE2 NATIVE API GENERAL CONTEXT FUNCTIONS

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       void pcre2_general_context_free(pcre2_general_context *gcontext);


PCRE2 NATIVE API COMPILE CONTEXT FUNCTIONS

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const unsigned char *tables);

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
         uint32_t extra_options);

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);


PCRE2 NATIVE API MATCH CONTEXT FUNCTIONS

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
         uint32_t value);


PCRE2 NATIVE API STRING EXTRACTION FUNCTIONS

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       void pcre2_substring_list_free(PCRE2_SPTR *list);

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);


PCRE2 NATIVE API STRING SUBSTITUTION FUNCTION

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacementzfP,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
         PCRE2_SIZE *outlengthptr);


PCRE2 NATIVE API JIT FUNCTIONS

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize,
         PCRE2_SIZE maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);


PCRE2 NATIVE API SERIALIZATION FUNCTIONS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint8_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(const pcre2_code **codes,
         int32_t number_of_codes, uint8_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);


PCRE2 NATIVE API AUXILIARY FUNCTIONS

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       const unsigned char *pcre2_maketables(pcre2_general_context *gcontext);

       int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       int pcre2_config(uint32_t what, void *where);


PCRE2 NATIVE API OBSOLETE FUNCTIONS

       int pcre2_set_recursion_limit(pcre2_match_context *mcontext,
         uint32_t value);

       int pcre2_set_recursion_memory_management(
         pcre2_match_context *mcontext,
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       These  functions became obsolete at release 10.30 and are retained only
       for backward compatibility. They should not be used in  new  code.  The
       first  is  replaced by pcre2_set_depth_limit(); the second is no longer
       needed and has no effect (it always returns zero).


PCRE2 EXPERIMENTAL PATTERN CONVERSION FUNCTIONS

       pcre2_convert_context *pcre2_convert_context_create(
         pcre2_general_context *gcontext);

       pcre2_convert_context *pcre2_convert_context_copy(
         pcre2_convert_context *cvcontext);

       void pcre2_convert_context_free(pcre2_convert_context *cvcontext);

       int pcre2_set_glob_escape(pcre2_convert_context *cvcontext,
         uint32_t escape_char);

       int pcre2_set_glob_separator(pcre2_convert_context *cvcontext,
         uint32_t separator_char);

       int pcre2_pattern_convert(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, PCRE2_UCHAR **buffer,
         PCRE2_SIZE *blength, pcre2_convert_context *cvcontext);

       void pcre2_converted_pattern_free(PCRE2_UCHAR *converted_pattern);

       These functions provide a way of  converting  non-PCRE2  patterns  into
       patterns  that  can  be  processed by pcre2_compile(). This facility is
       experimental and may be changed in future releases. At present, "globs"
       and  POSIX  basic  and  extended patterns can be converted. Details are
       given in the pcre2convert documentation.


PCRE2 8-BIT, 16-BIT, AND 32-BIT LIBRARIES

       There are three PCRE2 libraries, supporting 8-bit, 16-bit,  and  32-bit
       code  units,  respectively.  However,  there  is  just one header file,
       pcre2.h.  This contains the function prototypes and  other  definitions
       for all three libraries. One, two, or all three can be installed simul-
       taneously. On Unix-like systems the libraries  are  called  libpcre2-8,
       libpcre2-16, and libpcre2-32, and they can also co-exist with the orig-
       inal PCRE libraries.

       Character strings are passed to and from a PCRE2 library as a  sequence
       of  unsigned  integers  in  code  units of the appropriate width. Every
       PCRE2 function comes in three different forms, one  for  each  library,
       for example:

         pcre2_compile_8()
         pcre2_compile_16()
         pcre2_compile_32()

       There are also three different sets of data types:

         PCRE2_UCHAR8, PCRE2_UCHAR16, PCRE2_UCHAR32
         PCRE2_SPTR8,  PCRE2_SPTR16,  PCRE2_SPTR32

       The  UCHAR  types define unsigned code units of the appropriate widths.
       For example, PCRE2_UCHAR16 is usually defined as `uint16_t'.  The  SPTR
       types  are  constant  pointers  to the equivalent UCHAR types, that is,
       they are pointers to vectors of unsigned code units.

       Many applications use only one code unit width. For their  convenience,
       macros are defined whose names are the generic forms such as pcre2_com-
       pile() and  PCRE2_SPTR.  These  macros  use  the  value  of  the  macro
       PCRE2_CODE_UNIT_WIDTH  to generate the appropriate width-specific func-
       tion and macro names.  PCRE2_CODE_UNIT_WIDTH is not defined by default.
       An  application  must  define  it  to  be 8, 16, or 32 before including
       pcre2.h in order to make use of the generic names.

       Applications that use more than one code unit width can be linked  with
       more  than  one PCRE2 library, but must define PCRE2_CODE_UNIT_WIDTH to
       be 0 before including pcre2.h, and then use the  real  function  names.
       Any  code  that  is to be included in an environment where the value of
       PCRE2_CODE_UNIT_WIDTH is unknown should  also  use  the  real  function
       names. (Unfortunately, it is not possible in C code to save and restore
       the value of a macro.)

       If PCRE2_CODE_UNIT_WIDTH is not defined  before  including  pcre2.h,  a
       compiler error occurs.

       When  using  multiple  libraries  in an application, you must take care
       when processing any particular pattern to use  only  functions  from  a
       single  library.   For example, if you want to run a match using a pat-
       tern that was compiled with pcre2_compile_16(), you  must  do  so  with
       pcre2_match_16(), not pcre2_match_8() or pcre2_match_32().

       In  the  function summaries above, and in the rest of this document and
       other PCRE2 documents, functions and data  types  are  described  using
       their generic names, without the _8, _16, or _32 suffix.


PCRE2 API OVERVIEW

       PCRE2  has  its  own  native  API, which is described in this document.
       There are also some wrapper functions for the 8-bit library that corre-
       spond  to the POSIX regular expression API, but they do not give access
       to all the functionality of PCRE2. They are described in the pcre2posix
       documentation. Both these APIs define a set of C function calls.

       The  native  API  C data types, function prototypes, option values, and
       error codes are defined in the header file pcre2.h, which also contains
       definitions of PCRE2_MAJOR and PCRE2_MINOR, the major and minor release
       numbers for the library. Applications can use these to include  support
       for different releases of PCRE2.

       In a Windows environment, if you want to statically link an application
       program against a non-dll PCRE2 library, you must  define  PCRE2_STATIC
       before including pcre2.h.

       The  functions pcre2_compile() and pcre2_match() are used for compiling
       and matching regular expressions in a Perl-compatible manner. A  sample
       program that demonstrates the simplest way of using them is provided in
       the file called pcre2demo.c in the PCRE2 source distribution. A listing
       of  this  program  is  given  in  the  pcre2demo documentation, and the
       pcre2sample documentation describes how to compile and run it.

       The compiling and matching functions recognize various options that are
       passed as bits in an options argument. There are also some more compli-
       cated  parameters  such  as  custom  memory  management  functions  and
       resource  limits  that  are passed in "contexts" (which are just memory
       blocks, described below). Simple applications do not need to  make  use
       of contexts.

       Just-in-time  (JIT)  compiler  support  is an optional feature of PCRE2
       that can be built in  appropriate  hardware  environments.  It  greatly
       speeds  up  the  matching  performance  of  many patterns. Programs can
       request that it be used if  available  by  calling  pcre2_jit_compile()
       after a pattern has been successfully compiled by pcre2_compile(). This
       does nothing if JIT support is not available.

       More complicated programs might need to  make  use  of  the  specialist
       functions    pcre2_jit_stack_create(),    pcre2_jit_stack_free(),   and
       pcre2_jit_stack_assign() in order to  control  the  JIT  code's  memory
       usage.

       JIT matching is automatically used by pcre2_match() if it is available,
       unless the PCRE2_NO_JIT option is set. There is also a direct interface
       for  JIT  matching,  which gives improved performance at the expense of
       less sanity checking. The JIT-specific functions are discussed  in  the
       pcre2jit documentation.

       A  second  matching function, pcre2_dfa_match(), which is not Perl-com-
       patible, is also provided. This uses  a  different  algorithm  for  the
       matching.  The  alternative  algorithm finds all possible matches (at a
       given point in the subject), and scans the subject  just  once  (unless
       there  are  lookaround  assertions).  However,  this algorithm does not
       return captured substrings. A description of  the  two  matching  algo-
       rithms   and  their  advantages  and  disadvantages  is  given  in  the
       pcre2matching   documentation.   There   is   no   JIT   support    for
       pcre2_dfa_match().

       In  addition  to  the  main compiling and matching functions, there are
       convenience functions for extracting captured substrings from a subject
       string that has been matched by pcre2_match(). They are:

         pcre2_substring_copy_byname()
         pcre2_substring_copy_bynumber()
         pcre2_substring_get_byname()
         pcre2_substring_get_bynumber()
         pcre2_substring_list_get()
         pcre2_substring_length_byname()
         pcre2_substring_length_bynumber()
         pcre2_substring_nametable_scan()
         pcre2_substring_number_from_name()

       pcre2_substring_free()  and  pcre2_substring_list_free()  are also pro-
       vided, to free memory used for extracted strings. If  either  of  these
       functions  is called with a NULL argument, the function returns immedi-
       ately without doing anything.

       The function pcre2_substitute() can be called to match  a  pattern  and
       return  a  copy of the subject string with substitutions for parts that
       were matched.

       Functions whose names begin with pcre2_serialize_ are used  for  saving
       compiled patterns on disc or elsewhere, and reloading them later.

       Finally,  there  are functions for finding out information about a com-
       piled pattern (pcre2_pattern_info()) and about the  configuration  with
       which PCRE2 was built (pcre2_config()).

       Functions  with  names  ending with _free() are used for freeing memory
       blocks of various sorts. In all cases, if one  of  these  functions  is
       called with a NULL argument, it does nothing.


STRING LENGTHS AND OFFSETS

       The  PCRE2  API  uses  string  lengths and offsets into strings of code
       units in several places. These values are always  of  type  PCRE2_SIZE,
       which  is an unsigned integer type, currently always defined as size_t.
       The largest  value  that  can  be  stored  in  such  a  type  (that  is
       ~(PCRE2_SIZE)0)  is reserved as a special indicator for zero-terminated
       strings and unset offsets.  Therefore, the longest string that  can  be
       handled is one less than this maximum.


NEWLINES

       PCRE2 supports five different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, or any Unicode newline sequence. The Unicode newline  sequences
       are  the  three just mentioned, plus the single characters VT (vertical
       tab, U+000B), FF (form feed, U+000C), NEL (next line, U+0085), LS (line
       separator, U+2028), and PS (paragraph separator, U+2029).

       Each  of  the first three conventions is used by at least one operating
       system as its standard newline sequence. When PCRE2 is built, a default
       can be specified.  If it is not, the default is set to LF, which is the
       Unix standard. However, the newline convention can  be  changed  by  an
       application  when  calling  pcre2_compile(),  or it can be specified by
       special text at the start of the pattern  itself;  this  overrides  any
       other  settings.  See  the pcre2pattern page for details of the special
       character sequences.

       In the PCRE2 documentation the word "newline"  is  used  to  mean  "the
       character or pair of characters that indicate a line break". The choice
       of newline convention affects the handling of the dot, circumflex,  and
       dollar metacharacters, the handling of #-comments in /x mode, and, when
       CRLF is a recognized line ending sequence, the match position  advance-
       ment for a non-anchored pattern. There is more detail about this in the
       section on pcre2_match() options below.

       The choice of newline convention does not affect the interpretation  of
       the \n or \r escape sequences, nor does it affect what \R matches; this
       has its own separate convention.


MULTITHREADING

       In a multithreaded application it is important to keep  thread-specific
       data  separate  from data that can be shared between threads. The PCRE2
       library code itself is thread-safe: it contains  no  static  or  global
       variables.  The  API  is  designed to be fairly simple for non-threaded
       applications while at the same time ensuring that multithreaded  appli-
       cations can use it.

       There are several different blocks of data that are used to pass infor-
       mation between the application and the PCRE2 libraries.

   The compiled pattern

       A pointer to the compiled form of a pattern is  returned  to  the  user
       when pcre2_compile() is successful. The data in the compiled pattern is
       fixed, and does not change when the pattern is matched.  Therefore,  it
       is  thread-safe, that is, the same compiled pattern can be used by more
       than one thread simultaneously. For example, an application can compile
       all its patterns at the start, before forking off multiple threads that
       use them. However, if the just-in-time (JIT)  optimization  feature  is
       being  used,  it needs separate memory stack areas for each thread. See
       the pcre2jit documentation for more details.

       In a more complicated situation, where patterns are compiled only  when
       they  are  first needed, but are still shared between threads, pointers
       to compiled patterns must be protected  from  simultaneous  writing  by
       multiple threads, at least until a pattern has been compiled. The logic
       can be something like this:

         Get a read-only (shared) lock (mutex) for pointer
         if (pointer == NULL)
           {
           Get a write (unique) lock for pointer
           pointer = pcre2_compile(...
           }
         Release the lock
         Use pointer in pcre2_match()

       Of course, testing for compilation errors should also  be  included  in
       the code.

       If JIT is being used, but the JIT compilation is not being done immedi-
       ately, (perhaps waiting to see if the pattern  is  used  often  enough)
       similar logic is required. JIT compilation updates a pointer within the
       compiled code block, so a thread must gain unique write access  to  the
       pointer     before    calling    pcre2_jit_compile().    Alternatively,
       pcre2_code_copy()  or  pcre2_code_copy_with_tables()  can  be  used  to
       obtain  a private copy of the compiled code before calling the JIT com-
       piler.

   Context blocks

       The next main section below introduces the idea of "contexts" in  which
       PCRE2 functions are called. A context is nothing more than a collection
       of parameters that control the way PCRE2 operates. Grouping a number of
       parameters together in a context is a convenient way of passing them to
       a PCRE2 function without using lots of arguments. The  parameters  that
       are  stored  in  contexts  are in some sense "advanced features" of the
       API. Many straightforward applications will not need to use contexts.

       In a multithreaded application, if the parameters in a context are val-
       ues  that  are  never  changed, the same context can be used by all the
       threads. However, if any thread needs to change any value in a context,
       it must make its own thread-specific copy.

   Match blocks

       The  matching  functions need a block of memory for storing the results
       of a match. This includes details of what was matched, as well as addi-
       tional  information  such as the name of a (*MARK) setting. Each thread
       must provide its own copy of this memory.


PCRE2 CONTEXTS

       Some PCRE2 functions have a lot of parameters, many of which  are  used
       only  by  specialist  applications,  for example, those that use custom
       memory management or non-standard character tables.  To  keep  function
       argument  lists  at a reasonable size, and at the same time to keep the
       API extensible, "uncommon" parameters are passed to  certain  functions
       in  a  context instead of directly. A context is just a block of memory
       that holds the parameter values.  Applications  that  do  not  need  to
       adjust  any  of  the  context  parameters  can pass NULL when a context
       pointer is required.

       There are three different types of context: a general context  that  is
       relevant  for  several  PCRE2 operations, a compile-time context, and a
       match-time context.

   The general context

       At present, this context just  contains  pointers  to  (and  data  for)
       external  memory  management  functions  that  are  called from several
       places in the PCRE2 library. The context is named `general' rather than
       specifically  `memory'  because in future other fields may be added. If
       you do not want to supply your own custom memory management  functions,
       you  do not need to bother with a general context. A general context is
       created by:

       pcre2_general_context *pcre2_general_context_create(
         void *(*private_malloc)(PCRE2_SIZE, void *),
         void (*private_free)(void *, void *), void *memory_data);

       The two function pointers specify custom memory  management  functions,
       whose prototypes are:

         void *private_malloc(PCRE2_SIZE, void *);
         void  private_free(void *, void *);

       Whenever code in PCRE2 calls these functions, the final argument is the
       value of memory_data. Either of the first two arguments of the creation
       function  may be NULL, in which case the system memory management func-
       tions malloc() and free() are used. (This is not currently  useful,  as
       there  are  no  other  fields in a general context, but in future there
       might be.)  The private_malloc() function  is  used  (if  supplied)  to
       obtain  memory  for storing the context, and all three values are saved
       as part of the context.

       Whenever PCRE2 creates a data block of any kind, the block  contains  a
       pointer  to the free() function that matches the malloc() function that
       was used. When the time comes to  free  the  block,  this  function  is
       called.

       A general context can be copied by calling:

       pcre2_general_context *pcre2_general_context_copy(
         pcre2_general_context *gcontext);

       The memory used for a general context should be freed by calling:

       void pcre2_general_context_free(pcre2_general_context *gcontext);

       If  this  function  is  passed  a NULL argument, it returns immediately
       without doing anything.

   The compile context

       A compile context is required if you want to provide an external  func-
       tion  for  stack  checking  during compilation or to change the default
       values of any of the following compile-time parameters:

         What \R matches (Unicode newlines or CR, LF, CRLF only)
         PCRE2's character tables
         The newline character sequence
         The compile time nested parentheses limit
         The maximum length of the pattern string
         The extra options bits (none set by default)

       A compile context is also required if you are using custom memory  man-
       agement.   If  none of these apply, just pass NULL as the context argu-
       ment of pcre2_compile().

       A compile context is created, copied, and freed by the following  func-
       tions:

       pcre2_compile_context *pcre2_compile_context_create(
         pcre2_general_context *gcontext);

       pcre2_compile_context *pcre2_compile_context_copy(
         pcre2_compile_context *ccontext);

       void pcre2_compile_context_free(pcre2_compile_context *ccontext);

       A  compile  context  is created with default values for its parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_bsr(pcre2_compile_context *ccontext,
         uint32_t value);

       The  value  must  be PCRE2_BSR_ANYCRLF, to specify that \R matches only
       CR, LF, or CRLF, or PCRE2_BSR_UNICODE, to specify that \R  matches  any
       Unicode line ending sequence. The value is used by the JIT compiler and
       by  the  two  interpreted   matching   functions,   pcre2_match()   and
       pcre2_dfa_match().

       int pcre2_set_character_tables(pcre2_compile_context *ccontext,
         const unsigned char *tables);

       The  value  must  be  the result of a call to pcre2_maketables(), whose
       only argument is a general context. This function builds a set of char-
       acter tables in the current locale.

       int pcre2_set_compile_extra_options(pcre2_compile_context *ccontext,
         uint32_t extra_options);

       As  PCRE2  has developed, almost all the 32 option bits that are avail-
       able in the options argument of pcre2_compile() have been used  up.  To
       avoid  running  out, the compile context contains a set of extra option
       bits which are used for some newer, assumed rarer, options. This  func-
       tion  sets  those bits. It always sets all the bits (either on or off).
       It does not modify any existing  setting.  The  available  options  are
       defined in the section entitled "Extra compile options" below.

       int pcre2_set_max_pattern_length(pcre2_compile_context *ccontext,
         PCRE2_SIZE value);

       This  sets a maximum length, in code units, for any pattern string that
       is compiled with this context. If the pattern is longer,  an  error  is
       generated.   This facility is provided so that applications that accept
       patterns from external sources can limit their size. The default is the
       largest  number  that  a  PCRE2_SIZE variable can hold, which is effec-
       tively unlimited.

       int pcre2_set_newline(pcre2_compile_context *ccontext,
         uint32_t value);

       This specifies which characters or character sequences are to be recog-
       nized  as newlines. The value must be one of PCRE2_NEWLINE_CR (carriage
       return only), PCRE2_NEWLINE_LF (linefeed only), PCRE2_NEWLINE_CRLF (the
       two-character  sequence  CR followed by LF), PCRE2_NEWLINE_ANYCRLF (any
       of the above), PCRE2_NEWLINE_ANY (any  Unicode  newline  sequence),  or
       PCRE2_NEWLINE_NUL (the NUL character, that is a binary zero).

       A pattern can override the value set in the compile context by starting
       with a sequence such as (*CRLF). See the pcre2pattern page for details.

       When   a   pattern   is   compiled   with   the    PCRE2_EXTENDED    or
       PCRE2_EXTENDED_MORE option, the newline convention affects the recogni-
       tion of the end of internal comments starting  with  #.  The  value  is
       saved  with the compiled pattern for subsequent use by the JIT compiler
       and by  the  two  interpreted  matching  functions,  pcre2_match()  and
       pcre2_dfa_match().

       int pcre2_set_parens_nest_limit(pcre2_compile_context *ccontext,
         uint32_t value);

       This  parameter  adjusts  the  limit,  set when PCRE2 is built (default
       250), on the depth of parenthesis nesting  in  a  pattern.  This  limit
       stops  rogue  patterns  using  up too much system stack when being com-
       piled. The limit applies to parentheses of all kinds, not just  captur-
       ing parentheses.

       int pcre2_set_compile_recursion_guard(pcre2_compile_context *ccontext,
         int (*guard_function)(uint32_t, void *), void *user_data);

       There  is at least one application that runs PCRE2 in threads with very
       limited system stack, where running out of stack is to  be  avoided  at
       all  costs. The parenthesis limit above cannot take account of how much
       stack is actually available during compilation. For  a  finer  control,
       you  can  supply  a  function  that  is called whenever pcre2_compile()
       starts to compile a parenthesized part of a pattern. This function  can
       check  the  actual  stack  size  (or anything else that it wants to, of
       course).

       The first argument to the callout function gives the current  depth  of
       nesting,  and  the second is user data that is set up by the last argu-
       ment  of  pcre2_set_compile_recursion_guard().  The  callout   function
       should return zero if all is well, or non-zero to force an error.

   The match context

       A match context is required if you want to:

         Set up a callout function
         Set an offset limit for matching an unanchored pattern
         Change the limit on the amount of heap used when matching
         Change the backtracking match limit
         Change the backtracking depth limit
         Set custom memory management specifically for the match

       If  none  of  these  apply,  just  pass NULL as the context argument of
       pcre2_match(), pcre2_dfa_match(), or pcre2_jit_match().

       A match context is created, copied, and freed by  the  following  func-
       tions:

       pcre2_match_context *pcre2_match_context_create(
         pcre2_general_context *gcontext);

       pcre2_match_context *pcre2_match_context_copy(
         pcre2_match_context *mcontext);

       void pcre2_match_context_free(pcre2_match_context *mcontext);

       A  match  context  is  created  with default values for its parameters.
       These can be changed by calling the following functions, which return 0
       on success, or PCRE2_ERROR_BADDATA if invalid data is detected.

       int pcre2_set_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_callout_block *, void *),
         void *callout_data);

       This  sets  up a callout function for PCRE2 to call at specified points
       during a matching operation. Details are given in the pcre2callout doc-
       umentation.

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       This  sets up a callout function for PCRE2 to call after each substitu-
       tion made by pcre2_substitute(). Details are given in the section enti-
       tled "Creating a new string with substitutions" below.

       int pcre2_set_offset_limit(pcre2_match_context *mcontext,
         PCRE2_SIZE value);

       The  offset_limit  parameter  limits  how  far an unanchored search can
       advance in the subject string. The default value  is  PCRE2_UNSET.  The
       pcre2_match()      and      pcre2_dfa_match()      functions     return
       PCRE2_ERROR_NOMATCH if a match with a starting point before or  at  the
       given  offset  is  not  found. The pcre2_substitute() function makes no
       more substitutions.

       For example, if the pattern /abc/ is matched against "123abc"  with  an
       offset  limit  less  than 3, the result is PCRE2_ERROR_NOMATCH. A match
       can never be  found  if  the  startoffset  argument  of  pcre2_match(),
       pcre2_dfa_match(),  or  pcre2_substitute()  is  greater than the offset
       limit set in the match context.

       When using this  facility,  you  must  set  the  PCRE2_USE_OFFSET_LIMIT
       option when calling pcre2_compile() so that when JIT is in use, differ-
       ent code can be compiled. If a match  is  started  with  a  non-default
       match  limit when PCRE2_USE_OFFSET_LIMIT is not set, an error is gener-
       ated.

       The offset limit facility can be used to track progress when  searching
       large  subject  strings or to limit the extent of global substitutions.
       See also the PCRE2_FIRSTLINE option, which requires a  match  to  start
       before  or  at  the first newline that follows the start of matching in
       the subject. If this is set with an offset limit, a match must occur in
       the first line and also within the offset limit. In other words, which-
       ever limit comes first is used.

       int pcre2_set_heap_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The heap_limit parameter specifies, in units of kibibytes (1024 bytes),
       the  maximum  amount  of heap memory that pcre2_match() may use to hold
       backtracking information when running an interpretive match. This limit
       also applies to pcre2_dfa_match(), which may use the heap when process-
       ing patterns with a lot of nested pattern recursion or  lookarounds  or
       atomic groups. This limit does not apply to matching with the JIT opti-
       mization, which has  its  own  memory  control  arrangements  (see  the
       pcre2jit  documentation for more details). If the limit is reached, the
       negative error code  PCRE2_ERROR_HEAPLIMIT  is  returned.  The  default
       limit  can be set when PCRE2 is built; if it is not, the default is set
       very large and is essentially "unlimited".

       A value for the heap limit may also be supplied by an item at the start
       of a pattern of the form

         (*LIMIT_HEAP=ddd)

       where  ddd  is  a  decimal  number.  However, such a setting is ignored
       unless ddd is less than the limit set by the  caller  of  pcre2_match()
       or, if no such limit is set, less than the default.

       The  pcre2_match() function starts out using a 20KiB vector on the sys-
       tem stack for recording backtracking points. The more nested backtrack-
       ing  points  there  are (that is, the deeper the search tree), the more
       memory is needed.  Heap memory is used only if the  initial  vector  is
       too small. If the heap limit is set to a value less than 21 (in partic-
       ular, zero) no heap memory will be used. In this  case,  only  patterns
       that  do not have a lot of nested backtracking can be successfully pro-
       cessed.

       Similarly, for pcre2_dfa_match(), a vector on the system stack is  used
       when  processing pattern recursions, lookarounds, or atomic groups, and
       only if this is not big enough is heap memory used. In this case,  too,
       setting a value of zero disables the use of the heap.

       int pcre2_set_match_limit(pcre2_match_context *mcontext,
         uint32_t value);

       The  match_limit  parameter  provides  a means of preventing PCRE2 from
       using up too many computing resources when processing patterns that are
       not going to match, but which have a very large number of possibilities
       in their search trees. The classic  example  is  a  pattern  that  uses
       nested unlimited repeats.

       There  is an internal counter in pcre2_match() that is incremented each
       time round its main matching loop. If  this  value  reaches  the  match
       limit, pcre2_match() returns the negative value PCRE2_ERROR_MATCHLIMIT.
       This has the effect of limiting the amount  of  backtracking  that  can
       take place. For patterns that are not anchored, the count restarts from
       zero for each position in the subject string. This limit  also  applies
       to pcre2_dfa_match(), though the counting is done in a different way.

       When  pcre2_match() is called with a pattern that was successfully pro-
       cessed by pcre2_jit_compile(), the way in which matching is executed is
       entirely  different. However, there is still the possibility of runaway
       matching that goes on for a very long  time,  and  so  the  match_limit
       value  is  also used in this case (but in a different way) to limit how
       long the matching can continue.

       The default value for the limit can be set when  PCRE2  is  built;  the
       default  default  is 10 million, which handles all but the most extreme
       cases. A value for the match limit may also be supplied by an  item  at
       the start of a pattern of the form

         (*LIMIT_MATCH=ddd)

       where  ddd  is  a  decimal  number.  However, such a setting is ignored
       unless ddd is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.

       int pcre2_set_depth_limit(pcre2_match_context *mcontext,
         uint32_t value);

       This   parameter   limits   the   depth   of   nested  backtracking  in
       pcre2_match().  Each time a nested backtracking point is passed, a  new
       memory "frame" is used to remember the state of matching at that point.
       Thus, this parameter indirectly limits the amount  of  memory  that  is
       used  in  a  match.  However,  because  the size of each memory "frame"
       depends on the number of capturing parentheses, the actual memory limit
       varies  from pattern to pattern. This limit was more useful in versions
       before 10.30, where function recursion was used for backtracking.

       The depth limit is not relevant, and is ignored, when matching is  done
       using JIT compiled code. However, it is supported by pcre2_dfa_match(),
       which uses it to limit the depth of nested internal recursive  function
       calls  that implement atomic groups, lookaround assertions, and pattern
       recursions. This limits, indirectly, the amount of system stack that is
       used.  It  was  more useful in versions before 10.32, when stack memory
       was used for local workspace vectors for recursive function calls. From
       version  10.32,  only local variables are allocated on the stack and as
       each call uses only a few hundred bytes, even a small stack can support
       quite a lot of recursion.

       If  the  depth  of  internal  recursive function calls is great enough,
       local workspace vectors are allocated on the heap  from  version  10.32
       onwards,  so  the depth limit also indirectly limits the amount of heap
       memory that is used. A recursive pattern such as /(.(?2))((?1)|)/, when
       matched  to a very long string using pcre2_dfa_match(), can use a great
       deal of memory. However, it is probably  better  to  limit  heap  usage
       directly by calling pcre2_set_heap_limit().

       The  default  value for the depth limit can be set when PCRE2 is built;
       if it is not, the default is set to the same value as the  default  for
       the   match   limit.   If  the  limit  is  exceeded,  pcre2_match()  or
       pcre2_dfa_match() returns PCRE2_ERROR_DEPTHLIMIT. A value for the depth
       limit  may also be supplied by an item at the start of a pattern of the
       form

         (*LIMIT_DEPTH=ddd)

       where ddd is a decimal number.  However,  such  a  setting  is  ignored
       unless ddd is less than the limit set by the caller of pcre2_match() or
       pcre2_dfa_match() or, if no such limit is set, less than the default.


CHECKING BUILD-TIME OPTIONS

       int pcre2_config(uint32_t what, void *where);

       The function pcre2_config() makes it possible for  a  PCRE2  client  to
       discover  which  optional  features  have  been compiled into the PCRE2
       library. The pcre2build documentation  has  more  details  about  these
       optional features.

       The  first  argument  for pcre2_config() specifies which information is
       required. The second argument is a pointer to  memory  into  which  the
       information  is  placed.  If  NULL  is passed, the function returns the
       amount of memory that is needed  for  the  requested  information.  For
       calls  that  return  numerical  values,  the  value  is  in bytes; when
       requesting these values, where should point  to  appropriately  aligned
       memory.  For calls that return strings, the required length is given in
       code units, not counting the terminating zero.

       When requesting information, the returned value from pcre2_config()  is
       non-negative  on success, or the negative error code PCRE2_ERROR_BADOP-
       TION if the value in the first argument is not recognized. The  follow-
       ing information is available:

         PCRE2_CONFIG_BSR

       The  output  is a uint32_t integer whose value indicates what character
       sequences the \R  escape  sequence  matches  by  default.  A  value  of
       PCRE2_BSR_UNICODE  means  that  \R  matches  any  Unicode  line  ending
       sequence; a value of PCRE2_BSR_ANYCRLF means that \R matches  only  CR,
       LF, or CRLF. The default can be overridden when a pattern is compiled.

         PCRE2_CONFIG_COMPILED_WIDTHS

       The  output  is a uint32_t integer whose lower bits indicate which code
       unit widths were selected when PCRE2 was  built.  The  1-bit  indicates
       8-bit  support, and the 2-bit and 4-bit indicate 16-bit and 32-bit sup-
       port, respectively.

         PCRE2_CONFIG_DEPTHLIMIT

       The output is a uint32_t integer that gives the default limit  for  the
       depth  of  nested  backtracking in pcre2_match() or the depth of nested
       recursions, lookarounds, and atomic groups in  pcre2_dfa_match().  Fur-
       ther details are given with pcre2_set_depth_limit() above.

         PCRE2_CONFIG_HEAPLIMIT

       The  output is a uint32_t integer that gives, in kibibytes, the default
       limit  for  the  amount  of  heap  memory  used  by  pcre2_match()   or
       pcre2_dfa_match().      Further      details     are     given     with
       pcre2_set_heap_limit() above.

         PCRE2_CONFIG_JIT

       The output is a uint32_t integer that is set  to  one  if  support  for
       just-in-time compiling is available; otherwise it is set to zero.

         PCRE2_CONFIG_JITTARGET

       The  where  argument  should point to a buffer that is at least 48 code
       units long.  (The  exact  length  required  can  be  found  by  calling
       pcre2_config()  with  where  set  to NULL.) The buffer is filled with a
       string that contains the name of the architecture  for  which  the  JIT
       compiler  is  configured,  for  example  "x86  32bit  (little  endian +
       unaligned)". If JIT support is not available, PCRE2_ERROR_BADOPTION  is
       returned,  otherwise the number of code units used is returned. This is
       the length of the string, plus one unit for the terminating zero.

         PCRE2_CONFIG_LINKSIZE

       The output is a uint32_t integer that contains the number of bytes used
       for  internal  linkage  in  compiled regular expressions. When PCRE2 is
       configured, the value can be set to 2, 3, or 4, with the default  being
       2.  This is the value that is returned by pcre2_config(). However, when
       the 16-bit library is compiled, a value of 3 is rounded up  to  4,  and
       when  the  32-bit  library  is compiled, internal linkages always use 4
       bytes, so the configured value is not relevant.

       The default value of 2 for the 8-bit and 16-bit libraries is sufficient
       for  all but the most massive patterns, since it allows the size of the
       compiled pattern to be up to 65535  code  units.  Larger  values  allow
       larger  regular  expressions to be compiled by those two libraries, but
       at the expense of slower matching.

         PCRE2_CONFIG_MATCHLIMIT

       The output is a uint32_t integer that gives the default match limit for
       pcre2_match().  Further  details are given with pcre2_set_match_limit()
       above.

         PCRE2_CONFIG_NEWLINE

       The output is a uint32_t integer  whose  value  specifies  the  default
       character  sequence that is recognized as meaning "newline". The values
       are:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF
         PCRE2_NEWLINE_NUL      The NUL character (binary zero)

       The default should normally correspond to  the  standard  sequence  for
       your operating system.

         PCRE2_CONFIG_NEVER_BACKSLASH_C

       The  output  is  a uint32_t integer that is set to one if the use of \C
       was permanently disabled when PCRE2 was built; otherwise it is  set  to
       zero.

         PCRE2_CONFIG_PARENSLIMIT

       The  output is a uint32_t integer that gives the maximum depth of nest-
       ing of parentheses (of any kind) in a pattern. This limit is imposed to
       cap  the  amount of system stack used when a pattern is compiled. It is
       specified when PCRE2 is built; the default is 250. This limit does  not
       take  into  account  the  stack that may already be used by the calling
       application. For  finer  control  over  compilation  stack  usage,  see
       pcre2_set_compile_recursion_guard().

         PCRE2_CONFIG_STACKRECURSE

       This parameter is obsolete and should not be used in new code. The out-
       put is a uint32_t integer that is always set to zero.

         PCRE2_CONFIG_UNICODE_VERSION

       The where argument should point to a buffer that is at  least  24  code
       units  long.  (The  exact  length  required  can  be  found  by calling
       pcre2_config() with where set to NULL.)  If  PCRE2  has  been  compiled
       without  Unicode  support,  the buffer is filled with the text "Unicode
       not supported". Otherwise, the Unicode  version  string  (for  example,
       "8.0.0")  is  inserted. The number of code units used is returned. This
       is the length of the string plus one unit for the terminating zero.

         PCRE2_CONFIG_UNICODE

       The output is a uint32_t integer that is set to one if Unicode  support
       is  available; otherwise it is set to zero. Unicode support implies UTF
       support.

         PCRE2_CONFIG_VERSION

       The where argument should point to a buffer that is at  least  24  code
       units  long.  (The  exact  length  required  can  be  found  by calling
       pcre2_config() with where set to NULL.) The buffer is filled  with  the
       PCRE2 version string, zero-terminated. The number of code units used is
       returned. This is the length of the string plus one unit for the termi-
       nating zero.


COMPILING A PATTERN

       pcre2_code *pcre2_compile(PCRE2_SPTR pattern, PCRE2_SIZE length,
         uint32_t options, int *errorcode, PCRE2_SIZE *erroroffset,
         pcre2_compile_context *ccontext);

       void pcre2_code_free(pcre2_code *code);

       pcre2_code *pcre2_code_copy(const pcre2_code *code);

       pcre2_code *pcre2_code_copy_with_tables(const pcre2_code *code);

       The  pcre2_compile() function compiles a pattern into an internal form.
       The pattern is defined by a pointer to a string of  code  units  and  a
       length  (in  code units). If the pattern is zero-terminated, the length
       can be specified  as  PCRE2_ZERO_TERMINATED.  The  function  returns  a
       pointer  to  a  block  of memory that contains the compiled pattern and
       related data, or NULL if an error occurred.

       If the compile context argument ccontext is NULL, memory for  the  com-
       piled  pattern  is  obtained  by  calling  malloc().  Otherwise,  it is
       obtained from the same memory function that was used  for  the  compile
       context.  The  caller must free the memory by calling pcre2_code_free()
       when it is no longer needed.  If pcre2_code_free()  is  called  with  a
       NULL argument, it returns immediately, without doing anything.

       The function pcre2_code_copy() makes a copy of the compiled code in new
       memory, using the same memory allocator as was used for  the  original.
       However,  if  the  code  has  been  processed  by the JIT compiler (see
       below), the JIT information cannot be copied (because it  is  position-
       dependent).  The new copy can initially be used only for non-JIT match-
       ing, though it can be passed to  pcre2_jit_compile()  if  required.  If
       pcre2_code_copy() is called with a NULL argument, it returns NULL.

       The pcre2_code_copy() function provides a way for individual threads in
       a multithreaded application to acquire a private copy  of  shared  com-
       piled  code.   However, it does not make a copy of the character tables
       used by the compiled pattern; the new pattern code points to  the  same
       tables  as  the original code.  (See "Locale Support" below for details
       of these character tables.) In many applications the  same  tables  are
       used  throughout, so this behaviour is appropriate. Nevertheless, there
       are occasions when a copy of a compiled pattern and the relevant tables
       are  needed.  The pcre2_code_copy_with_tables() provides this facility.
       Copies of both the code and the tables are  made,  with  the  new  code
       pointing  to the new tables. The memory for the new tables is automati-
       cally freed when pcre2_code_free() is called for the new  copy  of  the
       compiled  code.  If pcre2_code_copy_with_tables() is called with a NULL
       argument, it returns NULL.

       NOTE: When one of the matching functions is  called,  pointers  to  the
       compiled pattern and the subject string are set in the match data block
       so that they can be referenced by the  substring  extraction  functions
       after  a  successful match.  After running a match, you must not free a
       compiled pattern or a subject string until after all operations on  the
       match  data  block have taken place, unless, in the case of the subject
       string, you have used the PCRE2_COPY_MATCHED_SUBJECT option,  which  is
       described  in  the  section  entitled  "Option  bits for pcre2_match()"
       below.

       The options argument for pcre2_compile() contains various bit  settings
       that  affect  the  compilation.  It  should be zero if none of them are
       required. The available options are described below. Some of  them  (in
       particular,  those  that  are  compatible with Perl, but some others as
       well) can also be set and  unset  from  within  the  pattern  (see  the
       detailed description in the pcre2pattern documentation).

       For  those options that can be different in different parts of the pat-
       tern, the contents of the options argument specifies their settings  at
       the  start  of  compilation. The PCRE2_ANCHORED, PCRE2_ENDANCHORED, and
       PCRE2_NO_UTF_CHECK options can be set at the time of matching  as  well
       as at compile time.

       Some  additional  options  and  less  frequently  required compile-time
       parameters (for example, the newline setting) can be provided in a com-
       pile context (as described above).

       If errorcode or erroroffset is NULL, pcre2_compile() returns NULL imme-
       diately. Otherwise, the variables to which these point are  set  to  an
       error  code  and  an  offset (number of code units) within the pattern,
       respectively, when pcre2_compile() returns NULL because  a  compilation
       error has occurred. The values are not defined when compilation is suc-
       cessful and pcre2_compile() returns a non-NULL value.

       There are nearly 100 positive  error  codes  that  pcre2_compile()  may
       return  if  it finds an error in the pattern. There are also some nega-
       tive error codes that are used for invalid UTF strings. These  are  the
       same as given by pcre2_match() and pcre2_dfa_match(), and are described
       in the pcre2unicode page. There is no separate  documentation  for  the
       positive  error  codes,  because  the  textual  error messages that are
       obtained  by  calling  the  pcre2_get_error_message()   function   (see
       "Obtaining  a textual error message" below) should be self-explanatory.
       Macro names starting with PCRE2_ERROR_ are defined  for  both  positive
       and negative error codes in pcre2.h.

       The value returned in erroroffset is an indication of where in the pat-
       tern the error occurred. It is not necessarily the  furthest  point  in
       the  pattern  that  was  read. For example, after the error "lookbehind
       assertion is not fixed length", the error offset points to the start of
       the  failing assertion. For an invalid UTF-8 or UTF-16 string, the off-
       set is that of the first code unit of the failing character.

       Some errors are not detected until the whole pattern has been  scanned;
       in  these  cases,  the offset passed back is the length of the pattern.
       Note that the offset is in code units, not characters, even  in  a  UTF
       mode. It may sometimes point into the middle of a UTF-8 or UTF-16 char-
       acter.

       This code fragment shows a typical straightforward call  to  pcre2_com-
       pile():

         pcre2_code *re;
         PCRE2_SIZE erroffset;
         int errorcode;
         re = pcre2_compile(
           "^A.*Z",                /* the pattern */
           PCRE2_ZERO_TERMINATED,  /* the pattern is zero-terminated */
           0,                      /* default options */
           &errorcode,             /* for error code */
           &erroffset,             /* for error offset */
           NULL);                  /* no compile context */


   Main compile options

       The  following  names for option bits are defined in the pcre2.h header
       file:

         PCRE2_ANCHORED

       If this bit is set, the pattern is forced to be "anchored", that is, it
       is  constrained to match only at the first matching point in the string
       that is being searched (the "subject string"). This effect can also  be
       achieved  by appropriate constructs in the pattern itself, which is the
       only way to do it in Perl.

         PCRE2_ALLOW_EMPTY_CLASS

       By default, for compatibility with Perl, a closing square bracket  that
       immediately  follows  an opening one is treated as a data character for
       the class. When  PCRE2_ALLOW_EMPTY_CLASS  is  set,  it  terminates  the
       class, which therefore contains no characters and so can never match.

         PCRE2_ALT_BSUX

       This  option  request  alternative  handling of three escape sequences,
       which makes PCRE2's behaviour more like  ECMAscript  (aka  JavaScript).
       When it is set:

       (1) \U matches an upper case "U" character; by default \U causes a com-
       pile time error (Perl uses \U to upper case subsequent characters).

       (2) \u matches a lower case "u" character unless it is followed by four
       hexadecimal  digits,  in  which case the hexadecimal number defines the
       code point to match. By default, \u causes a compile time  error  (Perl
       uses it to upper case the following character).

       (3)  \x matches a lower case "x" character unless it is followed by two
       hexadecimal digits, in which case the hexadecimal  number  defines  the
       code  point  to  match. By default, as in Perl, a hexadecimal number is
       always expected after \x, but it may have zero, one, or two digits (so,
       for example, \xz matches a binary zero character followed by z).

       ECMAscript 6 added additional functionality to \u. This can be accessed
       using  the  PCRE2_EXTRA_ALT_BSUX  extra  option  (see  "Extra   compile
       options"  below).   Note  that this alternative escape handling applies
       only to patterns. Neither of these options affects  the  processing  of
       replacement strings passed to pcre2_substitute().

         PCRE2_ALT_CIRCUMFLEX

       In  multiline  mode  (when  PCRE2_MULTILINE  is  set),  the  circumflex
       metacharacter matches at the start of the subject (unless  PCRE2_NOTBOL
       is  set),  and  also  after  any internal newline. However, it does not
       match after a newline at the end of the subject, for compatibility with
       Perl.  If  you want a multiline circumflex also to match after a termi-
       nating newline, you must set PCRE2_ALT_CIRCUMFLEX.

         PCRE2_ALT_VERBNAMES

       By default, for compatibility with Perl, the name in any verb  sequence
       such  as  (*MARK:NAME)  is  any  sequence  of  characters that does not
       include a closing parenthesis. The name is not processed  in  any  way,
       and  it  is  not possible to include a closing parenthesis in the name.
       However, if the PCRE2_ALT_VERBNAMES option  is  set,  normal  backslash
       processing  is  applied  to  verb  names  and only an unescaped closing
       parenthesis terminates the name. A closing parenthesis can be  included
       in  a  name either as \) or between \Q and \E. If the PCRE2_EXTENDED or
       PCRE2_EXTENDED_MORE option is set with  PCRE2_ALT_VERBNAMES,  unescaped
       whitespace  in  verb  names  is  skipped and #-comments are recognized,
       exactly as in the rest of the pattern.

         PCRE2_AUTO_CALLOUT

       If this bit  is  set,  pcre2_compile()  automatically  inserts  callout
       items,  all  with  number 255, before each pattern item, except immedi-
       ately before or after an explicit callout in the pattern.  For  discus-
       sion of the callout facility, see the pcre2callout documentation.

         PCRE2_CASELESS

       If  this  bit is set, letters in the pattern match both upper and lower
       case letters in the subject. It is equivalent to Perl's /i option,  and
       it  can  be  changed  within  a  pattern  by  a (?i) option setting. If
       PCRE2_UTF is set, Unicode properties are used for all  characters  with
       more  than one other case, and for all characters whose code points are
       greater than U+007F. For lower valued characters with  only  one  other
       case,  a  lookup  table is used for speed. When PCRE2_UTF is not set, a
       lookup table is used for all code points less than 256, and higher code
       points  (available  only  in  16-bit or 32-bit mode) are treated as not
       having another case.

         PCRE2_DOLLAR_ENDONLY

       If this bit is set, a dollar metacharacter in the pattern matches  only
       at  the  end  of the subject string. Without this option, a dollar also
       matches immediately before a newline at the end of the string (but  not
       before  any other newlines). The PCRE2_DOLLAR_ENDONLY option is ignored
       if PCRE2_MULTILINE is set. There is no equivalent  to  this  option  in
       Perl, and no way to set it within a pattern.

         PCRE2_DOTALL

       If  this  bit  is  set,  a dot metacharacter in the pattern matches any
       character, including one that indicates a  newline.  However,  it  only
       ever matches one character, even if newlines are coded as CRLF. Without
       this option, a dot does not match when the current position in the sub-
       ject  is  at  a newline. This option is equivalent to Perl's /s option,
       and it can be changed within a pattern by a (?s) option setting. A neg-
       ative  class such as [^a] always matches newline characters, and the \N
       escape sequence always matches a non-newline character, independent  of
       the setting of PCRE2_DOTALL.

         PCRE2_DUPNAMES

       If  this  bit is set, names used to identify capture groups need not be
       unique.  This can be helpful for certain types of pattern  when  it  is
       known  that  only  one instance of the named group can ever be matched.
       There are more details of named capture  groups  below;  see  also  the
       pcre2pattern documentation.

         PCRE2_ENDANCHORED

       If  this  bit is set, the end of any pattern match must be right at the
       end of the string being searched (the "subject string"). If the pattern
       match succeeds by reaching (*ACCEPT), but does not reach the end of the
       subject, the match fails at the current starting point. For  unanchored
       patterns,  a  new  match is then tried at the next starting point. How-
       ever, if the match succeeds by reaching the end of the pattern, but not
       the  end  of  the subject, backtracking occurs and an alternative match
       may be found. Consider these two patterns:

         .(*ACCEPT)|..
         .|..

       If matched against "abc" with PCRE2_ENDANCHORED set, the first  matches
       "c"  whereas  the  second matches "bc". The effect of PCRE2_ENDANCHORED
       can also be achieved by appropriate constructs in the  pattern  itself,
       which is the only way to do it in Perl.

       For DFA matching with pcre2_dfa_match(), PCRE2_ENDANCHORED applies only
       to the first (that is, the  longest)  matched  string.  Other  parallel
       matches,  which are necessarily substrings of the first one, must obvi-
       ously end before the end of the subject.

         PCRE2_EXTENDED

       If this bit is set, most white space  characters  in  the  pattern  are
       totally  ignored  except when escaped or inside a character class. How-
       ever, white space is not allowed within  sequences  such  as  (?>  that
       introduce  various  parenthesized  groups, nor within numerical quanti-
       fiers such as {1,3}. Ignorable white space is permitted between an item
       and  a  following quantifier and between a quantifier and a following +
       that indicates possessiveness. PCRE2_EXTENDED is equivalent  to  Perl's
       /x option, and it can be changed within a pattern by a (?x) option set-
       ting.

       When PCRE2 is compiled without Unicode support,  PCRE2_EXTENDED  recog-
       nizes  as  white space only those characters with code points less than
       256 that are flagged as white space in its low-character table. The ta-
       ble is normally created by pcre2_maketables(), which uses the isspace()
       function to identify space characters. In most ASCII environments,  the
       relevant  characters  are  those  with code points 0x0009 (tab), 0x000A
       (linefeed), 0x000B (vertical tab), 0x000C (formfeed), 0x000D  (carriage
       return), and 0x0020 (space).

       When PCRE2 is compiled with Unicode support, in addition to these char-
       acters, five more Unicode "Pattern White Space" characters  are  recog-
       nized by PCRE2_EXTENDED. These are U+0085 (next line), U+200E (left-to-
       right mark), U+200F (right-to-left mark), U+2028 (line separator),  and
       U+2029  (paragraph  separator).  This  set of characters is the same as
       recognized by Perl's /x option. Note that the horizontal  and  vertical
       space  characters that are matched by the \h and \v escapes in patterns
       are a much bigger set.

       As well as ignoring most white space, PCRE2_EXTENDED also causes  char-
       acters  between  an  unescaped # outside a character class and the next
       newline, inclusive, to be ignored, which makes it possible  to  include
       comments inside complicated patterns. Note that the end of this type of
       comment is a literal newline sequence in the pattern; escape  sequences
       that happen to represent a newline do not count.

       Which characters are interpreted as newlines can be specified by a set-
       ting in the compile context that is passed to pcre2_compile() or  by  a
       special  sequence at the start of the pattern, as described in the sec-
       tion entitled "Newline conventions" in the pcre2pattern  documentation.
       A default is defined when PCRE2 is built.

         PCRE2_EXTENDED_MORE

       This  option  has  the  effect  of  PCRE2_EXTENDED,  but,  in addition,
       unescaped space and horizontal tab  characters  are  ignored  inside  a
       character  class.  Note: only these two characters are ignored, not the
       full set of pattern white space characters that are ignored  outside  a
       character  class.  PCRE2_EXTENDED_MORE  is  equivalent  to  Perl's  /xx
       option, and it can be changed within a pattern by a (?xx)  option  set-
       ting.

         PCRE2_FIRSTLINE

       If this option is set, the start of an unanchored pattern match must be
       before or at the first newline in  the  subject  string  following  the
       start  of  matching, though the matched text may continue over the new-
       line. If startoffset is non-zero, the limiting newline is not necessar-
       ily  the  first  newline  in  the  subject. For example, if the subject
       string is "abc\nxyz" (where \n represents a single-character newline) a
       pattern  match for "yz" succeeds with PCRE2_FIRSTLINE if startoffset is
       greater than 3. See also PCRE2_USE_OFFSET_LIMIT, which provides a  more
       general  limiting  facility.  If  PCRE2_FIRSTLINE is set with an offset
       limit, a match must occur in the first line and also within the  offset
       limit. In other words, whichever limit comes first is used.

         PCRE2_LITERAL

       If this option is set, all meta-characters in the pattern are disabled,
       and it is treated as a literal string. Matching literal strings with  a
       regular expression engine is not the most efficient way of doing it. If
       you are doing a lot of literal matching and  are  worried  about  effi-
       ciency, you should consider using other approaches. The only other main
       options  that  are  allowed  with  PCRE2_LITERAL  are:  PCRE2_ANCHORED,
       PCRE2_ENDANCHORED, PCRE2_AUTO_CALLOUT, PCRE2_CASELESS, PCRE2_FIRSTLINE,
       PCRE2_NO_START_OPTIMIZE,     PCRE2_NO_UTF_CHECK,     PCRE2_UTF,     and
       PCRE2_USE_OFFSET_LIMIT.  The  extra  options PCRE2_EXTRA_MATCH_LINE and
       PCRE2_EXTRA_MATCH_WORD are also supported. Any other options  cause  an
       error.

         PCRE2_MATCH_UNSET_BACKREF

       If  this  option  is  set,  a  backreference  to an unset capture group
       matches an empty string (by default this causes  the  current  matching
       alternative  to  fail).   A  pattern such as (\1)(a) succeeds when this
       option is set (assuming it can find an "a" in the subject), whereas  it
       fails  by  default,  for  Perl compatibility. Setting this option makes
       PCRE2 behave more like ECMAscript (aka JavaScript).

         PCRE2_MULTILINE

       By default, for the purposes of matching "start of line"  and  "end  of
       line",  PCRE2  treats the subject string as consisting of a single line
       of characters, even if it actually contains  newlines.  The  "start  of
       line"  metacharacter  (^)  matches only at the start of the string, and
       the "end of line" metacharacter ($) matches only  at  the  end  of  the
       string,  or  before  a  terminating  newline  (except  when  PCRE2_DOL-
       LAR_ENDONLY is set). Note, however, that unless  PCRE2_DOTALL  is  set,
       the "any character" metacharacter (.) does not match at a newline. This
       behaviour (for ^, $, and dot) is the same as Perl.

       When PCRE2_MULTILINE it is set, the "start of line" and "end  of  line"
       constructs  match  immediately following or immediately before internal
       newlines in the subject string, respectively, as well as  at  the  very
       start  and  end.  This is equivalent to Perl's /m option, and it can be
       changed within a pattern by a (?m) option setting. Note that the "start
       of line" metacharacter does not match after a newline at the end of the
       subject, for compatibility with Perl.  However, you can change this  by
       setting  the PCRE2_ALT_CIRCUMFLEX option. If there are no newlines in a
       subject string, or no occurrences of ^  or  $  in  a  pattern,  setting
       PCRE2_MULTILINE has no effect.

         PCRE2_NEVER_BACKSLASH_C

       This  option  locks out the use of \C in the pattern that is being com-
       piled.  This escape can  cause  unpredictable  behaviour  in  UTF-8  or
       UTF-16  modes,  because  it may leave the current matching point in the
       middle of a multi-code-unit character. This option  may  be  useful  in
       applications  that  process  patterns  from external sources. Note that
       there is also a build-time option that permanently locks out the use of
       \C.

         PCRE2_NEVER_UCP

       This  option  locks  out the use of Unicode properties for handling \B,
       \b, \D, \d, \S, \s, \W, \w, and some of the POSIX character classes, as
       described  for  the  PCRE2_UCP option below. In particular, it prevents
       the creator of the pattern from enabling this facility by starting  the
       pattern  with  (*UCP).  This  option may be useful in applications that
       process patterns from external sources. The option combination PCRE_UCP
       and PCRE_NEVER_UCP causes an error.

         PCRE2_NEVER_UTF

       This  option  locks out interpretation of the pattern as UTF-8, UTF-16,
       or UTF-32, depending on which library is in use. In particular, it pre-
       vents  the  creator of the pattern from switching to UTF interpretation
       by starting the pattern with (*UTF).  This  option  may  be  useful  in
       applications  that process patterns from external sources. The combina-
       tion of PCRE2_UTF and PCRE2_NEVER_UTF causes an error.

         PCRE2_NO_AUTO_CAPTURE

       If this option is set, it disables the use of numbered capturing paren-
       theses  in the pattern. Any opening parenthesis that is not followed by
       ? behaves as if it were followed by ?: but named parentheses can  still
       be used for capturing (and they acquire numbers in the usual way). This
       is the same as Perl's /n option.  Note that, when this option  is  set,
       references  to  capture  groups (backreferences or recursion/subroutine
       calls) may only refer to named groups, though the reference can  be  by
       name or by number.

         PCRE2_NO_AUTO_POSSESS

       If this option is set, it disables "auto-possessification", which is an
       optimization that, for example, turns a+b into a++b in order  to  avoid
       backtracks  into  a+ that can never be successful. However, if callouts
       are in use, auto-possessification means that some  callouts  are  never
       taken. You can set this option if you want the matching functions to do
       a full unoptimized search and run all the callouts, but  it  is  mainly
       provided for testing purposes.

         PCRE2_NO_DOTSTAR_ANCHOR

       If this option is set, it disables an optimization that is applied when
       .* is the first significant item in a top-level branch  of  a  pattern,
       and  all  the  other branches also start with .* or with \A or \G or ^.
       The optimization is automatically disabled for .* if it  is  inside  an
       atomic group or a capture group that is the subject of a backreference,
       or if the pattern contains (*PRUNE) or (*SKIP). When  the  optimization
       is   not   disabled,  such  a  pattern  is  automatically  anchored  if
       PCRE2_DOTALL is set for all the .* items and PCRE2_MULTILINE is not set
       for  any  ^ items. Otherwise, the fact that any match must start either
       at the start of the subject or following a newline is remembered.  Like
       other optimizations, this can cause callouts to be skipped.

         PCRE2_NO_START_OPTIMIZE

       This  is  an  option whose main effect is at matching time. It does not
       change what pcre2_compile() generates, but it does affect the output of
       the JIT compiler.

       There  are  a  number of optimizations that may occur at the start of a
       match, in order to speed up the process. For example, if  it  is  known
       that  an  unanchored  match must start with a specific code unit value,
       the matching code searches the subject for that value, and fails  imme-
       diately  if it cannot find it, without actually running the main match-
       ing function. This means that a special item such as (*COMMIT)  at  the
       start  of  a  pattern is not considered until after a suitable starting
       point for the match has been found.  Also,  when  callouts  or  (*MARK)
       items  are  in use, these "start-up" optimizations can cause them to be
       skipped if the pattern is never actually used. The  start-up  optimiza-
       tions  are  in effect a pre-scan of the subject that takes place before
       the pattern is run.

       The PCRE2_NO_START_OPTIMIZE option disables the start-up optimizations,
       possibly  causing  performance  to  suffer,  but ensuring that in cases
       where the result is "no match", the callouts do occur, and  that  items
       such as (*COMMIT) and (*MARK) are considered at every possible starting
       position in the subject string.

       Setting PCRE2_NO_START_OPTIMIZE may change the outcome  of  a  matching
       operation.  Consider the pattern

         (*COMMIT)ABC

       When  this  is compiled, PCRE2 records the fact that a match must start
       with the character "A". Suppose the subject  string  is  "DEFABC".  The
       start-up  optimization  scans along the subject, finds "A" and runs the
       first match attempt from there. The (*COMMIT) item means that the  pat-
       tern  must  match the current starting position, which in this case, it
       does. However, if the same match is  run  with  PCRE2_NO_START_OPTIMIZE
       set,  the  initial  scan  along the subject string does not happen. The
       first match attempt is run starting  from  "D"  and  when  this  fails,
       (*COMMIT)  prevents  any  further  matches  being tried, so the overall
       result is "no match".

       There are also other start-up optimizations.  For  example,  a  minimum
       length for the subject may be recorded. Consider the pattern

         (*MARK:A)(X|Y)

       The  minimum  length  for  a  match is one character. If the subject is
       "ABC", there will be attempts to match "ABC", "BC", and "C". An attempt
       to match an empty string at the end of the subject does not take place,
       because PCRE2 knows that the subject is  now  too  short,  and  so  the
       (*MARK)  is  never encountered. In this case, the optimization does not
       affect the overall match result, which is still "no match", but it does
       affect the auxiliary information that is returned.

         PCRE2_NO_UTF_CHECK

       When  PCRE2_UTF  is set, the validity of the pattern as a UTF string is
       automatically checked. There are  discussions  about  the  validity  of
       UTF-8  strings,  UTF-16 strings, and UTF-32 strings in the pcre2unicode
       document. If an invalid UTF sequence is found, pcre2_compile()  returns
       a negative error code.

       If  you  know  that your pattern is a valid UTF string, and you want to
       skip  this  check  for   performance   reasons,   you   can   set   the
       PCRE2_NO_UTF_CHECK  option.  When  it  is set, the effect of passing an
       invalid UTF string as a pattern is undefined. It may cause your program
       to crash or loop.

       Note  that  this  option  can  also  be  passed  to  pcre2_match()  and
       pcre_dfa_match(), to suppress UTF  validity  checking  of  the  subject
       string.

       Note also that setting PCRE2_NO_UTF_CHECK at compile time does not dis-
       able the error that is given if an escape sequence for an invalid  Uni-
       code  code  point is encountered in the pattern. In particular, the so-
       called "surrogate" code points (0xd800 to 0xdfff) are invalid.  If  you
       want  to  allow  escape  sequences  such  as  \x{d800}  you can set the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option, as described  in  the
       section  entitled "Extra compile options" below.  However, this is pos-
       sible only in UTF-8 and UTF-32 modes, because these values are not rep-
       resentable in UTF-16.

         PCRE2_UCP

       This option changes the way PCRE2 processes \B, \b, \D, \d, \S, \s, \W,
       \w, and some of the POSIX character classes.  By  default,  only  ASCII
       characters  are recognized, but if PCRE2_UCP is set, Unicode properties
       are used instead to classify characters. More details are given in  the
       section on generic character types in the pcre2pattern page. If you set
       PCRE2_UCP, matching one of the items it affects takes much longer.  The
       option  is  available only if PCRE2 has been compiled with Unicode sup-
       port (which is the default).

         PCRE2_UNGREEDY

       This option inverts the "greediness" of the quantifiers  so  that  they
       are  not greedy by default, but become greedy if followed by "?". It is
       not compatible with Perl. It can also be set by a (?U)  option  setting
       within the pattern.

         PCRE2_USE_OFFSET_LIMIT

       This option must be set for pcre2_compile() if pcre2_set_offset_limit()
       is going to be used to set a non-default offset limit in a  match  con-
       text  for  matches  that  use this pattern. An error is generated if an
       offset limit is set without this option.  For  more  details,  see  the
       description  of  pcre2_set_offset_limit() in the section that describes
       match contexts. See also the PCRE2_FIRSTLINE option above.

         PCRE2_UTF

       This option causes PCRE2 to regard both the  pattern  and  the  subject
       strings  that  are  subsequently processed as strings of UTF characters
       instead of single-code-unit strings. It  is  available  when  PCRE2  is
       built  to  include  Unicode  support (which is the default). If Unicode
       support is not available, the use of this  option  provokes  an  error.
       Details  of  how  PCRE2_UTF changes the behaviour of PCRE2 are given in
       the pcre2unicode page. In particular, note  that  it  changes  the  way
       PCRE2_CASELESS handles characters with code points greater than 127.

   Extra compile options

       The  option  bits  that  can be set in a compile context by calling the
       pcre2_set_compile_extra_options() function are as follows:

         PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES

       This option applies when compiling a pattern in UTF-8 or  UTF-32  mode.
       It  is  forbidden in UTF-16 mode, and ignored in non-UTF modes. Unicode
       "surrogate" code points in the range 0xd800 to 0xdfff are used in pairs
       in  UTF-16  to  encode  code points with values in the range 0x10000 to
       0x10ffff. The surrogates cannot therefore  be  represented  in  UTF-16.
       They can be represented in UTF-8 and UTF-32, but are defined as invalid
       code points, and cause errors if  encountered  in  a  UTF-8  or  UTF-32
       string that is being checked for validity by PCRE2.

       These  values also cause errors if encountered in escape sequences such
       as \x{d912} within a pattern. However, it seems that some applications,
       when  using  PCRE2  to  check for unwanted characters in UTF-8 strings,
       explicitly  test  for  the  surrogates  using  escape  sequences.   The
       PCRE2_NO_UTF_CHECK  option  does  not  disable  the  error that occurs,
       because it applies only to the testing of input strings for UTF  valid-
       ity.

       If  the extra option PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES is set, surro-
       gate code point values in UTF-8 and UTF-32 patterns no  longer  provoke
       errors  and are incorporated in the compiled pattern. However, they can
       only match subject characters if the matching function is  called  with
       PCRE2_NO_UTF_CHECK set.

         PCRE2_EXTRA_ALT_BSUX

       The  original option PCRE2_ALT_BSUX causes PCRE2 to process \U, \u, and
       \x in the way that ECMAscript (aka JavaScript) does.  Additional  func-
       tionality was defined by ECMAscript 6; setting PCRE2_EXTRA_ALT_BSUX has
       the effect of PCRE2_ALT_BSUX, but in addition it  recognizes  \u{hhh..}
       as a hexadecimal character code, where hhh.. is any number of hexadeci-
       mal digits.

         PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL

       This is a dangerous option. Use with care. By default, an  unrecognized
       escape  such  as \j or a malformed one such as \x{2z} causes a compile-
       time error when detected by pcre2_compile(). Perl is somewhat inconsis-
       tent  in  handling  such items: for example, \j is treated as a literal
       "j", and non-hexadecimal digits in \x{} are just ignored, though  warn-
       ings  are given in both cases if Perl's warning switch is enabled. How-
       ever, a malformed octal number after \o{  always  causes  an  error  in
       Perl.

       If  the  PCRE2_EXTRA_BAD_ESCAPE_IS_LITERAL  extra  option  is passed to
       pcre2_compile(), all unrecognized or  malformed  escape  sequences  are
       treated  as  single-character escapes. For example, \j is a literal "j"
       and \x{2z} is treated as  the  literal  string  "x{2z}".  Setting  this
       option  means  that  typos in patterns may go undetected and have unex-
       pected results. Also note that a sequence such as [\N{] is  interpreted
       as  a  malformed attempt at [\N{...}] and so is treated as [N{] whereas
       [\N] gives an error  because  an  unqualified  \N  is  a  valid  escape
       sequence  but is not supported in a character class. To reiterate: this
       is a dangerous option. Use with great care.

         PCRE2_EXTRA_ESCAPED_CR_IS_LF

       There are some legacy applications where the escape sequence  \r  in  a
       pattern  is expected to match a newline. If this option is set, \r in a
       pattern is converted to \n so that it matches a LF  (linefeed)  instead
       of  a CR (carriage return) character. The option does not affect a lit-
       eral CR in the pattern, nor does it affect CR specified as an  explicit
       code point such as \x{0D}.

         PCRE2_EXTRA_MATCH_LINE

       This  option  is  provided  for  use  by the -x option of pcre2grep. It
       causes the pattern only to match complete lines. This  is  achieved  by
       automatically  inserting  the  code for "^(?:" at the start of the com-
       piled pattern and ")$" at the end. Thus, when PCRE2_MULTILINE  is  set,
       the  matched  line  may  be  in  the middle of the subject string. This
       option can be used with PCRE2_LITERAL.

         PCRE2_EXTRA_MATCH_WORD

       This option is provided for use by  the  -w  option  of  pcre2grep.  It
       causes  the  pattern only to match strings that have a word boundary at
       the start and the end. This is achieved by automatically inserting  the
       code  for "\b(?:" at the start of the compiled pattern and ")\b" at the
       end. The option may be used with PCRE2_LITERAL. However, it is  ignored
       if PCRE2_EXTRA_MATCH_LINE is also set.


JUST-IN-TIME (JIT) COMPILATION

       int pcre2_jit_compile(pcre2_code *code, uint32_t options);

       int pcre2_jit_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       pcre2_jit_stack *pcre2_jit_stack_create(PCRE2_SIZE startsize,
         PCRE2_SIZE maxsize, pcre2_general_context *gcontext);

       void pcre2_jit_stack_assign(pcre2_match_context *mcontext,
         pcre2_jit_callback callback_function, void *callback_data);

       void pcre2_jit_stack_free(pcre2_jit_stack *jit_stack);

       These  functions  provide  support  for  JIT compilation, which, if the
       just-in-time compiler is available, further processes a  compiled  pat-
       tern into machine code that executes much faster than the pcre2_match()
       interpretive matching function. Full details are given in the  pcre2jit
       documentation.

       JIT  compilation  is  a heavyweight optimization. It can take some time
       for patterns to be analyzed, and for one-off matches  and  simple  pat-
       terns  the benefit of faster execution might be offset by a much slower
       compilation time.  Most (but not all) patterns can be optimized by  the
       JIT compiler.


LOCALE SUPPORT

       PCRE2  handles caseless matching, and determines whether characters are
       letters, digits, or whatever, by reference to a set of tables,  indexed
       by  character  code  point.  This applies only to characters whose code
       points are less than 256. By default, higher-valued code  points  never
       match  escapes  such as \w or \d.  However, if PCRE2 is built with Uni-
       code support, all characters can be tested with \p and \P, or, alterna-
       tively,  the  PCRE2_UCP  option  can be set when a pattern is compiled;
       this causes \w and friends to use Unicode property support  instead  of
       the built-in tables.

       The  use  of  locales  with Unicode is discouraged. If you are handling
       characters with code points greater than 128,  you  should  either  use
       Unicode support, or use locales, but not try to mix the two.

       PCRE2  contains  an  internal  set of character tables that are used by
       default.  These are sufficient for  many  applications.  Normally,  the
       internal tables recognize only ASCII characters. However, when PCRE2 is
       built, it is possible to cause the internal tables to be rebuilt in the
       default "C" locale of the local system, which may cause them to be dif-
       ferent.

       The internal tables can be overridden by tables supplied by the  appli-
       cation  that  calls  PCRE2.  These may be created in a different locale
       from the default.  As more and more applications change to  using  Uni-
       code, the need for this locale support is expected to die away.

       External  tables  are built by calling the pcre2_maketables() function,
       in the relevant locale. The result can be passed to pcre2_compile()  as
       often   as  necessary,  by  creating  a  compile  context  and  calling
       pcre2_set_character_tables() to set the  tables  pointer  therein.  For
       example,  to  build  and use tables that are appropriate for the French
       locale (where accented characters with  values  greater  than  128  are
       treated as letters), the following code could be used:

         setlocale(LC_CTYPE, "fr_FR");
         tables = pcre2_maketables(NULL);
         ccontext = pcre2_compile_context_create(NULL);
         pcre2_set_character_tables(ccontext, tables);
         re = pcre2_compile(..., ccontext);

       The  locale  name "fr_FR" is used on Linux and other Unix-like systems;
       if you are using Windows, the name for the French locale  is  "french".
       It  is the caller's responsibility to ensure that the memory containing
       the tables remains available for as long as it is needed.

       The pointer that is passed (via the compile context) to pcre2_compile()
       is  saved  with  the  compiled pattern, and the same tables are used by
       pcre2_match() and pcre_dfa_match(). Thus, for any single pattern,  com-
       pilation  and  matching  both  happen in the same locale, but different
       patterns can be processed in different locales.


INFORMATION ABOUT A COMPILED PATTERN

       int pcre2_pattern_info(const pcre2 *code, uint32_t what, void *where);

       The pcre2_pattern_info() function returns general information  about  a
       compiled pattern. For information about callouts, see the next section.
       The first argument for pcre2_pattern_info() is a pointer  to  the  com-
       piled pattern. The second argument specifies which piece of information
       is required, and the third argument is  a  pointer  to  a  variable  to
       receive  the data. If the third argument is NULL, the first argument is
       ignored, and the function returns the size in  bytes  of  the  variable
       that is required for the information requested. Otherwise, the yield of
       the function is zero for success, or one of the following negative num-
       bers:

         PCRE2_ERROR_NULL           the argument code was NULL
         PCRE2_ERROR_BADMAGIC       the "magic number" was not found
         PCRE2_ERROR_BADOPTION      the value of what was invalid
         PCRE2_ERROR_UNSET          the requested field is not set

       The  "magic  number" is placed at the start of each compiled pattern as
       an simple check against passing an arbitrary memory pointer. Here is  a
       typical  call of pcre2_pattern_info(), to obtain the length of the com-
       piled pattern:

         int rc;
         size_t length;
         rc = pcre2_pattern_info(
           re,               /* result of pcre2_compile() */
           PCRE2_INFO_SIZE,  /* what is required */
           &length);         /* where to put the data */

       The possible values for the second argument are defined in pcre2.h, and
       are as follows:

         PCRE2_INFO_ALLOPTIONS
         PCRE2_INFO_ARGOPTIONS
         PCRE2_INFO_EXTRAOPTIONS

       Return copies of the pattern's options. The third argument should point
       to a  uint32_t  variable.  PCRE2_INFO_ARGOPTIONS  returns  exactly  the
       options  that were passed to pcre2_compile(), whereas PCRE2_INFO_ALLOP-
       TIONS returns the compile options as modified by any  top-level  (*XXX)
       option  settings  such  as  (*UTF)  at the start of the pattern itself.
       PCRE2_INFO_EXTRAOPTIONS returns the extra options that were set in  the
       compile  context by calling the pcre2_set_compile_extra_options() func-
       tion.

       For  example,  if  the  pattern  /(*UTF)abc/  is  compiled   with   the
       PCRE2_EXTENDED   option,   the   result  for  PCRE2_INFO_ALLOPTIONS  is
       PCRE2_EXTENDED and PCRE2_UTF.  Option settings such as  (?i)  that  can
       change  within  a pattern do not affect the result of PCRE2_INFO_ALLOP-
       TIONS, even if they appear right at the start of the pattern. (This was
       different in some earlier releases.)

       A  pattern compiled without PCRE2_ANCHORED is automatically anchored by
       PCRE2 if the first significant item in every top-level branch is one of
       the following:

         ^     unless PCRE2_MULTILINE is set
         \A    always
         \G    always
         .*    sometimes - see below

       When  .* is the first significant item, anchoring is possible only when
       all the following are true:

         .* is not in an atomic group
         .* is not in a capture group that is the subject
              of a backreference
         PCRE2_DOTALL is in force for .*
         Neither (*PRUNE) nor (*SKIP) appears in the pattern
         PCRE2_NO_DOTSTAR_ANCHOR is not set

       For patterns that are auto-anchored, the PCRE2_ANCHORED bit is  set  in
       the options returned for PCRE2_INFO_ALLOPTIONS.

         PCRE2_INFO_BACKREFMAX

       Return  the  number  of  the  highest backreference in the pattern. The
       third argument should point to  an  uint32_t  variable.  Named  capture
       groups  acquire  numbers  as well as names, and these count towards the
       highest backreference. Backreferences such as \4 or  \g{12}  match  the
       captured characters of the given group, but in addition, the check that
       a capture group is set in a conditional group such as (?(3)a|b) is also
       a backreference.  Zero is returned if there are no backreferences.

         PCRE2_INFO_BSR

       The  output  is a uint32_t integer whose value indicates what character
       sequences the \R escape sequence matches. A value of  PCRE2_BSR_UNICODE
       means  that  \R  matches  any  Unicode line ending sequence; a value of
       PCRE2_BSR_ANYCRLF means that \R matches only CR, LF, or CRLF.

         PCRE2_INFO_CAPTURECOUNT

       Return the highest capture group number in  the  pattern.  In  patterns
       where (?| is not used, this is also the total number of capture groups.
       The third argument should point to an uint32_t variable.

         PCRE2_INFO_DEPTHLIMIT

       If the pattern set a backtracking depth limit by including an  item  of
       the  form  (*LIMIT_DEPTH=nnnn) at the start, the value is returned. The
       third argument should point to a uint32_t integer. If no such value has
       been   set,   the   call  to  pcre2_pattern_info()  returns  the  error
       PCRE2_ERROR_UNSET. Note that this limit will only be used during match-
       ing  if it is less than the limit set or defaulted by the caller of the
       match function.

         PCRE2_INFO_FIRSTBITMAP

       In the absence of a single first code unit for a non-anchored  pattern,
       pcre2_compile()  may construct a 256-bit table that defines a fixed set
       of values for the first code unit in any match. For example, a  pattern
       that  starts  with  [abc]  results in a table with three bits set. When
       code unit values greater than 255 are supported, the flag bit  for  255
       means  "any  code unit of value 255 or above". If such a table was con-
       structed, a pointer to it is returned. Otherwise NULL is returned.  The
       third argument should point to a const uint8_t * variable.

         PCRE2_INFO_FIRSTCODETYPE

       Return information about the first code unit of any matched string, for
       a non-anchored pattern. The third argument should point to an  uint32_t
       variable.  If there is a fixed first value, for example, the letter "c"
       from a pattern such as (cat|cow|coyote), 1 is returned, and  the  value
       can  be  retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is no fixed
       first value, but it is known that a match can occur only at  the  start
       of  the  subject  or following a newline in the subject, 2 is returned.
       Otherwise, and for anchored patterns, 0 is returned.

         PCRE2_INFO_FIRSTCODEUNIT

       Return the value of the first code unit of any  matched  string  for  a
       pattern  where  PCRE2_INFO_FIRSTCODETYPE returns 1; otherwise return 0.
       The third argument should point to an uint32_t variable. In  the  8-bit
       library,  the  value is always less than 256. In the 16-bit library the
       value can be up to 0xffff. In the 32-bit library  in  UTF-32  mode  the
       value can be up to 0x10ffff, and up to 0xffffffff when not using UTF-32
       mode.

         PCRE2_INFO_FRAMESIZE

       Return the size (in bytes) of the data frames that are used to remember
       backtracking  positions  when the pattern is processed by pcre2_match()
       without the use of JIT. The third argument should  point  to  a  size_t
       variable. The frame size depends on the number of capturing parentheses
       in the pattern. Each additional capture group adds two PCRE2_SIZE vari-
       ables.

         PCRE2_INFO_HASBACKSLASHC

       Return  1 if the pattern contains any instances of \C, otherwise 0. The
       third argument should point to an uint32_t variable.

         PCRE2_INFO_HASCRORLF

       Return 1 if the pattern contains any explicit  matches  for  CR  or  LF
       characters, otherwise 0. The third argument should point to an uint32_t
       variable. An explicit match is either a literal CR or LF character,  or
       \r  or  \n  or  one  of  the  equivalent  hexadecimal  or  octal escape
       sequences.

         PCRE2_INFO_HEAPLIMIT

       If the pattern set a heap memory limit by including an item of the form
       (*LIMIT_HEAP=nnnn) at the start, the value is returned. The third argu-
       ment should point to a uint32_t integer. If no such value has been set,
       the  call  to pcre2_pattern_info() returns the error PCRE2_ERROR_UNSET.
       Note that this limit will only be used during matching if  it  is  less
       than the limit set or defaulted by the caller of the match function.

         PCRE2_INFO_JCHANGED

       Return  1  if  the (?J) or (?-J) option setting is used in the pattern,
       otherwise 0. The third argument should point to an  uint32_t  variable.
       (?J)  and  (?-J) set and unset the local PCRE2_DUPNAMES option, respec-
       tively.

         PCRE2_INFO_JITSIZE

       If the compiled pattern was successfully  processed  by  pcre2_jit_com-
       pile(),  return  the  size  of  the JIT compiled code, otherwise return
       zero. The third argument should point to a size_t variable.

         PCRE2_INFO_LASTCODETYPE

       Returns 1 if there is a rightmost literal code unit that must exist  in
       any  matched string, other than at its start. The third argument should
       point to an uint32_t  variable.  If  there  is  no  such  value,  0  is
       returned.  When  1  is  returned,  the  code  unit  value itself can be
       retrieved using PCRE2_INFO_LASTCODEUNIT. For anchored patterns, a  last
       literal  value  is  recorded  only  if it follows something of variable
       length. For example, for the pattern /^a\d+z\d+/ the returned value  is
       1  (with  "z" returned from PCRE2_INFO_LASTCODEUNIT), but for /^a\dz\d/
       the returned value is 0.

         PCRE2_INFO_LASTCODEUNIT

       Return the value of the rightmost literal code unit that must exist  in
       any  matched  string,  other  than  at  its  start, for a pattern where
       PCRE2_INFO_LASTCODETYPE returns 1. Otherwise, return 0. The third argu-
       ment should point to an uint32_t variable.

         PCRE2_INFO_MATCHEMPTY

       Return  1  if the pattern might match an empty string, otherwise 0. The
       third argument should point to an uint32_t  variable.  When  a  pattern
       contains recursive subroutine calls it is not always possible to deter-
       mine whether or not it can match an empty string. PCRE2  takes  a  cau-
       tious approach and returns 1 in such cases.

         PCRE2_INFO_MATCHLIMIT

       If  the  pattern  set  a  match  limit by including an item of the form
       (*LIMIT_MATCH=nnnn) at the start, the  value  is  returned.  The  third
       argument  should point to a uint32_t integer. If no such value has been
       set,   the   call   to   pcre2_pattern_info()   returns    the    error
       PCRE2_ERROR_UNSET. Note that this limit will only be used during match-
       ing if it is less than the limit set or defaulted by the caller of  the
       match function.

         PCRE2_INFO_MAXLOOKBEHIND

       Return the number of characters (not code units) in the longest lookbe-
       hind assertion in the pattern. The third argument  should  point  to  a
       uint32_t  integer.  This information is useful when doing multi-segment
       matching using the partial matching facilities. Note  that  the  simple
       assertions \b and \B require a one-character lookbehind. \A also regis-
       ters a one-character lookbehind, though it does  not  actually  inspect
       the  previous  character. This is to ensure that at least one character
       from the old segment is retained when a new segment is processed.  Oth-
       erwise,  if  there  are  no  lookbehinds in the pattern, \A might match
       incorrectly at the start of a second or subsequent segment.

         PCRE2_INFO_MINLENGTH

       If a minimum length for matching  subject  strings  was  computed,  its
       value  is  returned.  Otherwise the returned value is 0. The value is a
       number of characters, which in UTF mode may be different from the  num-
       ber  of  code  units.   The  third argument should point to an uint32_t
       variable. The value is a lower bound to  the  length  of  any  matching
       string.  There  may  not be any strings of that length that do actually
       match, but every string that does match is at least that long.

         PCRE2_INFO_NAMECOUNT
         PCRE2_INFO_NAMEENTRYSIZE
         PCRE2_INFO_NAMETABLE

       PCRE2 supports the use of named as well as numbered capturing parenthe-
       ses.  The names are just an additional way of identifying the parenthe-
       ses, which still acquire numbers. Several convenience functions such as
       pcre2_substring_get_byname()  are provided for extracting captured sub-
       strings by name. It is also possible to extract the data  directly,  by
       first  converting  the  name to a number in order to access the correct
       pointers in the output vector (described with pcre2_match() below).  To
       do  the  conversion,  you  need to use the name-to-number map, which is
       described by these three values.

       The map consists of a number of  fixed-size  entries.  PCRE2_INFO_NAME-
       COUNT  gives  the number of entries, and PCRE2_INFO_NAMEENTRYSIZE gives
       the size of each entry in code units; both of these return  a  uint32_t
       value. The entry size depends on the length of the longest name.

       PCRE2_INFO_NAMETABLE returns a pointer to the first entry of the table.
       This is a PCRE2_SPTR pointer to a block of code  units.  In  the  8-bit
       library,  the  first two bytes of each entry are the number of the cap-
       turing parenthesis, most significant byte first. In the 16-bit library,
       the  pointer  points  to 16-bit code units, the first of which contains
       the parenthesis number. In the 32-bit library, the  pointer  points  to
       32-bit  code units, the first of which contains the parenthesis number.
       The rest of the entry is the corresponding name, zero terminated.

       The names are in alphabetical order. If (?| is used to create  multiple
       capture  groups  with  the  same number, as described in the section on
       duplicate group numbers in the pcre2pattern page,  the  groups  may  be
       given  the same name, but there is only one entry in the table. Differ-
       ent names for groups of the same number are not permitted.

       Duplicate names for capture groups with different numbers  are  permit-
       ted, but only if PCRE2_DUPNAMES is set. They appear in the table in the
       order in which they were found in the pattern. In the  absence  of  (?|
       this  is  the  order of increasing number; when (?| is used this is not
       necessarily the case because later capture groups may have  lower  num-
       bers.

       As  a  simple  example of the name/number table, consider the following
       pattern after compilation by the 8-bit library  (assume  PCRE2_EXTENDED
       is set, so white space - including newlines - is ignored):

         (?<date> (?<year>(\d\d)?\d\d) -
         (?<month>\d\d) - (?<day>\d\d) )

       There are four named capture groups, so the table has four entries, and
       each entry in the table is eight bytes long. The table is  as  follows,
       with non-printing bytes shows in hexadecimal, and undefined bytes shown
       as ??:

         00 01 d  a  t  e  00 ??
         00 05 d  a  y  00 ?? ??
         00 04 m  o  n  t  h  00
         00 02 y  e  a  r  00 ??

       When writing code to extract data from named capture groups  using  the
       name-to-number  map,  remember that the length of the entries is likely
       to be different for each compiled pattern.

         PCRE2_INFO_NEWLINE

       The output is one of the following uint32_t values:

         PCRE2_NEWLINE_CR       Carriage return (CR)
         PCRE2_NEWLINE_LF       Linefeed (LF)
         PCRE2_NEWLINE_CRLF     Carriage return, linefeed (CRLF)
         PCRE2_NEWLINE_ANY      Any Unicode line ending
         PCRE2_NEWLINE_ANYCRLF  Any of CR, LF, or CRLF
         PCRE2_NEWLINE_NUL      The NUL character (binary zero)

       This identifies the character sequence that will be recognized as mean-
       ing "newline" while matching.

         PCRE2_INFO_SIZE

       Return  the  size  of  the  compiled  pattern  in  bytes (for all three
       libraries). The third argument should point to a size_t variable.  This
       value  includes  the  size  of the general data block that precedes the
       code units of the compiled pattern itself. The value that is used  when
       pcre2_compile()  is  getting memory in which to place the compiled pat-
       tern may be slightly larger than the value  returned  by  this  option,
       because  there are cases where the code that calculates the size has to
       over-estimate. Processing a pattern with  the  JIT  compiler  does  not
       alter the value returned by this option.


INFORMATION ABOUT A PATTERN'S CALLOUTS

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might like to scan all the callouts in a  pattern  before  running  the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument is a pointer to a compiled pattern, the  second  points  to  a
       callback  function,  and the third is arbitrary user data. The callback
       function is called for every callout in the pattern  in  the  order  in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation block, and its second argument is the user_data  value  that  was
       passed  to  pcre2_callout_enumerate(). The contents of the callout enu-
       meration block are described in the pcre2callout  documentation,  which
       also gives further details about callouts.


SERIALIZATION AND PRECOMPILING

       It  is  possible  to  save  compiled patterns on disc or elsewhere, and
       reload them later, subject to a number of  restrictions.  The  host  on
       which  the  patterns  are  reloaded must be running the same version of
       PCRE2, with the same code unit width, and must also have the same endi-
       anness,  pointer  width,  and PCRE2_SIZE type. Before compiled patterns
       can be saved, they must be converted to a "serialized" form,  which  in
       the  case of PCRE2 is really just a bytecode dump.  The functions whose
       names begin with pcre2_serialize_ are used for converting to  and  from
       the  serialized form. They are described in the pcre2serialize documen-
       tation. Note that PCRE2 serialization does not  convert  compiled  pat-
       terns to an abstract format like Java or .NET serialization.


THE MATCH DATA BLOCK

       pcre2_match_data *pcre2_match_data_create(uint32_t ovecsize,
         pcre2_general_context *gcontext);

       pcre2_match_data *pcre2_match_data_create_from_pattern(
         const pcre2_code *code, pcre2_general_context *gcontext);

       void pcre2_match_data_free(pcre2_match_data *match_data);

       Information  about  a  successful  or unsuccessful match is placed in a
       match data block, which is an opaque  structure  that  is  accessed  by
       function  calls.  In particular, the match data block contains a vector
       of offsets into the subject string that define the matched part of  the
       subject  and  any  substrings  that were captured. This is known as the
       ovector.

       Before calling pcre2_match(), pcre2_dfa_match(),  or  pcre2_jit_match()
       you must create a match data block by calling one of the creation func-
       tions above. For pcre2_match_data_create(), the first argument  is  the
       number  of  pairs  of  offsets  in  the ovector. One pair of offsets is
       required to identify the string that matched the whole pattern, with an
       additional  pair for each captured substring. For example, a value of 4
       creates enough space to record the matched portion of the subject  plus
       three  captured  substrings. A minimum of at least 1 pair is imposed by
       pcre2_match_data_create(), so it is always possible to return the over-
       all matched string.

       The second argument of pcre2_match_data_create() is a pointer to a gen-
       eral context, which can specify custom memory management for  obtaining
       the memory for the match data block. If you are not using custom memory
       management, pass NULL, which causes malloc() to be used.

       For pcre2_match_data_create_from_pattern(), the  first  argument  is  a
       pointer to a compiled pattern. The ovector is created to be exactly the
       right size to hold all the substrings a pattern might capture. The sec-
       ond  argument is again a pointer to a general context, but in this case
       if NULL is passed, the memory is obtained using the same allocator that
       was used for the compiled pattern (custom or default).

       A  match  data block can be used many times, with the same or different
       compiled patterns. You can extract information from a match data  block
       after  a  match  operation  has  finished,  using  functions  that  are
       described in the sections on  matched  strings  and  other  match  data
       below.

       When  a  call  of  pcre2_match()  fails, valid data is available in the
       match   block   only   when   the   error    is    PCRE2_ERROR_NOMATCH,
       PCRE2_ERROR_PARTIAL,  or  one  of  the  error  codes for an invalid UTF
       string. Exactly what is available depends on the error, and is detailed
       below.

       When  one of the matching functions is called, pointers to the compiled
       pattern and the subject string are set in the match data block so  that
       they  can  be referenced by the extraction functions after a successful
       match. After running a match, you must not free a compiled pattern or a
       subject  string until after all operations on the match data block (for
       that match) have taken place,  unless,  in  the  case  of  the  subject
       string,  you  have used the PCRE2_COPY_MATCHED_SUBJECT option, which is
       described in the  section  entitled  "Option  bits  for  pcre2_match()"
       below.

       When  a match data block itself is no longer needed, it should be freed
       by calling pcre2_match_data_free(). If this function is called  with  a
       NULL argument, it returns immediately, without doing anything.


MATCHING A PATTERN: THE TRADITIONAL FUNCTION

       int pcre2_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext);

       The  function pcre2_match() is called to match a subject string against
       a compiled pattern, which is passed in the code argument. You can  call
       pcre2_match() with the same code argument as many times as you like, in
       order to find multiple matches in the subject string or to  match  dif-
       ferent subject strings with the same pattern.

       This  function  is  the  main  matching facility of the library, and it
       operates in a Perl-like manner. For specialist use  there  is  also  an
       alternative  matching function, which is described below in the section
       about the pcre2_dfa_match() function.

       Here is an example of a simple call to pcre2_match():

         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           md,             /* the match data block */
           NULL);          /* a match context; NULL means use defaults */

       If the subject string is zero-terminated, the length can  be  given  as
       PCRE2_ZERO_TERMINATED. A match context must be provided if certain less
       common matching parameters are to be changed. For details, see the sec-
       tion on the match context above.

   The string to be matched by pcre2_match()

       The  subject string is passed to pcre2_match() as a pointer in subject,
       a length in length, and a starting offset in  startoffset.  The  length
       and  offset  are  in  code units, not characters.  That is, they are in
       bytes for the 8-bit library, 16-bit code units for the 16-bit  library,
       and  32-bit  code units for the 32-bit library, whether or not UTF pro-
       cessing is enabled.

       If startoffset is greater than the length of the subject, pcre2_match()
       returns  PCRE2_ERROR_BADOFFSET.  When  the starting offset is zero, the
       search for a match starts at the beginning of the subject, and this  is
       by far the most common case. In UTF-8 or UTF-16 mode, the starting off-
       set must point to the start of a character, or to the end of  the  sub-
       ject  (in  UTF-32 mode, one code unit equals one character, so all off-
       sets are valid). Like the  pattern  string,  the  subject  may  contain
       binary zeros.

       A  non-zero  starting offset is useful when searching for another match
       in the same subject by calling pcre2_match()  again  after  a  previous
       success.   Setting  startoffset  differs  from passing over a shortened
       string and setting PCRE2_NOTBOL in the case of a  pattern  that  begins
       with any kind of lookbehind. For example, consider the pattern

         \Biss\B

       which  finds  occurrences  of "iss" in the middle of words. (\B matches
       only if the current position in the subject is not  a  word  boundary.)
       When applied to the string "Mississipi" the first call to pcre2_match()
       finds the first occurrence. If pcre2_match() is called again with  just
       the  remainder  of  the  subject,  namely  "issipi", it does not match,
       because \B is always false at the start of the subject, which is deemed
       to  be  a word boundary. However, if pcre2_match() is passed the entire
       string again, but with startoffset set to 4, it finds the second occur-
       rence  of "iss" because it is able to look behind the starting point to
       discover that it is preceded by a letter.

       Finding all the matches in a subject is tricky  when  the  pattern  can
       match an empty string. It is possible to emulate Perl's /g behaviour by
       first  trying  the  match  again  at  the   same   offset,   with   the
       PCRE2_NOTEMPTY_ATSTART  and  PCRE2_ANCHORED  options,  and then if that
       fails, advancing the starting  offset  and  trying  an  ordinary  match
       again.  There  is  some  code  that  demonstrates how to do this in the
       pcre2demo sample program. In the most general case, you have  to  check
       to  see  if the newline convention recognizes CRLF as a newline, and if
       so, and the current character is CR followed by LF, advance the  start-
       ing offset by two characters instead of one.

       If a non-zero starting offset is passed when the pattern is anchored, a
       single attempt to match at the given offset is made. This can only suc-
       ceed  if  the  pattern does not require the match to be at the start of
       the subject. In other words, the anchoring must be the result  of  set-
       ting  the PCRE2_ANCHORED option or the use of .* with PCRE2_DOTALL, not
       by starting the pattern with ^ or \A.

   Option bits for pcre2_match()

       The unused bits of the options argument for pcre2_match() must be zero.
       The    only    bits    that    may    be    set   are   PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT,      PCRE2_ENDANCHORED,       PCRE2_NOTBOL,
       PCRE2_NOTEOL,   PCRE2_NOTEMPTY,  PCRE2_NOTEMPTY_ATSTART,  PCRE2_NO_JIT,
       PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and  PCRE2_PARTIAL_SOFT.  Their
       action is described below.

       Setting  PCRE2_ANCHORED  or PCRE2_ENDANCHORED at match time is not sup-
       ported by the just-in-time (JIT) compiler. If it is set,  JIT  matching
       is  disabled  and  the interpretive code in pcre2_match() is run. Apart
       from PCRE2_NO_JIT (obviously), the remaining options are supported  for
       JIT matching.

         PCRE2_ANCHORED

       The PCRE2_ANCHORED option limits pcre2_match() to matching at the first
       matching position. If a pattern was compiled  with  PCRE2_ANCHORED,  or
       turned  out to be anchored by virtue of its contents, it cannot be made
       unachored at matching time. Note that setting the option at match  time
       disables JIT matching.

         PCRE2_COPY_MATCHED_SUBJECT

       By  default,  a  pointer to the subject is remembered in the match data
       block so that, after a successful match, it can be  referenced  by  the
       substring  extraction  functions.  This means that the subject's memory
       must not be freed until all such  operations  are  complete.  For  some
       applications  where  the  lifetime of the subject string is not guaran-
       teed, it may be necessary to make a copy of the subject string, but  it
       is wasteful to do this unless the match is successful. After a success-
       ful match, if PCRE2_COPY_MATCHED_SUBJECT is set, the subject is  copied
       and  the  new  pointer is remembered in the match data block instead of
       the original subject pointer. The memory allocator that  was  used  for
       the  match  block  itself is used. The copy is automatically freed when
       pcre2_match_data_free() is called to free the match data block.  It  is
       also automatically freed if the match data block is re-used for another
       match operation.

         PCRE2_ENDANCHORED

       If the PCRE2_ENDANCHORED option is set, any string  that  pcre2_match()
       matches  must be right at the end of the subject string. Note that set-
       ting the option at match time disables JIT matching.

         PCRE2_NOTBOL

       This option specifies that first character of the subject string is not
       the  beginning  of  a  line, so the circumflex metacharacter should not
       match before it. Setting this without  having  set  PCRE2_MULTILINE  at
       compile time causes circumflex never to match. This option affects only
       the behaviour of the circumflex metacharacter. It does not affect \A.

         PCRE2_NOTEOL

       This option specifies that the end of the subject string is not the end
       of  a line, so the dollar metacharacter should not match it nor (except
       in multiline mode) a newline immediately before it. Setting this  with-
       out  having  set PCRE2_MULTILINE at compile time causes dollar never to
       match. This option affects only the behaviour of the dollar metacharac-
       ter. It does not affect \Z or \z.

         PCRE2_NOTEMPTY

       An empty string is not considered to be a valid match if this option is
       set. If there are alternatives in the pattern, they are tried.  If  all
       the  alternatives  match  the empty string, the entire match fails. For
       example, if the pattern

         a?b?

       is applied to a string not beginning with "a" or  "b",  it  matches  an
       empty string at the start of the subject. With PCRE2_NOTEMPTY set, this
       match is not valid, so pcre2_match() searches further into  the  string
       for occurrences of "a" or "b".

         PCRE2_NOTEMPTY_ATSTART

       This  is  like PCRE2_NOTEMPTY, except that it locks out an empty string
       match only at the first matching position, that is, at the start of the
       subject  plus  the  starting offset. An empty string match later in the
       subject is permitted.  If the pattern is anchored,  such  a  match  can
       occur only if the pattern contains \K.

         PCRE2_NO_JIT

       By   default,   if   a  pattern  has  been  successfully  processed  by
       pcre2_jit_compile(), JIT is automatically used  when  pcre2_match()  is
       called  with  options  that JIT supports. Setting PCRE2_NO_JIT disables
       the use of JIT; it forces matching to be done by the interpreter.

         PCRE2_NO_UTF_CHECK

       When PCRE2_UTF is set at compile time, the validity of the subject as a
       UTF  string  is  checked  by default when pcre2_match() is subsequently
       called.  If a non-zero starting offset is given, the check  is  applied
       only  to that part of the subject that could be inspected during match-
       ing, and there is a check that the starting offset points to the  first
       code  unit of a character or to the end of the subject. If there are no
       lookbehind assertions in the pattern, the check starts at the  starting
       offset.  Otherwise,  it  starts at the length of the longest lookbehind
       before the starting offset, or at the start of the subject if there are
       not  that  many  characters  before  the starting offset. Note that the
       sequences \b and \B are one-character lookbehinds.

       The check is carried out before any other processing takes place, and a
       negative  error  code is returned if the check fails. There are several
       UTF error codes for each code unit width,  corresponding  to  different
       problems  with  the code unit sequence. There are discussions about the
       validity of UTF-8 strings, UTF-16 strings, and UTF-32  strings  in  the
       pcre2unicode page.

       If  you  know  that  your  subject is valid, and you want to skip these
       checks for performance reasons,  you  can  set  the  PCRE2_NO_UTF_CHECK
       option  when  calling  pcre2_match(). You might want to do this for the
       second and subsequent calls to pcre2_match() if you are making repeated
       calls to find other matches in the same subject string.

       Warning:  When  PCRE2_NO_UTF_CHECK  is  set,  the  effect of passing an
       invalid string as a subject, or an invalid  value  of  startoffset,  is
       undefined.  Your program may crash or loop indefinitely.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These  options  turn  on  the partial matching feature. A partial match
       occurs if the end of the subject string is  reached  successfully,  but
       there  are not enough subject characters to complete the match. If this
       happens when PCRE2_PARTIAL_SOFT (but not  PCRE2_PARTIAL_HARD)  is  set,
       matching  continues  by  testing any remaining alternatives. Only if no
       complete match can be found is PCRE2_ERROR_PARTIAL returned instead  of
       PCRE2_ERROR_NOMATCH.  In other words, PCRE2_PARTIAL_SOFT specifies that
       the caller is prepared to handle a partial match, but only if  no  com-
       plete match can be found.

       If  PCRE2_PARTIAL_HARD is set, it overrides PCRE2_PARTIAL_SOFT. In this
       case, if a partial match is found,  pcre2_match()  immediately  returns
       PCRE2_ERROR_PARTIAL,  without  considering  any  other alternatives. In
       other words, when PCRE2_PARTIAL_HARD is set, a partial match is consid-
       ered to be more important that an alternative complete match.

       There is a more detailed discussion of partial and multi-segment match-
       ing, with examples, in the pcre2partial documentation.


NEWLINE HANDLING WHEN MATCHING

       When PCRE2 is built, a default newline convention is set; this is  usu-
       ally  the standard convention for the operating system. The default can
       be overridden in a compile context by calling  pcre2_set_newline().  It
       can  also be overridden by starting a pattern string with, for example,
       (*CRLF), as described in the section  on  newline  conventions  in  the
       pcre2pattern  page. During matching, the newline choice affects the be-
       haviour of the dot, circumflex, and dollar metacharacters. It may  also
       alter  the  way  the  match starting position is advanced after a match
       failure for an unanchored pattern.

       When PCRE2_NEWLINE_CRLF, PCRE2_NEWLINE_ANYCRLF, or PCRE2_NEWLINE_ANY is
       set  as  the  newline convention, and a match attempt for an unanchored
       pattern fails when the current starting position is at a CRLF sequence,
       and  the  pattern contains no explicit matches for CR or LF characters,
       the match position is advanced by two characters  instead  of  one,  in
       other words, to after the CRLF.

       The above rule is a compromise that makes the most common cases work as
       expected. For example, if the pattern  is  .+A  (and  the  PCRE2_DOTALL
       option is not set), it does not match the string "\r\nA" because, after
       failing at the start, it skips both the CR and the LF before  retrying.
       However,  the  pattern  [\r\n]A does match that string, because it con-
       tains an explicit CR or LF reference, and so advances only by one char-
       acter after the first failure.

       An explicit match for CR of LF is either a literal appearance of one of
       those characters in the pattern, or one of the \r or \n  or  equivalent
       octal or hexadecimal escape sequences. Implicit matches such as [^X] do
       not count, nor does \s, even though it includes CR and LF in the  char-
       acters that it matches.

       Notwithstanding  the above, anomalous effects may still occur when CRLF
       is a valid newline sequence and explicit \r or \n escapes appear in the
       pattern.


HOW PCRE2_MATCH() RETURNS A STRING AND CAPTURED SUBSTRINGS

       uint32_t pcre2_get_ovector_count(pcre2_match_data *match_data);

       PCRE2_SIZE *pcre2_get_ovector_pointer(pcre2_match_data *match_data);

       In  general, a pattern matches a certain portion of the subject, and in
       addition, further substrings from the subject  may  be  picked  out  by
       parenthesized  parts  of  the  pattern.  Following the usage in Jeffrey
       Friedl's book, this is called "capturing"  in  what  follows,  and  the
       phrase  "capture  group" (Perl terminology) is used for a fragment of a
       pattern that picks out a substring. PCRE2 supports several other  kinds
       of parenthesized group that do not cause substrings to be captured. The
       pcre2_pattern_info() function can be used to find out how many  capture
       groups there are in a compiled pattern.

       You  can  use  auxiliary functions for accessing captured substrings by
       number or by name, as described in sections below.

       Alternatively, you can make direct use of the vector of PCRE2_SIZE val-
       ues,  called  the  ovector,  which  contains  the  offsets  of captured
       strings.  It  is  part  of  the  match  data   block.    The   function
       pcre2_get_ovector_pointer()  returns  the  address  of the ovector, and
       pcre2_get_ovector_count() returns the number of pairs of values it con-
       tains.

       Within the ovector, the first in each pair of values is set to the off-
       set of the first code unit of a substring, and the second is set to the
       offset  of the first code unit after the end of a substring. These val-
       ues are always code unit offsets, not character offsets. That is,  they
       are  byte  offsets  in  the 8-bit library, 16-bit offsets in the 16-bit
       library, and 32-bit offsets in the 32-bit library.

       After a partial match  (error  return  PCRE2_ERROR_PARTIAL),  only  the
       first  pair  of  offsets  (that is, ovector[0] and ovector[1]) are set.
       They identify the part of the subject that was partially  matched.  See
       the pcre2partial documentation for details of partial matching.

       After  a  fully  successful match, the first pair of offsets identifies
       the portion of the subject string that was matched by the  entire  pat-
       tern.  The  next  pair is used for the first captured substring, and so
       on. The value returned by pcre2_match() is one more  than  the  highest
       numbered  pair  that  has been set. For example, if two substrings have
       been captured, the returned value is 3. If there are no  captured  sub-
       strings, the return value from a successful match is 1, indicating that
       just the first pair of offsets has been set.

       If a pattern uses the \K escape sequence within a  positive  assertion,
       the reported start of a successful match can be greater than the end of
       the match.  For example, if the pattern  (?=ab\K)  is  matched  against
       "ab", the start and end offset values for the match are 2 and 0.

       If  a  capture group is matched repeatedly within a single match opera-
       tion, it is the last portion of the subject that  it  matched  that  is
       returned.

       If the ovector is too small to hold all the captured substring offsets,
       as much as possible is filled in, and the function returns a  value  of
       zero.  If captured substrings are not of interest, pcre2_match() may be
       called with a match data block whose ovector is of minimum length (that
       is, one pair).

       It  is  possible for capture group number n+1 to match some part of the
       subject when group n has not been used at  all.  For  example,  if  the
       string "abc" is matched against the pattern (a|(z))(bc) the return from
       the function is 4, and groups 1 and 3 are matched, but 2 is  not.  When
       this  happens,  both values in the offset pairs corresponding to unused
       groups are set to PCRE2_UNSET.

       Offset values that correspond to  unused  groups  at  the  end  of  the
       expression  are  also  set  to  PCRE2_UNSET. For example, if the string
       "abc" is matched against the pattern (abc)(x(yz)?)? groups 2 and 3  are
       not  matched.  The  return  from the function is 2, because the highest
       used capture group number is 1. The offsets  for  for  the  second  and
       third  capture groupss (assuming the vector is large enough, of course)
       are set to PCRE2_UNSET.

       Elements in the ovector that do not correspond to capturing parentheses
       in the pattern are never changed. That is, if a pattern contains n cap-
       turing parentheses, no more than ovector[0] to ovector[2n+1] are set by
       pcre2_match().  The  other  elements retain whatever values they previ-
       ously had. After a failed match attempt, the contents  of  the  ovector
       are unchanged.


OTHER INFORMATION ABOUT A MATCH

       PCRE2_SPTR pcre2_get_mark(pcre2_match_data *match_data);

       PCRE2_SIZE pcre2_get_startchar(pcre2_match_data *match_data);

       As  well as the offsets in the ovector, other information about a match
       is retained in the match data block and can be retrieved by  the  above
       functions  in  appropriate  circumstances.  If they are called at other
       times, the result is undefined.

       After a successful match, a partial match (PCRE2_ERROR_PARTIAL),  or  a
       failure  to  match (PCRE2_ERROR_NOMATCH), a mark name may be available.
       The function pcre2_get_mark() can be called to access this name,  which
       can  be  specified  in  the  pattern by any of the backtracking control
       verbs, not just (*MARK). The same function applies to all the verbs. It
       returns a pointer to the zero-terminated name, which is within the com-
       piled pattern. If no name is available, NULL is returned. The length of
       the  name  (excluding  the terminating zero) is stored in the code unit
       that precedes the name. You should use this length instead  of  relying
       on the terminating zero if the name might contain a binary zero.

       After  a  successful  match, the name that is returned is the last mark
       name encountered on the matching path through the pattern. Instances of
       backtracking  verbs  without  names do not count. Thus, for example, if
       the matching path contains (*MARK:A)(*PRUNE), the name "A" is returned.
       After  a  "no  match"  or a partial match, the last encountered name is
       returned. For example, consider this pattern:

         ^(*MARK:A)((*MARK:B)a|b)c

       When it matches "bc", the returned name is A. The B mark is  "seen"  in
       the  first  branch of the group, but it is not on the matching path. On
       the other hand, when this pattern fails to  match  "bx",  the  returned
       name is B.

       Warning:  By  default, certain start-of-match optimizations are used to
       give a fast "no match" result in some situations. For example,  if  the
       anchoring  is removed from the pattern above, there is an initial check
       for the presence of "c" in the  subject  before  running  the  matching
       engine. This check fails for "bx", causing a match failure without see-
       ing any marks. You can disable the start-of-match optimizations by set-
       ting  the  PCRE2_NO_START_OPTIMIZE  option  for  pcre2_compile()  or by
       starting the pattern with (*NO_START_OPT).

       After a successful match, a partial match, or one of  the  invalid  UTF
       errors  (for example, PCRE2_ERROR_UTF8_ERR5), pcre2_get_startchar() can
       be called. After a successful or partial match it returns the code unit
       offset  of  the character at which the match started. For a non-partial
       match, this can be different to the value of ovector[0] if the  pattern
       contains  the  \K escape sequence. After a partial match, however, this
       value is always the same as ovector[0] because \K does not  affect  the
       result of a partial match.

       After  a UTF check failure, pcre2_get_startchar() can be used to obtain
       the code unit offset of the invalid UTF character. Details are given in
       the pcre2unicode page.


ERROR RETURNS FROM pcre2_match()

       If  pcre2_match() fails, it returns a negative number. This can be con-
       verted to a text string by calling the pcre2_get_error_message()  func-
       tion  (see  "Obtaining a textual error message" below).  Negative error
       codes are also returned by other functions,  and  are  documented  with
       them.  The codes are given names in the header file. If UTF checking is
       in force and an invalid UTF subject string is detected, one of a number
       of  UTF-specific negative error codes is returned. Details are given in
       the pcre2unicode page. The following are the other errors that  may  be
       returned by pcre2_match():

         PCRE2_ERROR_NOMATCH

       The subject string did not match the pattern.

         PCRE2_ERROR_PARTIAL

       The  subject  string did not match, but it did match partially. See the
       pcre2partial documentation for details of partial matching.

         PCRE2_ERROR_BADMAGIC

       PCRE2 stores a 4-byte "magic number" at the start of the compiled code,
       to  catch  the case when it is passed a junk pointer. This is the error
       that is returned when the magic number is not present.

         PCRE2_ERROR_BADMODE

       This error is given when a compiled pattern is passed to a function  in
       a  library  of a different code unit width, for example, a pattern com-
       piled by the 8-bit library is passed to  a  16-bit  or  32-bit  library
       function.

         PCRE2_ERROR_BADOFFSET

       The value of startoffset was greater than the length of the subject.

         PCRE2_ERROR_BADOPTION

       An unrecognized bit was set in the options argument.

         PCRE2_ERROR_BADUTFOFFSET

       The UTF code unit sequence that was passed as a subject was checked and
       found to be valid (the PCRE2_NO_UTF_CHECK option was not set), but  the
       value  of startoffset did not point to the beginning of a UTF character
       or the end of the subject.

         PCRE2_ERROR_CALLOUT

       This error is never generated by pcre2_match() itself. It  is  provided
       for  use  by  callout  functions  that  want  to cause pcre2_match() or
       pcre2_callout_enumerate() to return a distinctive error code.  See  the
       pcre2callout documentation for details.

         PCRE2_ERROR_DEPTHLIMIT

       The nested backtracking depth limit was reached.

         PCRE2_ERROR_HEAPLIMIT

       The heap limit was reached.

         PCRE2_ERROR_INTERNAL

       An  unexpected  internal error has occurred. This error could be caused
       by a bug in PCRE2 or by overwriting of the compiled pattern.

         PCRE2_ERROR_JIT_STACKLIMIT

       This error is returned when a pattern  that  was  successfully  studied
       using  JIT  is being matched, but the memory available for the just-in-
       time processing stack is not large enough. See the pcre2jit  documenta-
       tion for more details.

         PCRE2_ERROR_MATCHLIMIT

       The backtracking match limit was reached.

         PCRE2_ERROR_NOMEMORY

       If  a  pattern contains many nested backtracking points, heap memory is
       used to remember them. This error is given when the  memory  allocation
       function  (default  or  custom)  fails.  Note  that  a different error,
       PCRE2_ERROR_HEAPLIMIT, is given if the amount of memory needed  exceeds
       the    heap   limit.   PCRE2_ERROR_NOMEMORY   is   also   returned   if
       PCRE2_COPY_MATCHED_SUBJECT is set and memory allocation fails.

         PCRE2_ERROR_NULL

       Either the code, subject, or match_data argument was passed as NULL.

         PCRE2_ERROR_RECURSELOOP

       This error is returned when  pcre2_match()  detects  a  recursion  loop
       within  the  pattern. Specifically, it means that either the whole pat-
       tern or a capture group has been called recursively for the second time
       at  the  same position in the subject string. Some simple patterns that
       might do this are detected and faulted at compile time, but  more  com-
       plicated  cases,  in particular mutual recursions between two different
       groups, cannot be detected until matching is attempted.


OBTAINING A TEXTUAL ERROR MESSAGE

       int pcre2_get_error_message(int errorcode, PCRE2_UCHAR *buffer,
         PCRE2_SIZE bufflen);

       A text message for an error code  from  any  PCRE2  function  (compile,
       match,  or  auxiliary)  can be obtained by calling pcre2_get_error_mes-
       sage(). The code is passed as the first argument,  with  the  remaining
       two  arguments  specifying  a  code  unit buffer and its length in code
       units, into which the text message is placed. The message  is  returned
       in  code  units  of the appropriate width for the library that is being
       used.

       The returned message is terminated with a trailing zero, and the  func-
       tion  returns  the  number  of  code units used, excluding the trailing
       zero.  If  the  error  number  is  unknown,  the  negative  error  code
       PCRE2_ERROR_BADDATA  is  returned. If the buffer is too small, the mes-
       sage is truncated (but still with a trailing zero),  and  the  negative
       error  code PCRE2_ERROR_NOMEMORY is returned.  None of the messages are
       very long; a buffer size of 120 code units is ample.


EXTRACTING CAPTURED SUBSTRINGS BY NUMBER

       int pcre2_substring_length_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_SIZE *length);

       int pcre2_substring_copy_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR *buffer,
         PCRE2_SIZE *bufflen);

       int pcre2_substring_get_bynumber(pcre2_match_data *match_data,
         uint32_t number, PCRE2_UCHAR **bufferptr,
         PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       Captured substrings can be accessed directly by using  the  ovector  as
       described above.  For convenience, auxiliary functions are provided for
       extracting  captured  substrings  as  new,  separate,   zero-terminated
       strings. A substring that contains a binary zero is correctly extracted
       and has a further zero added on the end, but  the  result  is  not,  of
       course, a C string.

       The functions in this section identify substrings by number. The number
       zero refers to the entire matched substring, with higher numbers refer-
       ring  to  substrings  captured by parenthesized groups. After a partial
       match, only substring zero is available.  An  attempt  to  extract  any
       other  substring  gives the error PCRE2_ERROR_PARTIAL. The next section
       describes similar functions for extracting captured substrings by name.

       If a pattern uses the \K escape sequence within a  positive  assertion,
       the reported start of a successful match can be greater than the end of
       the match.  For example, if the pattern  (?=ab\K)  is  matched  against
       "ab",  the  start  and  end offset values for the match are 2 and 0. In
       this situation, calling these functions with a  zero  substring  number
       extracts a zero-length empty string.

       You  can  find the length in code units of a captured substring without
       extracting it by calling pcre2_substring_length_bynumber().  The  first
       argument  is a pointer to the match data block, the second is the group
       number, and the third is a pointer to a variable into which the  length
       is  placed.  If  you just want to know whether or not the substring has
       been captured, you can pass the third argument as NULL.

       The pcre2_substring_copy_bynumber() function  copies  a  captured  sub-
       string  into  a supplied buffer, whereas pcre2_substring_get_bynumber()
       copies it into new memory, obtained using the  same  memory  allocation
       function  that  was  used for the match data block. The first two argu-
       ments of these functions are a pointer to the match data  block  and  a
       capture group number.

       The final arguments of pcre2_substring_copy_bynumber() are a pointer to
       the buffer and a pointer to a variable that contains its length in code
       units.  This is updated to contain the actual number of code units used
       for the extracted substring, excluding the terminating zero.

       For pcre2_substring_get_bynumber() the third and fourth arguments point
       to  variables that are updated with a pointer to the new memory and the
       number of code units that comprise the substring, again  excluding  the
       terminating  zero.  When  the substring is no longer needed, the memory
       should be freed by calling pcre2_substring_free().

       The return value from all these functions is zero  for  success,  or  a
       negative  error  code.  If  the pattern match failed, the match failure
       code is returned.  If a substring number  greater  than  zero  is  used
       after  a partial match, PCRE2_ERROR_PARTIAL is returned. Other possible
       error codes are:

         PCRE2_ERROR_NOMEMORY

       The buffer was too small for  pcre2_substring_copy_bynumber(),  or  the
       attempt to get memory failed for pcre2_substring_get_bynumber().

         PCRE2_ERROR_NOSUBSTRING

       There  is  no  substring  with that number in the pattern, that is, the
       number is greater than the number of capturing parentheses.

         PCRE2_ERROR_UNAVAILABLE

       The substring number, though not greater than the number of captures in
       the pattern, is greater than the number of slots in the ovector, so the
       substring could not be captured.

         PCRE2_ERROR_UNSET

       The substring did not participate in the match.  For  example,  if  the
       pattern  is  (abc)|(def) and the subject is "def", and the ovector con-
       tains at least two capturing slots, substring number 1 is unset.


EXTRACTING A LIST OF ALL CAPTURED SUBSTRINGS

       int pcre2_substring_list_get(pcre2_match_data *match_data,
         PCRE2_UCHAR ***listptr, PCRE2_SIZE **lengthsptr);

       void pcre2_substring_list_free(PCRE2_SPTR *list);

       The pcre2_substring_list_get() function  extracts  all  available  sub-
       strings  and  builds  a  list of pointers to them. It also (optionally)
       builds a second list that  contains  their  lengths  (in  code  units),
       excluding a terminating zero that is added to each of them. All this is
       done in a single block of memory that is obtained using the same memory
       allocation function that was used to get the match data block.

       This  function  must be called only after a successful match. If called
       after a partial match, the error code PCRE2_ERROR_PARTIAL is returned.

       The address of the memory block is returned via listptr, which is  also
       the start of the list of string pointers. The end of the list is marked
       by a NULL pointer. The address of the list of lengths is  returned  via
       lengthsptr.  If your strings do not contain binary zeros and you do not
       therefore need the lengths, you may supply NULL as the lengthsptr argu-
       ment  to  disable  the  creation of a list of lengths. The yield of the
       function is zero if all went well, or PCRE2_ERROR_NOMEMORY if the  mem-
       ory  block could not be obtained. When the list is no longer needed, it
       should be freed by calling pcre2_substring_list_free().

       If this function encounters a substring that is unset, which can happen
       when  capture  group  number  n+1 matches some part of the subject, but
       group n has not been used at all, it returns an empty string. This  can
       be distinguished from a genuine zero-length substring by inspecting the
       appropriate offset in the ovector, which contain PCRE2_UNSET for  unset
       substrings, or by calling pcre2_substring_length_bynumber().


EXTRACTING CAPTURED SUBSTRINGS BY NAME

       int pcre2_substring_number_from_name(const pcre2_code *code,
         PCRE2_SPTR name);

       int pcre2_substring_length_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_SIZE *length);

       int pcre2_substring_copy_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen);

       int pcre2_substring_get_byname(pcre2_match_data *match_data,
         PCRE2_SPTR name, PCRE2_UCHAR **bufferptr, PCRE2_SIZE *bufflen);

       void pcre2_substring_free(PCRE2_UCHAR *buffer);

       To  extract a substring by name, you first have to find associated num-
       ber.  For example, for this pattern:

         (a+)b(?<xxx>\d+)...

       the number of the capture group called "xxx" is 2. If the name is known
       to be unique (PCRE2_DUPNAMES was not set), you can find the number from
       the name by calling pcre2_substring_number_from_name(). The first argu-
       ment  is the compiled pattern, and the second is the name. The yield of
       the function is the group number, PCRE2_ERROR_NOSUBSTRING if  there  is
       no  group  with that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if there is
       more than one group with that name.  Given the number, you can  extract
       the  substring  directly from the ovector, or use one of the "bynumber"
       functions described above.

       For convenience, there are also "byname" functions that  correspond  to
       the  "bynumber"  functions,  the  only difference being that the second
       argument is a name instead of a number. If PCRE2_DUPNAMES  is  set  and
       there are duplicate names, these functions scan all the groups with the
       given name, and return the captured  substring  from  the  first  named
       group that is set.

       If  there are no groups with the given name, PCRE2_ERROR_NOSUBSTRING is
       returned. If all groups with the name have  numbers  that  are  greater
       than  the  number  of  slots in the ovector, PCRE2_ERROR_UNAVAILABLE is
       returned. If there is at least one group with a slot  in  the  ovector,
       but no group is found to be set, PCRE2_ERROR_UNSET is returned.

       Warning: If the pattern uses the (?| feature to set up multiple capture
       groups with the same number, as described in the section  on  duplicate
       group numbers in the pcre2pattern page, you cannot use names to distin-
       guish the different capture groups, because names are not  included  in
       the  compiled  code.  The  matching process uses only numbers. For this
       reason, the use of different names for  groups  with  the  same  number
       causes an error at compile time.


CREATING A NEW STRING WITH SUBSTITUTIONS

       int pcre2_substitute(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext, PCRE2_SPTR replacement,
         PCRE2_SIZE rlength, PCRE2_UCHAR *outputbuffer,
         PCRE2_SIZE *outlengthptr);

       This  function calls pcre2_match() and then makes a copy of the subject
       string in outputbuffer, replacing one or more parts that  were  matched
       with the replacement string, whose length is supplied in rlength.  This
       can be given as PCRE2_ZERO_TERMINATED  for  a  zero-terminated  string.
       The  default is to perform just one replacement, but there is an option
       that requests multiple replacements (see PCRE2_SUBSTITUTE_GLOBAL  below
       for details).

       Matches  in  which  a  \K item in a lookahead in the pattern causes the
       match to end before it starts are not supported, and give  rise  to  an
       error return. For global replacements, matches in which \K in a lookbe-
       hind causes the match to start earlier than the point that was  reached
       in the previous iteration are also not supported.

       The  first  seven  arguments  of pcre2_substitute() are the same as for
       pcre2_match(), except that the partial matching options are not permit-
       ted,  and  match_data may be passed as NULL, in which case a match data
       block is obtained and freed within this function, using memory  manage-
       ment  functions from the match context, if provided, or else those that
       were used to allocate memory for the compiled code.

       If an external match_data block is provided,  its  contents  afterwards
       are  those  set by the final call to pcre2_match(). For global changes,
       this will have ended in a matching error. The contents of  the  ovector
       within the match data block may or may not have been changed.

       The  outlengthptr  argument  must point to a variable that contains the
       length, in code units, of the output buffer. If the  function  is  suc-
       cessful,  the value is updated to contain the length of the new string,
       excluding the trailing zero that is automatically added.

       If the function is not  successful,  the  value  set  via  outlengthptr
       depends  on  the  type  of  error. For syntax errors in the replacement
       string, the value is the offset in the  replacement  string  where  the
       error  was  detected.  For  other  errors,  the value is PCRE2_UNSET by
       default. This includes the case of the output buffer being  too  small,
       unless  PCRE2_SUBSTITUTE_OVERFLOW_LENGTH  is  set (see below), in which
       case the value is the minimum length needed, including  space  for  the
       trailing  zero.  Note  that  in  order  to compute the required length,
       pcre2_substitute() has  to  simulate  all  the  matching  and  copying,
       instead of giving an error return as soon as the buffer overflows. Note
       also that the length is in code units, not bytes.

       In the replacement string, which is interpreted as a UTF string in  UTF
       mode,  and  is  checked  for UTF validity unless the PCRE2_NO_UTF_CHECK
       option is set, a dollar character is an escape character that can spec-
       ify  the  insertion  of  characters  from  capture groups or names from
       (*MARK) or other control verbs in the pattern. The following forms  are
       always recognized:

         $$                  insert a dollar character
         $<n> or ${<n>}      insert the contents of group <n>
         $*MARK or ${*MARK}  insert a control verb name

       Either  a  group  number  or  a  group name can be given for <n>. Curly
       brackets are required only if the following character would  be  inter-
       preted as part of the number or name. The number may be zero to include
       the entire matched string.   For  example,  if  the  pattern  a(b)c  is
       matched  with "=abc=" and the replacement string "+$1$0$1+", the result
       is "=+babcb+=".

       $*MARK inserts the name from the last encountered backtracking  control
       verb  on the matching path that has a name. (*MARK) must always include
       a name, but the other verbs need not.  For  example,  in  the  case  of
       (*MARK:A)(*PRUNE) the name inserted is "A", but for (*MARK:A)(*PRUNE:B)
       the relevant name is "B". This facility can be used to  perform  simple
       simultaneous substitutions, as this pcre2test example shows:

         /(*MARK:pear)apple|(*MARK:orange)lemon/g,replace=${*MARK}
             apple lemon
          2: pear orange

       As  well as the usual options for pcre2_match(), a number of additional
       options can be set in the options argument of pcre2_substitute().

       PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject
       string,  replacing every matching substring. If this option is not set,
       only the first matching substring is replaced. The search  for  matches
       takes  place in the original subject string (that is, previous replace-
       ments do not affect it).  Iteration is  implemented  by  advancing  the
       startoffset  value  for  each search, which is always passed the entire
       subject string. If an offset limit is set in the match context, search-
       ing stops when that limit is reached.

       You  can  restrict  the effect of a global substitution to a portion of
       the subject string by setting either or both of startoffset and an off-
       set limit. Here is a pcre2test example:

         /B/g,replace=!,use_offset_limit
         ABC ABC ABC ABC\=offset=3,offset_limit=12
          2: ABC A!C A!C ABC

       When  continuing  with  global substitutions after matching a substring
       with zero length, an attempt to find a non-empty match at the same off-
       set is performed.  If this is not successful, the offset is advanced by
       one character except when CRLF is a valid newline sequence and the next
       two  characters are CR, LF. In this case, the offset is advanced by two
       characters.

       PCRE2_SUBSTITUTE_OVERFLOW_LENGTH changes what happens when  the  output
       buffer is too small. The default action is to return PCRE2_ERROR_NOMEM-
       ORY immediately. If this option  is  set,  however,  pcre2_substitute()
       continues to go through the motions of matching and substituting (with-
       out, of course, writing anything) in order to compute the size of  buf-
       fer  that  is  needed.  This  value is passed back via the outlengthptr
       variable,   with   the   result   of   the   function    still    being
       PCRE2_ERROR_NOMEMORY.

       Passing  a  buffer  size  of zero is a permitted way of finding out how
       much memory is needed for given substitution. However, this  does  mean
       that the entire operation is carried out twice. Depending on the appli-
       cation, it may be more efficient to allocate a large  buffer  and  free
       the   excess   afterwards,   instead  of  using  PCRE2_SUBSTITUTE_OVER-
       FLOW_LENGTH.

       PCRE2_SUBSTITUTE_UNKNOWN_UNSET causes references to capture groups that
       do not appear in the pattern to be treated as unset groups. This option
       should be used with care, because it means that a typo in a group  name
       or number no longer causes the PCRE2_ERROR_NOSUBSTRING error.

       PCRE2_SUBSTITUTE_UNSET_EMPTY  causes  unset  capture  groups (including
       unknown  groups  when  PCRE2_SUBSTITUTE_UNKNOWN_UNSET  is  set)  to  be
       treated  as  empty  strings  when  inserted as described above. If this
       option is not set, an attempt to  insert  an  unset  group  causes  the
       PCRE2_ERROR_UNSET  error.  This  option does not influence the extended
       substitution syntax described below.

       PCRE2_SUBSTITUTE_EXTENDED causes extra processing to be applied to  the
       replacement  string.  Without this option, only the dollar character is
       special, and only the group insertion forms  listed  above  are  valid.
       When PCRE2_SUBSTITUTE_EXTENDED is set, two things change:

       Firstly,  backslash in a replacement string is interpreted as an escape
       character. The usual forms such as \n or \x{ddd} can be used to specify
       particular  character codes, and backslash followed by any non-alphanu-
       meric character quotes that character. Extended quoting  can  be  coded
       using \Q...\E, exactly as in pattern strings.

       There  are  also four escape sequences for forcing the case of inserted
       letters.  The insertion mechanism has three states:  no  case  forcing,
       force upper case, and force lower case. The escape sequences change the
       current state: \U and \L change to upper or lower case forcing, respec-
       tively,  and  \E (when not terminating a \Q quoted sequence) reverts to
       no case forcing. The sequences \u and \l force the next  character  (if
       it  is  a  letter)  to  upper or lower case, respectively, and then the
       state automatically reverts to no case forcing. Case forcing applies to
       all  inserted  characters, including those from capture groups and let-
       ters within \Q...\E quoted sequences.

       Note that case forcing sequences such as \U...\E do not nest. For exam-
       ple,  the  result of processing "\Uaa\LBB\Ecc\E" is "AAbbcc"; the final
       \E  has  no   effect.   Note   also   that   the   PCRE2_ALT_BSUX   and
       PCRE2_EXTRA_ALT_BSUX  options  do not apply to not apply to replacement
       strings.

       The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to  add  more
       flexibility  to  capture  group  substitution. The syntax is similar to
       that used by Bash:

         ${<n>:-<string>}
         ${<n>:+<string1>:<string2>}

       As before, <n> may be a group number or a name. The first  form  speci-
       fies  a  default  value. If group <n> is set, its value is inserted; if
       not, <string> is expanded and the  result  inserted.  The  second  form
       specifies  strings that are expanded and inserted when group <n> is set
       or unset, respectively. The first form is just a  convenient  shorthand
       for

         ${<n>:+${<n>}:<string>}

       Backslash  can  be  used to escape colons and closing curly brackets in
       the replacement strings. A change of the case forcing  state  within  a
       replacement  string  remains  in  force  afterwards,  as  shown in this
       pcre2test example:

         /(some)?(body)/substitute_extended,replace=${1:+\U:\L}HeLLo
             body
          1: hello
             somebody
          1: HELLO

       The PCRE2_SUBSTITUTE_UNSET_EMPTY option does not affect these  extended
       substitutions.   However,   PCRE2_SUBSTITUTE_UNKNOWN_UNSET  does  cause
       unknown groups in the extended syntax forms to be treated as unset.

       If successful, pcre2_substitute()  returns  the  number  of  successful
       matches.  This  may  be  zero  if  no  matches were found, and is never
       greater than 1 unless PCRE2_SUBSTITUTE_GLOBAL is set.

       In the event of an error, a negative error code is returned. Except for
       PCRE2_ERROR_NOMATCH    (which   is   never   returned),   errors   from
       pcre2_match() are passed straight back.

       PCRE2_ERROR_NOSUBSTRING is returned for a non-existent substring inser-
       tion, unless PCRE2_SUBSTITUTE_UNKNOWN_UNSET is set.

       PCRE2_ERROR_UNSET is returned for an unset substring insertion (includ-
       ing an unknown substring when  PCRE2_SUBSTITUTE_UNKNOWN_UNSET  is  set)
       when  the  simple  (non-extended)  syntax  is  used  and  PCRE2_SUBSTI-
       TUTE_UNSET_EMPTY is not set.

       PCRE2_ERROR_NOMEMORY is returned  if  the  output  buffer  is  not  big
       enough. If the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option is set, the size
       of buffer that is needed is returned via outlengthptr. Note  that  this
       does not happen by default.

       PCRE2_ERROR_BADREPLACEMENT  is  used for miscellaneous syntax errors in
       the   replacement   string,   with   more   particular   errors   being
       PCRE2_ERROR_BADREPESCAPE  (invalid  escape  sequence), PCRE2_ERROR_REP-
       MISSINGBRACE (closing curly bracket not found),  PCRE2_ERROR_BADSUBSTI-
       TUTION   (syntax   error   in   extended   group   substitution),   and
       PCRE2_ERROR_BADSUBSPATTERN (the pattern match ended before  it  started
       or  the match started earlier than the current position in the subject,
       which can happen if \K is used in an assertion).

       As for all PCRE2 errors, a text message that describes the error can be
       obtained   by   calling  the  pcre2_get_error_message()  function  (see
       "Obtaining a textual error message" above).

   Substitution callouts

       int pcre2_set_substitute_callout(pcre2_match_context *mcontext,
         int (*callout_function)(pcre2_substitute_callout_block *, void *),
         void *callout_data);

       The pcre2_set_substitution_callout() function can be used to specify  a
       callout  function for pcre2_substitute(). This information is passed in
       a match context. The callout function is called after each substitution
       has been processed, but it can cause the replacement not to happen. The
       callout function is not called for simulated substitutions that  happen
       as a result of the PCRE2_SUBSTITUTE_OVERFLOW_LENGTH option.

       The first argument of the callout function is a pointer to a substitute
       callout block structure, which contains the following fields, not  nec-
       essarily in this order:

         uint32_t    version;
         uint32_t    subscount;
         PCRE2_SPTR  input;
         PCRE2_SPTR  output;
         PCRE2_SIZE *ovector;
         uint32_t    oveccount;
         PCRE2_SIZE  output_offsets[2];

       The  version field contains the version number of the block format. The
       current version is 0. The version number will  increase  in  future  if
       more  fields are added, but the intention is never to remove any of the
       existing fields.

       The subscount field is the number of the current match. It is 1 for the
       first callout, 2 for the second, and so on. The input and output point-
       ers are copies of the values passed to pcre2_substitute().

       The ovector field points to the ovector, which contains the  result  of
       the most recent match. The oveccount field contains the number of pairs
       that are set in the ovector, and is always greater than zero.

       The output_offsets vector contains the offsets of  the  replacement  in
       the  output  string. This has already been processed for dollar and (if
       requested) backslash substitutions as described above.

       The second argument of the callout function  is  the  value  passed  as
       callout_data  when  the  function was registered. The value returned by
       the callout function is interpreted as follows:

       If the value is zero, the replacement is accepted, and,  if  PCRE2_SUB-
       STITUTE_GLOBAL  is set, processing continues with a search for the next
       match. If the value  is  not  zero,  the  current  replacement  is  not
       accepted.  If the value is greater than zero, processing continues when
       PCRE2_SUBSTITUTE_GLOBAL is set. Otherwise (the value is less than  zero
       or  PCRE2_SUBSTITUTE_GLOBAL  is  not set), the the rest of the input is
       copied to the output and the call to pcre2_substitute() exits,  return-
       ing the number of matches so far.


DUPLICATE CAPTURE GROUP NAMES

       int pcre2_substring_nametable_scan(const pcre2_code *code,
         PCRE2_SPTR name, PCRE2_SPTR *first, PCRE2_SPTR *last);

       When  a  pattern  is compiled with the PCRE2_DUPNAMES option, names for
       capture groups are not required  to  be  unique.  Duplicate  names  are
       always  allowed  for  groups with the same number, created by using the
       (?| feature. Indeed, if such groups are named, they are required to use
       the same names.

       Normally,  patterns  that  use duplicate names are such that in any one
       match, only one of each set of identically-named  groups  participates.
       An example is shown in the pcre2pattern documentation.

       When   duplicates   are   present,   pcre2_substring_copy_byname()  and
       pcre2_substring_get_byname() return the first  substring  corresponding
       to   the   given   name   that   is  set.  Only  if  none  are  set  is
       PCRE2_ERROR_UNSET is returned.  The  pcre2_substring_number_from_name()
       function returns the error PCRE2_ERROR_NOUNIQUESUBSTRING when there are
       duplicate names.

       If you want to get full details of all captured substrings for a  given
       name,  you  must use the pcre2_substring_nametable_scan() function. The
       first argument is the compiled pattern, and the second is the name.  If
       the  third  and fourth arguments are NULL, the function returns a group
       number for a unique name, or PCRE2_ERROR_NOUNIQUESUBSTRING otherwise.

       When the third and fourth arguments are not NULL, they must be pointers
       to  variables  that are updated by the function. After it has run, they
       point to the first and last entries in the name-to-number table for the
       given  name,  and the function returns the length of each entry in code
       units. In both cases, PCRE2_ERROR_NOSUBSTRING is returned if there  are
       no entries for the given name.

       The format of the name table is described above in the section entitled
       Information about a pattern. Given all the  relevant  entries  for  the
       name,  you  can  extract  each of their numbers, and hence the captured
       data.


FINDING ALL POSSIBLE MATCHES AT ONE POSITION

       The traditional matching function uses a  similar  algorithm  to  Perl,
       which  stops when it finds the first match at a given point in the sub-
       ject. If you want to find all possible matches, or the longest possible
       match  at  a  given  position,  consider using the alternative matching
       function (see below) instead. If you cannot use the  alternative  func-
       tion, you can kludge it up by making use of the callout facility, which
       is described in the pcre2callout documentation.

       What you have to do is to insert a callout right at the end of the pat-
       tern.   When your callout function is called, extract and save the cur-
       rent matched substring. Then return 1, which  forces  pcre2_match()  to
       backtrack  and  try other alternatives. Ultimately, when it runs out of
       matches, pcre2_match() will yield PCRE2_ERROR_NOMATCH.


MATCHING A PATTERN: THE ALTERNATIVE FUNCTION

       int pcre2_dfa_match(const pcre2_code *code, PCRE2_SPTR subject,
         PCRE2_SIZE length, PCRE2_SIZE startoffset,
         uint32_t options, pcre2_match_data *match_data,
         pcre2_match_context *mcontext,
         int *workspace, PCRE2_SIZE wscount);

       The function pcre2_dfa_match() is called  to  match  a  subject  string
       against  a  compiled pattern, using a matching algorithm that scans the
       subject string just once (not counting lookaround assertions), and does
       not  backtrack.  This has different characteristics to the normal algo-
       rithm, and is not compatible with Perl. Some of the features  of  PCRE2
       patterns  are  not  supported.  Nevertheless, there are times when this
       kind of matching can be useful. For a discussion of  the  two  matching
       algorithms, and a list of features that pcre2_dfa_match() does not sup-
       port, see the pcre2matching documentation.

       The arguments for the pcre2_dfa_match() function are the  same  as  for
       pcre2_match(), plus two extras. The ovector within the match data block
       is used in a different way, and this is described below. The other com-
       mon  arguments  are used in the same way as for pcre2_match(), so their
       description is not repeated here.

       The two additional arguments provide workspace for  the  function.  The
       workspace  vector  should  contain at least 20 elements. It is used for
       keeping  track  of  multiple  paths  through  the  pattern  tree.  More
       workspace  is needed for patterns and subjects where there are a lot of
       potential matches.

       Here is an example of a simple call to pcre2_dfa_match():

         int wspace[20];
         pcre2_match_data *md = pcre2_match_data_create(4, NULL);
         int rc = pcre2_dfa_match(
           re,             /* result of pcre2_compile() */
           "some string",  /* the subject string */
           11,             /* the length of the subject string */
           0,              /* start at offset 0 in the subject */
           0,              /* default options */
           md,             /* the match data block */
           NULL,           /* a match context; NULL means use defaults */
           wspace,         /* working space vector */
           20);            /* number of elements (NOT size in bytes) */

   Option bits for pcre_dfa_match()

       The unused bits of the options argument for pcre2_dfa_match()  must  be
       zero.   The   only   bits   that   may   be   set  are  PCRE2_ANCHORED,
       PCRE2_COPY_MATCHED_SUBJECT,      PCRE2_ENDANCHORED,       PCRE2_NOTBOL,
       PCRE2_NOTEOL,          PCRE2_NOTEMPTY,          PCRE2_NOTEMPTY_ATSTART,
       PCRE2_NO_UTF_CHECK,       PCRE2_PARTIAL_HARD,       PCRE2_PARTIAL_SOFT,
       PCRE2_DFA_SHORTEST,  and  PCRE2_DFA_RESTART.  All  but the last four of
       these are exactly the same as for pcre2_match(), so  their  description
       is not repeated here.

         PCRE2_PARTIAL_HARD
         PCRE2_PARTIAL_SOFT

       These  have  the  same general effect as they do for pcre2_match(), but
       the details are slightly different. When PCRE2_PARTIAL_HARD is set  for
       pcre2_dfa_match(),  it  returns  PCRE2_ERROR_PARTIAL  if the end of the
       subject is reached and there is still at least one matching possibility
       that requires additional characters. This happens even if some complete
       matches have already been found. When PCRE2_PARTIAL_SOFT  is  set,  the
       return  code  PCRE2_ERROR_NOMATCH is converted into PCRE2_ERROR_PARTIAL
       if the end of the subject is  reached,  there  have  been  no  complete
       matches, but there is still at least one matching possibility. The por-
       tion of the string that was inspected when the  longest  partial  match
       was found is set as the first matching string in both cases. There is a
       more detailed discussion of partial and  multi-segment  matching,  with
       examples, in the pcre2partial documentation.

         PCRE2_DFA_SHORTEST

       Setting  the PCRE2_DFA_SHORTEST option causes the matching algorithm to
       stop as soon as it has found one match. Because of the way the alterna-
       tive  algorithm  works, this is necessarily the shortest possible match
       at the first possible matching point in the subject string.

         PCRE2_DFA_RESTART

       When pcre2_dfa_match() returns a partial match, it is possible to  call
       it again, with additional subject characters, and have it continue with
       the same match. The PCRE2_DFA_RESTART option requests this action; when
       it  is  set,  the workspace and wscount options must reference the same
       vector as before because data about the match so far is  left  in  them
       after a partial match. There is more discussion of this facility in the
       pcre2partial documentation.

   Successful returns from pcre2_dfa_match()

       When pcre2_dfa_match() succeeds, it may have matched more than one sub-
       string in the subject. Note, however, that all the matches from one run
       of the function start at the same point in  the  subject.  The  shorter
       matches  are all initial substrings of the longer matches. For example,
       if the pattern

         <.*>

       is matched against the string

         This is <something> <something else> <something further> no more

       the three matched strings are

         <something> <something else> <something further>
         <something> <something else>
         <something>

       On success, the yield of the function is a number  greater  than  zero,
       which  is  the  number  of  matched substrings. The offsets of the sub-
       strings are returned in the ovector, and can be extracted by number  in
       the  same way as for pcre2_match(), but the numbers bear no relation to
       any capture groups that may exist in the pattern, because DFA  matching
       does not support capturing.

       Calls  to  the  convenience  functions  that extract substrings by name
       return the error PCRE2_ERROR_DFA_UFUNC (unsupported function)  if  used
       after a DFA match. The convenience functions that extract substrings by
       number never return PCRE2_ERROR_NOSUBSTRING.

       The matched strings are stored in  the  ovector  in  reverse  order  of
       length;  that  is,  the longest matching string is first. If there were
       too many matches to fit into the ovector, the yield of the function  is
       zero, and the vector is filled with the longest matches.

       NOTE:  PCRE2's  "auto-possessification" optimization usually applies to
       character repeats at the end of a pattern (as well as internally).  For
       example,  the pattern "a\d+" is compiled as if it were "a\d++". For DFA
       matching, this means that only one possible  match  is  found.  If  you
       really  do  want multiple matches in such cases, either use an ungreedy
       repeat such as "a\d+?" or set  the  PCRE2_NO_AUTO_POSSESS  option  when
       compiling.

   Error returns from pcre2_dfa_match()

       The pcre2_dfa_match() function returns a negative number when it fails.
       Many of the errors are the same  as  for  pcre2_match(),  as  described
       above.  There are in addition the following errors that are specific to
       pcre2_dfa_match():

         PCRE2_ERROR_DFA_UITEM

       This return is given if pcre2_dfa_match() encounters  an  item  in  the
       pattern  that it does not support, for instance, the use of \C in a UTF
       mode or a backreference.

         PCRE2_ERROR_DFA_UCOND

       This return is given if pcre2_dfa_match() encounters a  condition  item
       that uses a backreference for the condition, or a test for recursion in
       a specific capture group. These are not supported.

         PCRE2_ERROR_DFA_WSSIZE

       This return is given if pcre2_dfa_match() runs  out  of  space  in  the
       workspace vector.

         PCRE2_ERROR_DFA_RECURSE

       When a recursion or subroutine call is processed, the matching function
       calls itself recursively, using private  memory  for  the  ovector  and
       workspace.   This  error  is given if the internal ovector is not large
       enough. This should be extremely rare, as a  vector  of  size  1000  is
       used.

         PCRE2_ERROR_DFA_BADRESTART

       When  pcre2_dfa_match()  is  called  with the PCRE2_DFA_RESTART option,
       some plausibility checks are made on the  contents  of  the  workspace,
       which  should  contain data about the previous partial match. If any of
       these checks fail, this error is given.


SEE ALSO

       pcre2build(3),   pcre2callout(3),    pcre2demo(3),    pcre2matching(3),
       pcre2partial(3), pcre2posix(3), pcre2sample(3), pcre2unicode(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 12 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2BUILD(3)              Library Functions Manual              PCRE2BUILD(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

BUILDING PCRE2

       PCRE2  is distributed with a configure script that can be used to build
       the library in Unix-like environments using the applications  known  as
       Autotools. Also in the distribution are files to support building using
       CMake instead of configure.  The  text  file  README  contains  general
       information  about  building  with Autotools (some of which is repeated
       below), and also has some comments about building on various  operating
       systems.  There  is a lot more information about building PCRE2 without
       using Autotools (including information about using CMake  and  building
       "by  hand")  in  the  text file called NON-AUTOTOOLS-BUILD.  You should
       consult this file as well as the README file if you are building  in  a
       non-Unix-like environment.


PCRE2 BUILD-TIME OPTIONS

       The rest of this document describes the optional features of PCRE2 that
       can be selected when the library is compiled. It  assumes  use  of  the
       configure  script,  where  the  optional features are selected or dese-
       lected by providing options to configure before running the  make  com-
       mand.  However,  the same options can be selected in both Unix-like and
       non-Unix-like environments if you are using CMake instead of  configure
       to build PCRE2.

       If  you  are not using Autotools or CMake, option selection can be done
       by editing the config.h file, or by passing parameter settings  to  the
       compiler, as described in NON-AUTOTOOLS-BUILD.

       The complete list of options for configure (which includes the standard
       ones such as the  selection  of  the  installation  directory)  can  be
       obtained by running

         ./configure --help

       The  following  sections include descriptions of "on/off" options whose
       names begin with --enable or --disable. Because of the way that config-
       ure  works, --enable and --disable always come in pairs, so the comple-
       mentary option always exists as well, but as it specifies the  default,
       it is not described.  Options that specify values have names that start
       with --with. At the end of a configure run, a summary of the configura-
       tion is output.


BUILDING 8-BIT, 16-BIT AND 32-BIT LIBRARIES

       By  default, a library called libpcre2-8 is built, containing functions
       that take string arguments contained in arrays  of  bytes,  interpreted
       either  as single-byte characters, or UTF-8 strings. You can also build
       two other libraries, called libpcre2-16 and libpcre2-32, which  process
       strings  that  are contained in arrays of 16-bit and 32-bit code units,
       respectively. These can be interpreted either as single-unit characters
       or  UTF-16/UTF-32 strings. To build these additional libraries, add one
       or both of the following to the configure command:

         --enable-pcre2-16
         --enable-pcre2-32

       If you do not want the 8-bit library, add

         --disable-pcre2-8

       as well. At least one of the three libraries must be built.  Note  that
       the  POSIX wrapper is for the 8-bit library only, and that pcre2grep is
       an 8-bit program. Neither of these are built if  you  select  only  the
       16-bit or 32-bit libraries.


BUILDING SHARED AND STATIC LIBRARIES

       The  Autotools PCRE2 building process uses libtool to build both shared
       and static libraries by default. You can suppress an  unwanted  library
       by adding one of

         --disable-shared
         --disable-static

       to the configure command.


UNICODE AND UTF SUPPORT

       By  default,  PCRE2 is built with support for Unicode and UTF character
       strings.  To build it without Unicode support, add

         --disable-unicode

       to the configure command. This setting applies to all three  libraries.
       It  is  not  possible  to  build  one library with Unicode support, and
       another without, in the same configuration.

       Of itself, Unicode support does not make PCRE2 treat strings as  UTF-8,
       UTF-16 or UTF-32. To do that, applications that use the library can set
       the PCRE2_UTF option when they call pcre2_compile() to compile  a  pat-
       tern.   Alternatively,  patterns  may be started with (*UTF) unless the
       application has locked this out by setting PCRE2_NEVER_UTF.

       UTF support allows the libraries to process character code points up to
       0x10ffff  in  the  strings that they handle. Unicode support also gives
       access to the Unicode properties of characters, using  pattern  escapes
       such as \P, \p, and \X. Only the general category properties such as Lu
       and Nd are supported. Details are given in the pcre2pattern  documenta-
       tion.

       Pattern escapes such as \d and \w do not by default make use of Unicode
       properties. The application can request that they  do  by  setting  the
       PCRE2_UCP  option.  Unless  the  application has set PCRE2_NEVER_UCP, a
       pattern may also request this by starting with (*UCP).


DISABLING THE USE OF \C

       The \C escape sequence, which matches a single code unit, even in a UTF
       mode,  can  cause unpredictable behaviour because it may leave the cur-
       rent matching point in the middle of a multi-code-unit  character.  The
       application  can  lock  it  out  by setting the PCRE2_NEVER_BACKSLASH_C
       option when calling pcre2_compile(). There is also a build-time option

         --enable-never-backslash-C

       (note the upper case C) which locks out the use of \C entirely.


JUST-IN-TIME COMPILER SUPPORT

       Just-in-time (JIT) compiler support is included in the build by  speci-
       fying

         --enable-jit

       This  support  is available only for certain hardware architectures. If
       this option is set for an unsupported architecture,  a  building  error
       occurs.  If in doubt, use

         --enable-jit=auto

       which  enables  JIT  only if the current hardware is supported. You can
       check if JIT is enabled in the configuration summary that is output  at
       the  end  of a configure run. If you are enabling JIT under SELinux you
       may also want to add

         --enable-jit-sealloc

       which enables the use of an execmem allocator in JIT that is compatible
       with  SELinux.  This  has  no  effect  if  JIT  is not enabled. See the
       pcre2jit documentation for a discussion of JIT usage. When JIT  support
       is enabled, pcre2grep automatically makes use of it, unless you add

         --disable-pcre2grep-jit

       to the "configure" command.


NEWLINE RECOGNITION

       By  default, PCRE2 interprets the linefeed (LF) character as indicating
       the end of a line. This is the normal newline  character  on  Unix-like
       systems.  You can compile PCRE2 to use carriage return (CR) instead, by
       adding

         --enable-newline-is-cr

       to the configure  command.  There  is  also  an  --enable-newline-is-lf
       option, which explicitly specifies linefeed as the newline character.

       Alternatively, you can specify that line endings are to be indicated by
       the two-character sequence CRLF (CR immediately followed by LF). If you
       want this, add

         --enable-newline-is-crlf

       to the configure command. There is a fourth option, specified by

         --enable-newline-is-anycrlf

       which  causes  PCRE2 to recognize any of the three sequences CR, LF, or
       CRLF as indicating a line ending. A fifth option, specified by

         --enable-newline-is-any

       causes PCRE2 to recognize any Unicode  newline  sequence.  The  Unicode
       newline sequences are the three just mentioned, plus the single charac-
       ters VT (vertical tab, U+000B), FF (form feed, U+000C), NEL (next line,
       U+0085),  LS  (line  separator,  U+2028),  and PS (paragraph separator,
       U+2029). The final option is

         --enable-newline-is-nul

       which causes NUL (binary zero) to be set  as  the  default  line-ending
       character.

       Whatever default line ending convention is selected when PCRE2 is built
       can be overridden by applications that use the library. At  build  time
       it is recommended to use the standard for your operating system.


WHAT \R MATCHES

       By  default,  the  sequence \R in a pattern matches any Unicode newline
       sequence, independently of what has been selected as  the  line  ending
       sequence. If you specify

         --enable-bsr-anycrlf

       the  default  is changed so that \R matches only CR, LF, or CRLF. What-
       ever is selected when PCRE2 is built can be overridden by  applications
       that use the library.


HANDLING VERY LARGE PATTERNS

       Within  a  compiled  pattern,  offset values are used to point from one
       part to another (for example, from an opening parenthesis to an  alter-
       nation  metacharacter).  By default, in the 8-bit and 16-bit libraries,
       two-byte values are used for these offsets, leading to a  maximum  size
       for a compiled pattern of around 64 thousand code units. This is suffi-
       cient to handle all but the most gigantic patterns. Nevertheless,  some
       people do want to process truly enormous patterns, so it is possible to
       compile PCRE2 to use three-byte or four-byte offsets by adding  a  set-
       ting such as

         --with-link-size=3

       to  the  configure command. The value given must be 2, 3, or 4. For the
       16-bit library, a value of 3 is rounded up to 4.  In  these  libraries,
       using  longer  offsets slows down the operation of PCRE2 because it has
       to load additional data when handling them. For the 32-bit library  the
       value  is  always 4 and cannot be overridden; the value of --with-link-
       size is ignored.


LIMITING PCRE2 RESOURCE USAGE

       The pcre2_match() function increments a counter each time it goes round
       its  main  loop. Putting a limit on this counter controls the amount of
       computing resource used by a single call to  pcre2_match().  The  limit
       can be changed at run time, as described in the pcre2api documentation.
       The default is 10 million, but this can be changed by adding a  setting
       such as

         --with-match-limit=500000

       to   the   configure   command.   This  setting  also  applies  to  the
       pcre2_dfa_match() matching function, and to JIT  matching  (though  the
       counting is done differently).

       The  pcre2_match() function starts out using a 20KiB vector on the sys-
       tem stack to record backtracking points. The more  nested  backtracking
       points there are (that is, the deeper the search tree), the more memory
       is needed. If the initial vector is not large enough,  heap  memory  is
       used,  up to a certain limit, which is specified in kibibytes (units of
       1024 bytes). The limit can be changed at run time, as described in  the
       pcre2api  documentation.  The default limit (in effect unlimited) is 20
       million. You can change this by a setting such as

         --with-heap-limit=500

       which limits the amount of heap to 500 KiB. This limit applies only  to
       interpretive matching in pcre2_match() and pcre2_dfa_match(), which may
       also use the heap for internal workspace  when  processing  complicated
       patterns.  This limit does not apply when JIT (which has its own memory
       arrangements) is used.

       You can also explicitly limit the depth of nested backtracking  in  the
       pcre2_match() interpreter. This limit defaults to the value that is set
       for --with-match-limit. You can set a lower default  limit  by  adding,
       for example,

         --with-match-limit_depth=10000

       to  the  configure  command.  This value can be overridden at run time.
       This depth limit indirectly limits the amount of heap  memory  that  is
       used,  but because the size of each backtracking "frame" depends on the
       number of capturing parentheses in a pattern, the amount of  heap  that
       is  used  before  the  limit is reached varies from pattern to pattern.
       This limit was more useful in versions  before  10.30,  where  function
       recursion was used for backtracking.

       As well as applying to pcre2_match(), the depth limit also controls the
       depth of recursive function calls in pcre2_dfa_match(). These are  used
       for  lookaround  assertions,  atomic  groups, and recursion within pat-
       terns.  The limit does not apply to JIT matching.


CREATING CHARACTER TABLES AT BUILD TIME

       PCRE2 uses fixed tables for processing characters whose code points are
       less than 256. By default, PCRE2 is built with a set of tables that are
       distributed in the file src/pcre2_chartables.c.dist. These  tables  are
       for ASCII codes only. If you add

         --enable-rebuild-chartables

       to  the  configure  command, the distributed tables are no longer used.
       Instead, a program called dftables is compiled and  run.  This  outputs
       the source for new set of tables, created in the default locale of your
       C run-time system. This method of replacing the tables does not work if
       you  are cross compiling, because dftables is run on the local host. If
       you need to create alternative tables when cross  compiling,  you  will
       have to do so "by hand".


USING EBCDIC CODE

       PCRE2  assumes  by default that it will run in an environment where the
       character code is ASCII or Unicode, which is a superset of ASCII.  This
       is the case for most computer operating systems. PCRE2 can, however, be
       compiled to run in an 8-bit EBCDIC environment by adding

         --enable-ebcdic --disable-unicode

       to the configure command. This setting implies --enable-rebuild-charta-
       bles.  You  should  only  use  it if you know that you are in an EBCDIC
       environment (for example, an IBM mainframe operating system).

       It is not possible to support both EBCDIC and UTF-8 codes in  the  same
       version  of  the  library. Consequently, --enable-unicode and --enable-
       ebcdic are mutually exclusive.

       The EBCDIC character that corresponds to an ASCII LF is assumed to have
       the  value  0x15 by default. However, in some EBCDIC environments, 0x25
       is used. In such an environment you should use

         --enable-ebcdic-nl25

       as well as, or instead of, --enable-ebcdic. The EBCDIC character for CR
       has  the  same  value  as in ASCII, namely, 0x0d. Whichever of 0x15 and
       0x25 is not chosen as LF is made to correspond to the Unicode NEL char-
       acter (which, in Unicode, is 0x85).

       The options that select newline behaviour, such as --enable-newline-is-
       cr, and equivalent run-time options, refer to these character values in
       an EBCDIC environment.


PCRE2GREP SUPPORT FOR EXTERNAL SCRIPTS

       By default, on non-Windows systems, pcre2grep supports the use of call-
       outs with string arguments within the patterns it is matching, in order
       to  run external scripts. For details, see the pcre2grep documentation.
       This support can be disabled by adding  --disable-pcre2grep-callout  to
       the configure command.


PCRE2GREP OPTIONS FOR COMPRESSED FILE SUPPORT

       By  default,  pcre2grep reads all files as plain text. You can build it
       so that it recognizes files whose names end in .gz or .bz2,  and  reads
       them with libz or libbz2, respectively, by adding one or both of

         --enable-pcre2grep-libz
         --enable-pcre2grep-libbz2

       to the configure command. These options naturally require that the rel-
       evant libraries are installed on your system. Configuration  will  fail
       if they are not.


PCRE2GREP BUFFER SIZE

       pcre2grep  uses an internal buffer to hold a "window" on the file it is
       scanning, in order to be able to output "before" and "after" lines when
       it finds a match. The default starting size of the buffer is 20KiB. The
       buffer itself is three times this size, but because of the  way  it  is
       used for holding "before" lines, the longest line that is guaranteed to
       be processable is the notional buffer size. If a longer line is encoun-
       tered,  pcre2grep  automatically  expands the buffer, up to a specified
       maximum size, whose default is 1MiB or the starting size, whichever  is
       the  larger. You can change the default parameter values by adding, for
       example,

         --with-pcre2grep-bufsize=51200
         --with-pcre2grep-max-bufsize=2097152

       to the configure command. The caller of pcre2grep  can  override  these
       values  by  using  --buffer-size  and  --max-buffer-size on the command
       line.


PCRE2TEST OPTION FOR LIBREADLINE SUPPORT

       If you add one of

         --enable-pcre2test-libreadline
         --enable-pcre2test-libedit

       to the configure command, pcre2test  is  linked  with  the  libreadline
       orlibedit library, respectively, and when its input is from a terminal,
       it reads it using the readline() function. This  provides  line-editing
       and  history  facilities.  Note that libreadline is GPL-licensed, so if
       you distribute a binary of pcre2test linked in this way, there  may  be
       licensing issues. These can be avoided by linking instead with libedit,
       which has a BSD licence.

       Setting --enable-pcre2test-libreadline causes the -lreadline option  to
       be  added to the pcre2test build. In many operating environments with a
       sytem-installed readline library this is sufficient. However,  in  some
       environments (e.g. if an unmodified distribution version of readline is
       in use), some extra configuration may be necessary.  The  INSTALL  file
       for libreadline says this:

         "Readline uses the termcap functions, but does not link with
         the termcap or curses library itself, allowing applications
         which link with readline the to choose an appropriate library."

       If  your environment has not been set up so that an appropriate library
       is automatically included, you may need to add something like

         LIBS="-ncurses"

       immediately before the configure command.


INCLUDING DEBUGGING CODE

       If you add

         --enable-debug

       to the configure command, additional debugging code is included in  the
       build. This feature is intended for use by the PCRE2 maintainers.


DEBUGGING WITH VALGRIND SUPPORT

       If you add

         --enable-valgrind

       to  the  configure command, PCRE2 will use valgrind annotations to mark
       certain memory regions as  unaddressable.  This  allows  it  to  detect
       invalid  memory  accesses,  and  is  mostly  useful for debugging PCRE2
       itself.


CODE COVERAGE REPORTING

       If your C compiler is gcc, you can build a version of  PCRE2  that  can
       generate a code coverage report for its test suite. To enable this, you
       must install lcov version 1.6 or above. Then specify

         --enable-coverage

       to the configure command and build PCRE2 in the usual way.

       Note that using ccache (a caching C compiler) is incompatible with code
       coverage  reporting. If you have configured ccache to run automatically
       on your system, you must set the environment variable

         CCACHE_DISABLE=1

       before running make to build PCRE2, so that ccache is not used.

       When --enable-coverage is used,  the  following  addition  targets  are
       added to the Makefile:

         make coverage

       This  creates  a  fresh coverage report for the PCRE2 test suite. It is
       equivalent to running "make coverage-reset", "make  coverage-baseline",
       "make check", and then "make coverage-report".

         make coverage-reset

       This zeroes the coverage counters, but does nothing else.

         make coverage-baseline

       This captures baseline coverage information.

         make coverage-report

       This creates the coverage report.

         make coverage-clean-report

       This  removes the generated coverage report without cleaning the cover-
       age data itself.

         make coverage-clean-data

       This removes the captured coverage data without removing  the  coverage
       files created at compile time (*.gcno).

         make coverage-clean

       This  cleans all coverage data including the generated coverage report.
       For more information about code coverage, see the gcov and  lcov  docu-
       mentation.


DISABLING THE Z AND T FORMATTING MODIFIERS

       The  C99  standard  defines formatting modifiers z and t for size_t and
       ptrdiff_t values, respectively. By default, PCRE2 uses these  modifiers
       in  environments  other  than  Microsoft Visual Studio when __STDC_VER-
       SION__ is defined and has a value greater  than  or  equal  to  199901L
       (indicating  C99).   However,  there  is  at least one environment that
       claims to be C99 but does not support these modifiers. If

         --disable-percent-zt

       is specified, no use is made of the z or t modifiers. Instead or %td or
       %zu, %lu is used, with a cast for size_t values.


SUPPORT FOR FUZZERS

       There  is  a  special  option for use by people who want to run fuzzing
       tests on PCRE2:

         --enable-fuzz-support

       At present this applies only to the 8-bit library. If set, it causes an
       extra  library  called  libpcre2-fuzzsupport.a  to  be  built,  but not
       installed. This contains a single function called  LLVMFuzzerTestOneIn-
       put()  whose  arguments are a pointer to a string and the length of the
       string. When called, this function tries to compile  the  string  as  a
       pattern,  and if that succeeds, to match it.  This is done both with no
       options and with some random options bits that are generated  from  the
       string.

       Setting  --enable-fuzz-support  also  causes  a binary called pcre2fuz-
       zcheck to be created. This is normally run under valgrind or used  when
       PCRE2 is compiled with address sanitizing enabled. It calls the fuzzing
       function and outputs information about what  it  is  doing.  The  input
       strings  are specified by arguments: if an argument starts with "=" the
       rest of it is a literal input string. Otherwise, it is assumed to be  a
       file name, and the contents of the file are the test string.


OBSOLETE OPTION

       In  versions  of  PCRE2 prior to 10.30, there were two ways of handling
       backtracking in the pcre2_match() function. The default was to use  the
       system stack, but if

         --disable-stack-for-recursion

       was  set,  memory on the heap was used. From release 10.30 onwards this
       has changed (the stack is no longer used)  and  this  option  now  does
       nothing except give a warning.


SEE ALSO

       pcre2api(3), pcre2-config(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 15 November 2018
       Copyright (c) 1997-2018 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2CALLOUT(3)            Library Functions Manual            PCRE2CALLOUT(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

       #include <pcre2.h>

       int (*pcre2_callout)(pcre2_callout_block *, void *);

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);


DESCRIPTION

       PCRE2  provides  a feature called "callout", which is a means of tempo-
       rarily passing control to the caller of PCRE2 in the middle of  pattern
       matching.  The caller of PCRE2 provides an external function by putting
       its entry point in a match  context  (see  pcre2_set_callout()  in  the
       pcre2api documentation).

       When  using the pcre2_substitute() function, an additional callout fea-
       ture is available. This does a callout after each change to the subject
       string and is described in the pcre2api documentation; the rest of this
       document is concerned with callouts during pattern matching.

       Within a regular expression, (?C<arg>) indicates a point at  which  the
       external  function  is  to  be  called. Different callout points can be
       identified by putting a number less than 256 after the  letter  C.  The
       default  value is zero.  Alternatively, the argument may be a delimited
       string. The starting delimiter must be one of ` ' " ^ % # $ {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing delimiter is }. If  the  ending  delimiter  is  needed  within  the
       string,  it  must be doubled. For example, this pattern has two callout
       points:

         (?C1)abc(?C"some ""arbitrary"" text")def

       If the PCRE2_AUTO_CALLOUT option bit is set when a pattern is compiled,
       PCRE2  automatically inserts callouts, all with number 255, before each
       item in the pattern except for immediately before or after an  explicit
       callout. For example, if PCRE2_AUTO_CALLOUT is used with the pattern

         A(?C3)B

       it is processed as if it were

         (?C255)A(?C3)B(?C255)

       Here is a more complicated example:

         A(\d{2}|--)

       With PCRE2_AUTO_CALLOUT, this pattern is processed as if it were

         (?C255)A(?C255)((?C255)\d{2}(?C255)|(?C255)-(?C255)-(?C255))(?C255)

       Notice  that  there  is a callout before and after each parenthesis and
       alternation bar. If the pattern contains a conditional group whose con-
       dition  is  an  assertion, an automatic callout is inserted immediately
       before the condition. Such a callout may also be  inserted  explicitly,
       for example:

         (?(?C9)(?=a)ab|de)  (?(?C%text%)(?!=d)ab|de)

       This  applies only to assertion conditions (because they are themselves
       independent groups).

       Callouts can be useful for tracking the progress of  pattern  matching.
       The pcre2test program has a pattern qualifier (/auto_callout) that sets
       automatic callouts.  When any callouts are  present,  the  output  from
       pcre2test  indicates  how  the pattern is being matched. This is useful
       information when you are trying to optimize the performance of  a  par-
       ticular pattern.


MISSING CALLOUTS

       You  should  be  aware  that, because of optimizations in the way PCRE2
       compiles and matches patterns, callouts sometimes do not happen exactly
       as you might expect.

   Auto-possessification

       At compile time, PCRE2 "auto-possessifies" repeated items when it knows
       that what follows cannot be part of the repeat. For example, a+[bc]  is
       compiled  as if it were a++[bc]. The pcre2test output when this pattern
       is compiled with PCRE2_ANCHORED and PCRE2_AUTO_CALLOUT and then applied
       to the string "aaaa" is:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
         No match

       This  indicates that when matching [bc] fails, there is no backtracking
       into a+ (because it is being treated as a++) and therefore the callouts
       that  would  be  taken for the backtracks do not occur. You can disable
       the  auto-possessify  feature  by  passing   PCRE2_NO_AUTO_POSSESS   to
       pcre2_compile(),  or  starting  the pattern with (*NO_AUTO_POSSESS). In
       this case, the output changes to this:

         --->aaaa
          +0 ^        a+
          +2 ^   ^    [bc]
          +2 ^  ^     [bc]
          +2 ^ ^      [bc]
          +2 ^^       [bc]
         No match

       This time, when matching [bc] fails, the matcher backtracks into a+ and
       tries again, repeatedly, until a+ itself fails.

   Automatic .* anchoring

       By default, an optimization is applied when .* is the first significant
       item in a pattern. If PCRE2_DOTALL is set, so that the  dot  can  match
       any  character,  the pattern is automatically anchored. If PCRE2_DOTALL
       is not set, a match can start only after an internal newline or at  the
       beginning of the subject, and pcre2_compile() remembers this. If a pat-
       tern has more than one top-level branch, automatic anchoring occurs  if
       all branches are anchorable.

       This  optimization is disabled, however, if .* is in an atomic group or
       if there is a backreference to the capture group in which  it  appears.
       It  is  also disabled if the pattern contains (*PRUNE) or (*SKIP). How-
       ever, the presence of callouts does not affect it.

       For example, if the pattern .*\d is  compiled  with  PCRE2_AUTO_CALLOUT
       and applied to the string "aa", the pcre2test output is:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
         No match

       This  shows  that all match attempts start at the beginning of the sub-
       ject. In other words, the pattern is anchored.  You  can  disable  this
       optimization  by passing PCRE2_NO_DOTSTAR_ANCHOR to pcre2_compile(), or
       starting the pattern with (*NO_DOTSTAR_ANCHOR). In this case, the  out-
       put changes to:

         --->aa
          +0 ^      .*
          +2 ^ ^    \d
          +2 ^^     \d
          +2 ^      \d
          +0  ^     .*
          +2  ^^    \d
          +2  ^     \d
         No match

       This  shows more match attempts, starting at the second subject charac-
       ter.  Another optimization, described in the next section,  means  that
       there is no subsequent attempt to match with an empty subject.

   Other optimizations

       Other  optimizations  that  provide fast "no match" results also affect
       callouts.  For example, if the pattern is

         ab(?C4)cd

       PCRE2 knows that any matching string must contain the  letter  "d".  If
       the  subject  string  is  "abyz",  the  lack of "d" means that matching
       doesn't ever start, and the callout is  never  reached.  However,  with
       "abyd", though the result is still no match, the callout is obeyed.

       For  most  patterns  PCRE2  also knows the minimum length of a matching
       string, and will immediately give a "no match" return without  actually
       running  a  match if the subject is not long enough, or, for unanchored
       patterns, if it has been scanned far enough.

       You can disable these optimizations by passing the PCRE2_NO_START_OPTI-
       MIZE  option  to  pcre2_compile(),  or  by  starting  the  pattern with
       (*NO_START_OPT). This slows down the matching process, but does  ensure
       that callouts such as the example above are obeyed.


THE CALLOUT INTERFACE

       During  matching,  when  PCRE2  reaches a callout point, if an external
       function is provided in the match context, it is called.  This  applies
       to  both normal, DFA, and JIT matching. The first argument to the call-
       out function is a pointer to a pcre2_callout block. The second argument
       is  the  void * callout data that was supplied when the callout was set
       up by calling pcre2_set_callout() (see the pcre2api documentation). The
       callout  block structure contains the following fields, not necessarily
       in this order:

         uint32_t      version;
         uint32_t      callout_number;
         uint32_t      capture_top;
         uint32_t      capture_last;
         uint32_t      callout_flags;
         PCRE2_SIZE   *offset_vector;
         PCRE2_SPTR    mark;
         PCRE2_SPTR    subject;
         PCRE2_SIZE    subject_length;
         PCRE2_SIZE    start_match;
         PCRE2_SIZE    current_position;
         PCRE2_SIZE    pattern_position;
         PCRE2_SIZE    next_item_length;
         PCRE2_SIZE    callout_string_offset;
         PCRE2_SIZE    callout_string_length;
         PCRE2_SPTR    callout_string;

       The version field contains the version number of the block format.  The
       current  version  is  2; the three callout string fields were added for
       version 1, and the callout_flags field for version 2. If you are  writ-
       ing  an  application  that  might  use an earlier release of PCRE2, you
       should check the version number before accessing any of  these  fields.
       The  version  number  will increase in future if more fields are added,
       but the intention is never to remove any of the existing fields.

   Fields for numerical callouts

       For a numerical callout, callout_string  is  NULL,  and  callout_number
       contains  the  number  of  the callout, in the range 0-255. This is the
       number that follows (?C for callouts that part of the  pattern;  it  is
       255 for automatically generated callouts.

   Fields for string callouts

       For  callouts with string arguments, callout_number is always zero, and
       callout_string points to the string that is contained within  the  com-
       piled pattern. Its length is given by callout_string_length. Duplicated
       ending delimiters that were present in the original pattern string have
       been turned into single characters, but there is no other processing of
       the callout string argument. An additional code unit containing  binary
       zero  is  present  after the string, but is not included in the length.
       The delimiter that was used to start the string is also  stored  within
       the  pattern, immediately before the string itself. You can access this
       delimiter as callout_string[-1] if you need it.

       The callout_string_offset field is the code unit offset to the start of
       the callout argument string within the original pattern string. This is
       provided for the benefit of applications such as script languages  that
       might need to report errors in the callout string within the pattern.

   Fields for all callouts

       The  remaining  fields in the callout block are the same for both kinds
       of callout.

       The offset_vector field is a pointer to a vector of  capturing  offsets
       (the "ovector"). You may read the elements in this vector, but you must
       not change any of them.

       For calls to pcre2_match(),  the  offset_vector  field  is  not  (since
       release  10.30)  a pointer to the actual ovector that was passed to the
       matching function in the match data block.  Instead  it  points  to  an
       internal  ovector  of a size large enough to hold all possible captured
       substrings in the pattern. Note that whenever a recursion or subroutine
       call  within  a pattern completes, the capturing state is reset to what
       it was before.

       The capture_last field contains the number of the  most  recently  cap-
       tured  substring,  and the capture_top field contains one more than the
       number of the highest numbered captured substring so far.  If  no  sub-
       strings  have yet been captured, the value of capture_last is 0 and the
       value of capture_top is 1. The values of these  fields  do  not  always
       differ   by   one;  for  example,  when  the  callout  in  the  pattern
       ((a)(b))(?C2) is taken, capture_last is 1 but capture_top is 4.

       The  contents  of  ovector[2]  to  ovector[<capture_top>*2-1]  can   be
       inspected in order to extract substrings that have been matched so far,
       in the same way as extracting substrings after a match  has  completed.
       The  values in ovector[0] and ovector[1] are always PCRE2_UNSET because
       the match is by definition not complete. Substrings that have not  been
       captured  but whose numbers are less than capture_top also have both of
       their ovector slots set to PCRE2_UNSET.

       For DFA matching, the offset_vector field points to  the  ovector  that
       was  passed  to the matching function in the match data block for call-
       outs at the top level, but to an internal ovector during the processing
       of  pattern  recursions, lookarounds, and atomic groups. However, these
       ovectors hold no useful information because pcre2_dfa_match() does  not
       support  substring  capturing. The value of capture_top is always 1 and
       the value of capture_last is always 0 for DFA matching.

       The subject and subject_length fields contain copies of the values that
       were passed to the matching function.

       The  start_match  field normally contains the offset within the subject
       at which the current match attempt  started.  However,  if  the  escape
       sequence  \K has been encountered, this value is changed to reflect the
       modified starting point. If the pattern is not  anchored,  the  callout
       function may be called several times from the same point in the pattern
       for different starting points in the subject.

       The current_position field contains the offset within  the  subject  of
       the current match pointer.

       The pattern_position field contains the offset in the pattern string to
       the next item to be matched.

       The next_item_length field contains the length of the next item  to  be
       processed  in the pattern string. When the callout is at the end of the
       pattern, the length is zero.  When  the  callout  precedes  an  opening
       parenthesis, the length includes meta characters that follow the paren-
       thesis. For example, in a callout before an assertion  such  as  (?=ab)
       the  length  is  3. For an an alternation bar or a closing parenthesis,
       the length is one, unless a closing parenthesis is followed by a  quan-
       tifier, in which case its length is included.  (This changed in release
       10.23. In earlier releases, before an opening  parenthesis  the  length
       was  that of the entire group, and before an alternation bar or a clos-
       ing parenthesis the length was zero.)

       The pattern_position and next_item_length fields are intended  to  help
       in  distinguishing between different automatic callouts, which all have
       the same callout number. However, they are set for  all  callouts,  and
       are used by pcre2test to show the next item to be matched when display-
       ing callout information.

       In callouts from pcre2_match() the mark field contains a pointer to the
       zero-terminated  name of the most recently passed (*MARK), (*PRUNE), or
       (*THEN) item in the match, or NULL if no such items have  been  passed.
       Instances  of  (*PRUNE)  or  (*THEN) without a name do not obliterate a
       previous (*MARK). In callouts from the DFA matching function this field
       always contains NULL.

       The   callout_flags   field   is   always   zero   in   callouts   from
       pcre2_dfa_match() or when JIT is being used. When pcre2_match() without
       JIT is used, the following bits may be set:

         PCRE2_CALLOUT_STARTMATCH

       This  is set for the first callout after the start of matching for each
       new starting position in the subject.

         PCRE2_CALLOUT_BACKTRACK

       This is set if there has been a matching backtrack since  the  previous
       callout,  or  since  the start of matching if this is the first callout
       from a pcre2_match() run.

       Both bits are set when a backtrack has caused a "bumpalong"  to  a  new
       starting  position in the subject. Output from pcre2test does not indi-
       cate the presence of these bits unless the  callout_extra  modifier  is
       set.

       The information in the callout_flags field is provided so that applica-
       tions can track and tell their users how matching with backtracking  is
       done.  This  can be useful when trying to optimize patterns, or just to
       understand how PCRE2 works. There is no  support  in  pcre2_dfa_match()
       because  there is no backtracking in DFA matching, and there is no sup-
       port in JIT because JIT is all about maximimizing matching performance.
       In both these cases the callout_flags field is always zero.


RETURN VALUES FROM CALLOUTS

       The external callout function returns an integer to PCRE2. If the value
       is zero, matching proceeds as normal. If  the  value  is  greater  than
       zero,  matching  fails  at  the current point, but the testing of other
       matching possibilities goes ahead, just as if a lookahead assertion had
       failed. If the value is less than zero, the match is abandoned, and the
       matching function returns the negative value.

       Negative  values  should  normally  be   chosen   from   the   set   of
       PCRE2_ERROR_xxx  values.  In  particular,  PCRE2_ERROR_NOMATCH forces a
       standard "no match" failure. The error  number  PCRE2_ERROR_CALLOUT  is
       reserved  for  use by callout functions; it will never be used by PCRE2
       itself.


CALLOUT ENUMERATION

       int pcre2_callout_enumerate(const pcre2_code *code,
         int (*callback)(pcre2_callout_enumerate_block *, void *),
         void *user_data);

       A script language that supports the use of string arguments in callouts
       might  like  to  scan  all the callouts in a pattern before running the
       match. This can be done by calling pcre2_callout_enumerate(). The first
       argument  is  a  pointer  to a compiled pattern, the second points to a
       callback function, and the third is arbitrary user data.  The  callback
       function  is  called  for  every callout in the pattern in the order in
       which they appear. Its first argument is a pointer to a callout enumer-
       ation  block,  and  its second argument is the user_data value that was
       passed to pcre2_callout_enumerate(). The data block contains  the  fol-
       lowing fields:

         version                Block version number
         pattern_position       Offset to next item in pattern
         next_item_length       Length of next item in pattern
         callout_number         Number for numbered callouts
         callout_string_offset  Offset to string within pattern
         callout_string_length  Length of callout string
         callout_string         Points to callout string or is NULL

       The  version  number is currently 0. It will increase if new fields are
       ever added to the block. The remaining fields are  the  same  as  their
       namesakes  in  the pcre2_callout block that is used for callouts during
       matching, as described above.

       Note that the value of pattern_position is  unique  for  each  callout.
       However,  if  a callout occurs inside a group that is quantified with a
       non-zero minimum or a fixed maximum, the group is replicated inside the
       compiled  pattern.  For example, a pattern such as /(a){2}/ is compiled
       as if it were /(a)(a)/. This means that the callout will be  enumerated
       more  than  once,  but with the same value for pattern_position in each
       case.

       The callback function should normally return zero. If it returns a non-
       zero value, scanning the pattern stops, and that value is returned from
       pcre2_callout_enumerate().


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 03 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2COMPAT(3)             Library Functions Manual             PCRE2COMPAT(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

DIFFERENCES BETWEEN PCRE2 AND PERL

       This document describes the differences in the ways that PCRE2 and Perl
       handle regular expressions. The differences  described  here  are  with
       respect  to Perl versions 5.26, but as both Perl and PCRE2 are continu-
       ally changing, the information may sometimes be out of date.

       1. PCRE2 has only a subset of Perl's Unicode support. Details  of  what
       it does have are given in the pcre2unicode page.

       2.  Like  Perl, PCRE2 allows repeat quantifiers on parenthesized asser-
       tions, but they do not mean what you might think. For example, (?!a){3}
       does  not  assert  that  the next three characters are not "a". It just
       asserts that the next character is not "a" three times  (in  principle;
       PCRE2  optimizes this to run the assertion just once). Perl allows some
       repeat quantifiers on other  assertions,  for  example,  \b*  (but  not
       \b{3}), but these do not seem to have any use.

       3.  Capture groups that occur inside negative lookaround assertions are
       counted, but their entries in the offsets vector are set  only  when  a
       negative  assertion is a condition that has a matching branch (that is,
       the condition is false).

       4. The following Perl escape sequences are not supported: \F,  \l,  \L,
       \u, \U, and \N when followed by a character name. \N on its own, match-
       ing a non-newline character, and \N{U+dd..}, matching  a  Unicode  code
       point,  are  supported.  The  escapes that modify the case of following
       letters are implemented by Perl's general string-handling and  are  not
       part of its pattern matching engine. If any of these are encountered by
       PCRE2, an error is generated by default.  However,  if  either  of  the
       PCRE2_ALT_BSUX  or  PCRE2_EXTRA_ALT_BSUX  options is set, \U and \u are
       interpreted as ECMAScript interprets them.

       5. The Perl escape sequences \p, \P, and \X are supported only if PCRE2
       is built with Unicode support (the default). The properties that can be
       tested with \p and \P are limited to the  general  category  properties
       such  as  Lu and Nd, script names such as Greek or Han, and the derived
       properties Any and L&.  PCRE2 does support the Cs (surrogate) property,
       which  Perl  does  not; the Perl documentation says "Because Perl hides
       the need for the user to understand the internal representation of Uni-
       code  characters, there is no need to implement the somewhat messy con-
       cept of surrogates."

       6. PCRE2 supports the \Q...\E escape for quoting substrings. Characters
       in between are treated as literals. However, this is slightly different
       from Perl in that $ and @ are  also  handled  as  literals  inside  the
       quotes. In Perl, they cause variable interpolation (but of course PCRE2
       does not have variables). Also,  Perl  does  "double-quotish  backslash
       interpolation" on any backslashes between \Q and \E which, its documen-
       tation says, "may lead to confusing results". PCRE2 treats a  backslash
       between  \Q  and  \E  just like any other character. Note the following
       examples:

           Pattern            PCRE2 matches     Perl matches

           \Qabc$xyz\E        abc$xyz           abc followed by the
                                                  contents of $xyz
           \Qabc\$xyz\E       abc\$xyz          abc\$xyz
           \Qabc\E\$\Qxyz\E   abc$xyz           abc$xyz
           \QA\B\E            A\B               A\B
           \Q\\E              \                 \\E

       The \Q...\E sequence is recognized both inside  and  outside  character
       classes.

       7.   Fairly  obviously,  PCRE2  does  not  support  the  (?{code})  and
       (??{code}) constructions. However, PCRE2 does have a "callout" feature,
       which allows an external function to be called during pattern matching.
       See the pcre2callout documentation for details.

       8. Subroutine calls (whether recursive or not) were treated  as  atomic
       groups  up to PCRE2 release 10.23, but from release 10.30 this changed,
       and backtracking into subroutine calls is now supported, as in Perl.

       9. If any of the backtracking control verbs are used in a group that is
       called  as  a  subroutine (whether or not recursively), their effect is
       confined to that group; it does not extend to the surrounding  pattern.
       This  is  not  always  the  case  in Perl. In particular, if (*THEN) is
       present in a group that is called as a subroutine, its action  is  lim-
       ited  to  that  group, even if the group does not contain any | charac-
       ters. Note that such groups are processed  as  anchored  at  the  point
       where they are tested.

       10.  If a pattern contains more than one backtracking control verb, the
       first one that is backtracked onto acts. For example,  in  the  pattern
       A(*COMMIT)B(*PRUNE)C  a  failure in B triggers (*COMMIT), but a failure
       in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
       it is the same as PCRE2, but there are cases where it differs.

       11.  Most  backtracking  verbs in assertions have their normal actions.
       They are not confined to the assertion.

       12. There are some differences that are concerned with the settings  of
       captured  strings  when  part  of  a  pattern is repeated. For example,
       matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
       unset, but in PCRE2 it is set to "b".

       13.  PCRE2's  handling  of duplicate capture group numbers and names is
       not as general as Perl's. This is a consequence of the fact  the  PCRE2
       works  internally  just with numbers, using an external table to trans-
       late between numbers and  names.  In  particular,  a  pattern  such  as
       (?|(?<a>A)|(?<b>B),  where  the two capture groups have the same number
       but different names, is not supported, and causes an error  at  compile
       time. If it were allowed, it would not be possible to distinguish which
       group matched, because both names map to capture  group  number  1.  To
       avoid this confusing situation, an error is given at compile time.

       14. Perl used to recognize comments in some places that PCRE2 does not,
       for example, between the ( and ? at the start of a  group.  If  the  /x
       modifier  is  set,  Perl allowed white space between ( and ? though the
       latest Perls give an error (for a while it was just deprecated).  There
       may still be some cases where Perl behaves differently.

       15.  Perl,  when  in warning mode, gives warnings for character classes
       such as [A-\d] or [a-[:digit:]]. It then treats the hyphens  as  liter-
       als. PCRE2 has no warning features, so it gives an error in these cases
       because they are almost certainly user mistakes.

       16. In PCRE2, the upper/lower case character properties Lu and  Ll  are
       not  affected when case-independent matching is specified. For example,
       \p{Lu} always matches an upper case letter. I think Perl has changed in
       this  respect; in the release at the time of writing (5.24), \p{Lu} and
       \p{Ll} match all letters, regardless of case, when case independence is
       specified.

       17.  PCRE2  provides  some  extensions  to  the Perl regular expression
       facilities.  Perl 5.10 includes new features that are  not  in  earlier
       versions  of  Perl,  some  of which (such as named parentheses) were in
       PCRE2 for some time before. This list is with respect to Perl 5.26:

       (a) Although lookbehind assertions in PCRE2  must  match  fixed  length
       strings,  each alternative branch of a lookbehind assertion can match a
       different length of string. Perl requires them all  to  have  the  same
       length.

       (b) From PCRE2 10.23, backreferences to groups of fixed length are sup-
       ported in lookbehinds, provided that there is no possibility of  refer-
       encing  a  non-unique  number or name. Perl does not support backrefer-
       ences in lookbehinds.

       (c) If PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set,  the
       $ meta-character matches only at the very end of the string.

       (d)  A  backslash  followed  by  a  letter  with  no special meaning is
       faulted. (Perl can be made to issue a warning.)

       (e) If PCRE2_UNGREEDY is set, the greediness of the repetition  quanti-
       fiers is inverted, that is, by default they are not greedy, but if fol-
       lowed by a question mark they are.

       (f) PCRE2_ANCHORED can be used at matching time to force a  pattern  to
       be tried only at the first matching position in the subject string.

       (g)     The     PCRE2_NOTBOL,    PCRE2_NOTEOL,    PCRE2_NOTEMPTY    and
       PCRE2_NOTEMPTY_ATSTART options have no Perl equivalents.

       (h) The \R escape sequence can be restricted to match only CR,  LF,  or
       CRLF by the PCRE2_BSR_ANYCRLF option.

       (i)  The  callout  facility is PCRE2-specific. Perl supports codeblocks
       and variable interpolation, but not general hooks on every match.

       (j) The partial matching facility is PCRE2-specific.

       (k) The alternative matching function (pcre2_dfa_match() matches  in  a
       different way and is not Perl-compatible.

       (l)  PCRE2 recognizes some special sequences such as (*CR) or (*NO_JIT)
       at the start of a pattern that  set  overall  options  that  cannot  be
       changed within the pattern.

       18.  The  Perl  /a modifier restricts /d numbers to pure ascii, and the
       /aa modifier restricts /i  case-insensitive  matching  to  pure  ascii,
       ignoring  Unicode  rules.  This  separation  cannot be represented with
       PCRE2_UCP.

       19. Perl has different limits than PCRE2. See the pcre2limit documenta-
       tion for details. Perl went with 5.10 from recursion to iteration keep-
       ing the intermediate matches on the heap, which is ~10% slower but does
       not  fall into any stack-overflow limit. PCRE2 made a similar change at
       release 10.30, and also has many build-time and  run-time  customizable
       limits.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 12 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2JIT(3)                Library Functions Manual                PCRE2JIT(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 JUST-IN-TIME COMPILER SUPPORT

       Just-in-time  compiling  is a heavyweight optimization that can greatly
       speed up pattern matching. However, it comes at the cost of extra  pro-
       cessing  before  the  match is performed, so it is of most benefit when
       the same pattern is going to be matched many times. This does not  nec-
       essarily  mean many calls of a matching function; if the pattern is not
       anchored, matching attempts may take place many times at various  posi-
       tions in the subject, even for a single call. Therefore, if the subject
       string is very long, it may still pay  to  use  JIT  even  for  one-off
       matches.  JIT  support  is  available  for all of the 8-bit, 16-bit and
       32-bit PCRE2 libraries.

       JIT support applies only to the  traditional  Perl-compatible  matching
       function.   It  does  not apply when the DFA matching function is being
       used. The code for this support was written by Zoltan Herczeg.


AVAILABILITY OF JIT SUPPORT

       JIT support is an optional feature of  PCRE2.  The  "configure"  option
       --enable-jit  (or  equivalent  CMake  option) must be set when PCRE2 is
       built if you want to use JIT. The support is limited to  the  following
       hardware platforms:

         ARM 32-bit (v5, v7, and Thumb2)
         ARM 64-bit
         Intel x86 32-bit and 64-bit
         MIPS 32-bit and 64-bit
         Power PC 32-bit and 64-bit
         SPARC 32-bit

       If --enable-jit is set on an unsupported platform, compilation fails.

       A  program  can  tell if JIT support is available by calling pcre2_con-
       fig() with the PCRE2_CONFIG_JIT option. The result is  1  when  JIT  is
       available,  and 0 otherwise. However, a simple program does not need to
       check this in order to use JIT. The API is implemented in  a  way  that
       falls  back  to the interpretive code if JIT is not available. For pro-
       grams that need the best possible performance, there is  also  a  "fast
       path" API that is JIT-specific.


SIMPLE USE OF JIT

       To  make use of the JIT support in the simplest way, all you have to do
       is to call pcre2_jit_compile() after successfully compiling  a  pattern
       with pcre2_compile(). This function has two arguments: the first is the
       compiled pattern pointer that was returned by pcre2_compile(), and  the
       second  is  zero  or  more of the following option bits: PCRE2_JIT_COM-
       PLETE, PCRE2_JIT_PARTIAL_HARD, or PCRE2_JIT_PARTIAL_SOFT.

       If JIT support is not available, a  call  to  pcre2_jit_compile()  does
       nothing  and returns PCRE2_ERROR_JIT_BADOPTION. Otherwise, the compiled
       pattern is passed to the JIT compiler, which turns it into machine code
       that executes much faster than the normal interpretive code, but yields
       exactly the same results. The returned value  from  pcre2_jit_compile()
       is zero on success, or a negative error code.

       There  is  a limit to the size of pattern that JIT supports, imposed by
       the size of machine stack that it uses. The exact rules are  not  docu-
       mented  because  they  may  change at any time, in particular, when new
       optimizations are introduced.  If a pattern  is  too  big,  a  call  to
       pcre2_jit_compile() returns PCRE2_ERROR_NOMEMORY.

       PCRE2_JIT_COMPLETE  requests the JIT compiler to generate code for com-
       plete matches. If you want to run partial matches using the  PCRE2_PAR-
       TIAL_HARD  or  PCRE2_PARTIAL_SOFT  options of pcre2_match(), you should
       set one or both of  the  other  options  as  well  as,  or  instead  of
       PCRE2_JIT_COMPLETE. The JIT compiler generates different optimized code
       for each of the three modes (normal, soft partial, hard partial).  When
       pcre2_match()  is  called,  the appropriate code is run if it is avail-
       able. Otherwise, the pattern is matched using interpretive code.

       You can call pcre2_jit_compile() multiple times for the  same  compiled
       pattern.  It does nothing if it has previously compiled code for any of
       the option bits. For example, you can call it once with  PCRE2_JIT_COM-
       PLETE  and  (perhaps  later,  when  you find you need partial matching)
       again with PCRE2_JIT_COMPLETE and PCRE2_JIT_PARTIAL_HARD. This time  it
       will ignore PCRE2_JIT_COMPLETE and just compile code for partial match-
       ing. If pcre2_jit_compile() is called with no option bits set, it imme-
       diately returns zero. This is an alternative way of testing whether JIT
       is available.

       At present, it is not possible to free JIT compiled  code  except  when
       the entire compiled pattern is freed by calling pcre2_code_free().

       In  some circumstances you may need to call additional functions. These
       are described in the  section  entitled  "Controlling  the  JIT  stack"
       below.

       There are some pcre2_match() options that are not supported by JIT, and
       there are also some pattern items that JIT cannot handle.  Details  are
       given  below.  In  both cases, matching automatically falls back to the
       interpretive code. If you want to know whether JIT  was  actually  used
       for  a particular match, you should arrange for a JIT callback function
       to be set up as described in the section entitled "Controlling the  JIT
       stack"  below,  even  if  you  do  not need to supply a non-default JIT
       stack. Such a callback function is called whenever JIT code is about to
       be  obeyed.  If the match-time options are not right for JIT execution,
       the callback function is not obeyed.

       If the JIT compiler finds an unsupported item, no JIT  data  is  gener-
       ated.  You  can find out if JIT matching is available after compiling a
       pattern by calling  pcre2_pattern_info()  with  the  PCRE2_INFO_JITSIZE
       option.  A non-zero result means that JIT compilation was successful. A
       result of 0 means that JIT support is not available, or the pattern was
       not  processed by pcre2_jit_compile(), or the JIT compiler was not able
       to handle the pattern.


UNSUPPORTED OPTIONS AND PATTERN ITEMS

       The pcre2_match() options that  are  supported  for  JIT  matching  are
       PCRE2_COPY_MATCHED_SUBJECT, PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY,
       PCRE2_NOTEMPTY_ATSTART,  PCRE2_NO_UTF_CHECK,  PCRE2_PARTIAL_HARD,   and
       PCRE2_PARTIAL_SOFT.  The  PCRE2_ANCHORED  and PCRE2_ENDANCHORED options
       are not supported at match time.

       If the PCRE2_NO_JIT option is passed to pcre2_match() it  disables  the
       use of JIT, forcing matching by the interpreter code.

       The  only  unsupported  pattern items are \C (match a single data unit)
       when running in a UTF mode, and a callout immediately before an  asser-
       tion condition in a conditional group.


RETURN VALUES FROM JIT MATCHING

       When a pattern is matched using JIT matching, the return values are the
       same as those given by the interpretive pcre2_match()  code,  with  the
       addition  of one new error code: PCRE2_ERROR_JIT_STACKLIMIT. This means
       that the memory used for the JIT stack was insufficient. See  "Control-
       ling the JIT stack" below for a discussion of JIT stack usage.

       The  error  code  PCRE2_ERROR_MATCHLIMIT is returned by the JIT code if
       searching a very large pattern tree goes on for too long, as it  is  in
       the  same circumstance when JIT is not used, but the details of exactly
       what is counted are not the same. The PCRE2_ERROR_DEPTHLIMIT error code
       is never returned when JIT matching is used.


CONTROLLING THE JIT STACK

       When the compiled JIT code runs, it needs a block of memory to use as a
       stack.  By default, it uses 32KiB on the machine stack.  However,  some
       large   or   complicated  patterns  need  more  than  this.  The  error
       PCRE2_ERROR_JIT_STACKLIMIT is given when there  is  not  enough  stack.
       Three  functions  are provided for managing blocks of memory for use as
       JIT stacks. There is further discussion about the use of JIT stacks  in
       the section entitled "JIT stack FAQ" below.

       The  pcre2_jit_stack_create()  function  creates a JIT stack. Its argu-
       ments are a starting size, a maximum size, and a general  context  (for
       memory  allocation  functions, or NULL for standard memory allocation).
       It returns a pointer to an opaque structure of type pcre2_jit_stack, or
       NULL  if there is an error. The pcre2_jit_stack_free() function is used
       to free a stack that is no longer needed. If its argument is NULL, this
       function  returns immediately, without doing anything. (For the techni-
       cally minded: the address space is allocated by mmap or  VirtualAlloc.)
       A  maximum  stack size of 512KiB to 1MiB should be more than enough for
       any pattern.

       The pcre2_jit_stack_assign() function specifies which  stack  JIT  code
       should use. Its arguments are as follows:

         pcre2_match_context  *mcontext
         pcre2_jit_callback    callback
         void                 *data

       The first argument is a pointer to a match context. When this is subse-
       quently passed to a matching function, its information determines which
       JIT stack is used. If this argument is NULL, the function returns imme-
       diately, without doing anything. There are three cases for  the  values
       of the other two options:

         (1) If callback is NULL and data is NULL, an internal 32KiB block
             on the machine stack is used. This is the default when a match
             context is created.

         (2) If callback is NULL and data is not NULL, data must be
             a pointer to a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

         (3) If callback is not NULL, it must point to a function that is
             called with data as an argument at the start of matching, in
             order to set up a JIT stack. If the return from the callback
             function is NULL, the internal 32KiB stack is used; otherwise the
             return value must be a valid JIT stack, the result of calling
             pcre2_jit_stack_create().

       A  callback function is obeyed whenever JIT code is about to be run; it
       is not obeyed when pcre2_match() is called with options that are incom-
       patible  for JIT matching. A callback function can therefore be used to
       determine whether a match operation was  executed  by  JIT  or  by  the
       interpreter.

       You may safely use the same JIT stack for more than one pattern (either
       by assigning directly or by callback), as  long  as  the  patterns  are
       matched sequentially in the same thread. Currently, the only way to set
       up non-sequential matches in one thread is to use callouts: if a  call-
       out  function starts another match, that match must use a different JIT
       stack to the one used for currently suspended match(es).

       In a multithread application, if you do not specify a JIT stack, or  if
       you  assign  or  pass  back  NULL from a callback, that is thread-safe,
       because each thread has its own machine stack. However, if  you  assign
       or  pass  back a non-NULL JIT stack, this must be a different stack for
       each thread so that the application is thread-safe.

       Strictly speaking, even more is allowed. You can assign the  same  non-
       NULL  stack  to a match context that is used by any number of patterns,
       as long as they are not used for matching by multiple  threads  at  the
       same  time.  For  example, you could use the same stack in all compiled
       patterns, with a global mutex in the callback to wait until  the  stack
       is available for use. However, this is an inefficient solution, and not
       recommended.

       This is a suggestion for how a multithreaded program that needs to  set
       up non-default JIT stacks might operate:

         During thread initalization
           thread_local_var = pcre2_jit_stack_create(...)

         During thread exit
           pcre2_jit_stack_free(thread_local_var)

         Use a one-line callback function
           return thread_local_var

       All  the  functions  described in this section do nothing if JIT is not
       available.


JIT STACK FAQ

       (1) Why do we need JIT stacks?

       PCRE2 (and JIT) is a recursive, depth-first engine, so it needs a stack
       where  the local data of the current node is pushed before checking its
       child nodes.  Allocating real machine stack on some platforms is diffi-
       cult. For example, the stack chain needs to be updated every time if we
       extend the stack on PowerPC.  Although it  is  possible,  its  updating
       time overhead decreases performance. So we do the recursion in memory.

       (2) Why don't we simply allocate blocks of memory with malloc()?

       Modern  operating  systems  have  a  nice  feature: they can reserve an
       address space instead of allocating memory. We can safely allocate mem-
       ory  pages  inside  this address space, so the stack could grow without
       moving memory data (this is important because of pointers). Thus we can
       allocate 1MiB address space, and use only a single memory page (usually
       4KiB) if that is enough. However, we can still grow up to 1MiB  anytime
       if needed.

       (3) Who "owns" a JIT stack?

       The owner of the stack is the user program, not the JIT studied pattern
       or anything else. The user program must ensure that if a stack is being
       used by pcre2_match(), (that is, it is assigned to a match context that
       is passed to the pattern currently running), that  stack  must  not  be
       used  by any other threads (to avoid overwriting the same memory area).
       The best practice for multithreaded programs is to allocate a stack for
       each thread, and return this stack through the JIT callback function.

       (4) When should a JIT stack be freed?

       You can free a JIT stack at any time, as long as it will not be used by
       pcre2_match() again. When you assign the stack to a match context, only
       a  pointer  is  set. There is no reference counting or any other magic.
       You can free compiled patterns, contexts, and stacks in any order, any-
       time.  Just  do not call pcre2_match() with a match context pointing to
       an already freed stack, as that will cause SEGFAULT. (Also, do not free
       a  stack  currently  used  by pcre2_match() in another thread). You can
       also replace the stack in a context at any time when it is not in  use.
       You should free the previous stack before assigning a replacement.

       (5)  Should  I  allocate/free  a  stack every time before/after calling
       pcre2_match()?

       No, because this is too costly in  terms  of  resources.  However,  you
       could  implement  some clever idea which release the stack if it is not
       used in let's say two minutes. The JIT callback  can  help  to  achieve
       this without keeping a list of patterns.

       (6)  OK, the stack is for long term memory allocation. But what happens
       if a pattern causes stack overflow with a stack of 1MiB? Is  that  1MiB
       kept until the stack is freed?

       Especially  on embedded sytems, it might be a good idea to release mem-
       ory sometimes without freeing the stack. There is no API  for  this  at
       the  moment.  Probably a function call which returns with the currently
       allocated memory for any stack and another which allows releasing  mem-
       ory (shrinking the stack) would be a good idea if someone needs this.

       (7) This is too much of a headache. Isn't there any better solution for
       JIT stack handling?

       No, thanks to Windows. If POSIX threads were used everywhere, we  could
       throw out this complicated API.


FREEING JIT SPECULATIVE MEMORY

       void pcre2_jit_free_unused_memory(pcre2_general_context *gcontext);

       The JIT executable allocator does not free all memory when it is possi-
       ble.  It expects new allocations, and keeps some free memory around  to
       improve  allocation  speed. However, in low memory conditions, it might
       be better to free all possible memory. You can cause this to happen  by
       calling  pcre2_jit_free_unused_memory(). Its argument is a general con-
       text, for custom memory management, or NULL for standard memory manage-
       ment.


EXAMPLE CODE

       This  is  a  single-threaded example that specifies a JIT stack without
       using a callback. A real program should include  error  checking  after
       all the function calls.

         int rc;
         pcre2_code *re;
         pcre2_match_data *match_data;
         pcre2_match_context *mcontext;
         pcre2_jit_stack *jit_stack;

         re = pcre2_compile(pattern, PCRE2_ZERO_TERMINATED, 0,
           &errornumber, &erroffset, NULL);
         rc = pcre2_jit_compile(re, PCRE2_JIT_COMPLETE);
         mcontext = pcre2_match_context_create(NULL);
         jit_stack = pcre2_jit_stack_create(32*1024, 512*1024, NULL);
         pcre2_jit_stack_assign(mcontext, NULL, jit_stack);
         match_data = pcre2_match_data_create(re, 10);
         rc = pcre2_match(re, subject, length, 0, 0, match_data, mcontext);
         /* Process result */

         pcre2_code_free(re);
         pcre2_match_data_free(match_data);
         pcre2_match_context_free(mcontext);
         pcre2_jit_stack_free(jit_stack);


JIT FAST PATH API

       Because the API described above falls back to interpreted matching when
       JIT is not available, it is convenient for programs  that  are  written
       for  general  use  in  many  environments.  However,  calling  JIT  via
       pcre2_match() does have a performance impact. Programs that are written
       for  use  where  JIT  is known to be available, and which need the best
       possible performance, can instead use a "fast path"  API  to  call  JIT
       matching  directly instead of calling pcre2_match() (obviously only for
       patterns that have been successfully processed by pcre2_jit_compile()).

       The fast path  function  is  called  pcre2_jit_match(),  and  it  takes
       exactly  the  same  arguments  as  pcre2_match().  However, the subject
       string must be specified with a length;  PCRE2_ZERO_TERMINATED  is  not
       supported.   Unsupported  option  bits  (for  example,  PCRE2_ANCHORED,
       PCRE2_ENDANCHORED and PCRE2_COPY_MATCHED_SUBJECT) are  ignored,  as  is
       the  PCRE2_NO_JIT  option.  The  return values are also the same as for
       pcre2_match(), plus PCRE2_ERROR_JIT_BADOPTION if a matching mode  (par-
       tial or complete) is requested that was not compiled.

       When  you call pcre2_match(), as well as testing for invalid options, a
       number of other sanity checks are performed on the arguments. For exam-
       ple, if the subject pointer is NULL, an immediate error is given. Also,
       unless PCRE2_NO_UTF_CHECK is set, a UTF subject string  is  tested  for
       validity.  In the interests of speed, these checks do not happen on the
       JIT fast path, and if invalid data is passed, the result is undefined.

       Bypassing the sanity checks and the  pcre2_match()  wrapping  can  give
       speedups of more than 10%.


SEE ALSO

       pcre2api(3)


AUTHOR

       Philip Hazel (FAQ by Zoltan Herczeg)
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 16 October 2018
       Copyright (c) 1997-2018 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2LIMITS(3)             Library Functions Manual             PCRE2LIMITS(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SIZE AND OTHER LIMITATIONS

       There are some size limitations in PCRE2 but it is hoped that they will
       never in practice be relevant.

       The maximum size of a compiled pattern  is  approximately  64  thousand
       code units for the 8-bit and 16-bit libraries if PCRE2 is compiled with
       the  default  internal  linkage  size,  which  is  2  bytes  for  these
       libraries.  If  you  want to process regular expressions that are truly
       enormous, you can compile PCRE2 with an internal linkage size of 3 or 4
       (when  building  the  16-bit  library,  3  is rounded up to 4). See the
       README file in the source distribution and the pcre2build documentation
       for  details.  In  these cases the limit is substantially larger.  How-
       ever, the speed of execution is slower.  In  the  32-bit  library,  the
       internal linkage size is always 4.

       The maximum length of a source pattern string is essentially unlimited;
       it is the largest number a PCRE2_SIZE variable can hold.  However,  the
       program that calls pcre2_compile() can specify a smaller limit.

       The maximum length (in code units) of a subject string is one less than
       the largest number a PCRE2_SIZE variable can  hold.  PCRE2_SIZE  is  an
       unsigned  integer  type,  usually  defined as size_t. Its maximum value
       (that is ~(PCRE2_SIZE)0) is reserved as a special indicator  for  zero-
       terminated strings and unset offsets.

       All values in repeating quantifiers must be less than 65536.

       The maximum length of a lookbehind assertion is 65535 characters.

       There  is no limit to the number of parenthesized groups, but there can
       be no more than 65535 capture groups, and there is a limit to the depth
       of  nesting  of parenthesized subpatterns of all kinds. This is imposed
       in order to limit the amount of system stack used at compile time.  The
       default limit can be specified when PCRE2 is built; if not, the default
       is set to  250.  An  application  can  change  this  limit  by  calling
       pcre2_set_parens_nest_limit() to set the limit in a compile context.

       The  maximum length of name for a named capture group is 32 code units,
       and the maximum number of such groups is 10000.

       The maximum length of a  name  in  a  (*MARK),  (*PRUNE),  (*SKIP),  or
       (*THEN)  verb  is  255  code units for the 8-bit library and 65535 code
       units for the 16-bit and 32-bit libraries.

       The maximum length of a string argument to a  callout  is  the  largest
       number a 32-bit unsigned integer can hold.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 02 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2MATCHING(3)           Library Functions Manual           PCRE2MATCHING(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 MATCHING ALGORITHMS

       This document describes the two different algorithms that are available
       in PCRE2 for matching a compiled regular  expression  against  a  given
       subject  string.  The  "standard"  algorithm is the one provided by the
       pcre2_match() function. This works in the same as  as  Perl's  matching
       function,  and  provide a Perl-compatible matching operation. The just-
       in-time (JIT) optimization that is described in the pcre2jit documenta-
       tion is compatible with this function.

       An alternative algorithm is provided by the pcre2_dfa_match() function;
       it operates in a different way, and is not Perl-compatible. This alter-
       native  has  advantages  and  disadvantages  compared with the standard
       algorithm, and these are described below.

       When there is only one possible way in which a given subject string can
       match  a pattern, the two algorithms give the same answer. A difference
       arises, however, when there are multiple possibilities. For example, if
       the pattern

         ^<.*>

       is matched against the string

         <something> <something else> <something further>

       there are three possible answers. The standard algorithm finds only one
       of them, whereas the alternative algorithm finds all three.


REGULAR EXPRESSIONS AS TREES

       The set of strings that are matched by a regular expression can be rep-
       resented  as  a  tree structure. An unlimited repetition in the pattern
       makes the tree of infinite size, but it is still a tree.  Matching  the
       pattern  to a given subject string (from a given starting point) can be
       thought of as a search of the tree.  There are two  ways  to  search  a
       tree:  depth-first  and  breadth-first, and these correspond to the two
       matching algorithms provided by PCRE2.


THE STANDARD MATCHING ALGORITHM

       In the terminology of Jeffrey Friedl's book "Mastering Regular  Expres-
       sions",  the  standard  algorithm  is an "NFA algorithm". It conducts a
       depth-first search of the pattern tree. That is, it  proceeds  along  a
       single path through the tree, checking that the subject matches what is
       required. When there is a mismatch, the algorithm  tries  any  alterna-
       tives  at  the  current point, and if they all fail, it backs up to the
       previous branch point in the  tree,  and  tries  the  next  alternative
       branch  at  that  level.  This often involves backing up (moving to the
       left) in the subject string as well.  The  order  in  which  repetition
       branches  are  tried  is controlled by the greedy or ungreedy nature of
       the quantifier.

       If a leaf node is reached, a matching string has  been  found,  and  at
       that  point the algorithm stops. Thus, if there is more than one possi-
       ble match, this algorithm returns the first one that it finds.  Whether
       this  is the shortest, the longest, or some intermediate length depends
       on the way the greedy and ungreedy repetition quantifiers are specified
       in the pattern.

       Because  it  ends  up  with a single path through the tree, it is rela-
       tively straightforward for this algorithm to keep  track  of  the  sub-
       strings  that  are  matched  by portions of the pattern in parentheses.
       This provides support for capturing parentheses and backreferences.


THE ALTERNATIVE MATCHING ALGORITHM

       This algorithm conducts a breadth-first search of  the  tree.  Starting
       from  the  first  matching  point  in the subject, it scans the subject
       string from left to right, once, character by character, and as it does
       this,  it remembers all the paths through the tree that represent valid
       matches. In Friedl's terminology, this is a kind  of  "DFA  algorithm",
       though  it is not implemented as a traditional finite state machine (it
       keeps multiple states active simultaneously).

       Although the general principle of this matching algorithm  is  that  it
       scans  the subject string only once, without backtracking, there is one
       exception: when a lookaround assertion is encountered,  the  characters
       following  or  preceding  the  current  point  have to be independently
       inspected.

       The scan continues until either the end of the subject is  reached,  or
       there  are  no more unterminated paths. At this point, terminated paths
       represent the different matching possibilities (if there are none,  the
       match  has  failed).   Thus,  if there is more than one possible match,
       this algorithm finds all of them, and in particular, it finds the long-
       est.  The  matches are returned in decreasing order of length. There is
       an option to stop the algorithm after the first match (which is  neces-
       sarily the shortest) is found.

       Note that all the matches that are found start at the same point in the
       subject. If the pattern

         cat(er(pillar)?)?

       is matched against the string "the caterpillar catchment",  the  result
       is  the  three  strings "caterpillar", "cater", and "cat" that start at
       the fifth character of the subject. The algorithm  does  not  automati-
       cally move on to find matches that start at later positions.

       PCRE2's "auto-possessification" optimization usually applies to charac-
       ter repeats at the end of a pattern (as well as internally). For  exam-
       ple, the pattern "a\d+" is compiled as if it were "a\d++" because there
       is no point even considering the possibility of backtracking  into  the
       repeated  digits.  For  DFA matching, this means that only one possible
       match is found. If you really do want multiple matches in  such  cases,
       either  use  an ungreedy repeat ("a\d+?") or set the PCRE2_NO_AUTO_POS-
       SESS option when compiling.

       There are a number of features of PCRE2 regular  expressions  that  are
       not  supported  or behave differently in the alternative matching func-
       tion. Those that are not supported cause an error if encountered.

       1. Because the algorithm finds all  possible  matches,  the  greedy  or
       ungreedy  nature  of  repetition quantifiers is not relevant (though it
       may affect auto-possessification, as just described). During  matching,
       greedy  and  ungreedy  quantifiers are treated in exactly the same way.
       However, possessive quantifiers can make a difference when what follows
       could  also  match  what  is  quantified, for example in a pattern like
       this:

         ^a++\w!

       This pattern matches "aaab!" but not "aaa!", which would be matched  by
       a  non-possessive quantifier. Similarly, if an atomic group is present,
       it is matched as if it were a standalone pattern at the current  point,
       and  the  longest match is then "locked in" for the rest of the overall
       pattern.

       2. When dealing with multiple paths through the tree simultaneously, it
       is  not  straightforward  to  keep track of captured substrings for the
       different matching possibilities, and PCRE2's  implementation  of  this
       algorithm does not attempt to do this. This means that no captured sub-
       strings are available.

       3. Because no substrings are captured, backreferences within  the  pat-
       tern are not supported.

       4.  For  the same reason, conditional expressions that use a backrefer-
       ence as the condition or test for a specific group  recursion  are  not
       supported.

       5. Again for the same reason, script runs are not supported.

       6.  Because  many  paths  through the tree may be active, the \K escape
       sequence, which resets the start of the match when encountered (but may
       be on some paths and not on others), is not supported.

       7.  Callouts  are  supported, but the value of the capture_top field is
       always 1, and the value of the capture_last field is always 0.

       8. The \C escape sequence, which (in  the  standard  algorithm)  always
       matches  a  single  code  unit, even in a UTF mode, is not supported in
       these modes, because the alternative algorithm moves through  the  sub-
       ject  string  one  character  (not code unit) at a time, for all active
       paths through the tree.

       9. Except for (*FAIL), the backtracking control verbs such as  (*PRUNE)
       are  not  supported.  (*FAIL)  is supported, and behaves like a failing
       negative assertion.


ADVANTAGES OF THE ALTERNATIVE ALGORITHM

       Using the alternative matching algorithm provides the following  advan-
       tages:

       1. All possible matches (at a single point in the subject) are automat-
       ically found, and in particular, the longest match is  found.  To  find
       more than one match using the standard algorithm, you have to do kludgy
       things with callouts.

       2. Because the alternative algorithm  scans  the  subject  string  just
       once, and never needs to backtrack (except for lookbehinds), it is pos-
       sible to pass very long subject strings to  the  matching  function  in
       several pieces, checking for partial matching each time. Although it is
       also possible to do multi-segment matching  using  the  standard  algo-
       rithm,  by  retaining  partially matched substrings, it is more compli-
       cated. The pcre2partial documentation gives details of partial matching
       and discusses multi-segment matching.


DISADVANTAGES OF THE ALTERNATIVE ALGORITHM

       The alternative algorithm suffers from a number of disadvantages:

       1.  It  is  substantially  slower  than the standard algorithm. This is
       partly because it has to search for all possible matches, but  is  also
       because it is less susceptible to optimization.

       2.  Capturing parentheses, backreferences, and script runs are not sup-
       ported.

       3. Although atomic groups are supported, their use does not provide the
       performance advantage that it does for the standard algorithm.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 10 October 2018
       Copyright (c) 1997-2018 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2PARTIAL(3)            Library Functions Manual            PCRE2PARTIAL(3)



NAME
       PCRE2 - Perl-compatible regular expressions

PARTIAL MATCHING IN PCRE2

       In  normal  use  of  PCRE2,  if  the subject string that is passed to a
       matching function matches as far as it goes, but is too short to  match
       the  entire pattern, PCRE2_ERROR_NOMATCH is returned. There are circum-
       stances where it might be helpful to distinguish this case  from  other
       cases in which there is no match.

       Consider, for example, an application where a human is required to type
       in data for a field with specific formatting requirements.  An  example
       might be a date in the form ddmmmyy, defined by this pattern:

         ^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$

       If the application sees the user's keystrokes one by one, and can check
       that what has been typed so far is potentially valid,  it  is  able  to
       raise  an  error  as  soon  as  a  mistake  is made, by beeping and not
       reflecting the character that has been typed, for example. This immedi-
       ate  feedback is likely to be a better user interface than a check that
       is delayed until the entire string has been entered.  Partial  matching
       can  also be useful when the subject string is very long and is not all
       available at once.

       PCRE2 supports partial matching by means of the PCRE2_PARTIAL_SOFT  and
       PCRE2_PARTIAL_HARD  options,  which  can be set when calling a matching
       function.  The difference between the two options is whether or  not  a
       partial match is preferred to an alternative complete match, though the
       details differ between the two types  of  matching  function.  If  both
       options are set, PCRE2_PARTIAL_HARD takes precedence.

       If  you  want to use partial matching with just-in-time optimized code,
       you must call pcre2_jit_compile() with one or both of these options:

         PCRE2_JIT_PARTIAL_SOFT
         PCRE2_JIT_PARTIAL_HARD

       PCRE2_JIT_COMPLETE should also be set if you are going to run  non-par-
       tial  matches  on the same pattern. If the appropriate JIT mode has not
       been compiled, interpretive matching code is used.

       Setting a partial matching option  disables  two  of  PCRE2's  standard
       optimizations. PCRE2 remembers the last literal code unit in a pattern,
       and abandons matching immediately if it is not present in  the  subject
       string.  This  optimization  cannot  be  used for a subject string that
       might match only partially. PCRE2 also knows the minimum  length  of  a
       matching  string,  and  does not bother to run the matching function on
       shorter strings. This optimization is also disabled for partial  match-
       ing.


PARTIAL MATCHING USING pcre2_match()

       A  partial  match occurs during a call to pcre2_match() when the end of
       the subject string is reached successfully, but  matching  cannot  con-
       tinue because more characters are needed. However, at least one charac-
       ter in the subject must have been inspected. This  character  need  not
       form part of the final matched string; lookbehind assertions and the \K
       escape sequence provide ways of inspecting characters before the  start
       of  a matched string. The requirement for inspecting at least one char-
       acter exists because an empty string can  always  be  matched;  without
       such  a  restriction  there would always be a partial match of an empty
       string at the end of the subject.

       When a partial match is returned, the first two elements in the ovector
       point to the portion of the subject that was matched, but the values in
       the rest of the ovector are undefined. The appearance of \K in the pat-
       tern has no effect for a partial match. Consider this pattern:

         /abc\K123/

       If it is matched against "456abc123xyz" the result is a complete match,
       and the ovector defines the matched string as "123", because \K  resets
       the  "start  of  match" point. However, if a partial match is requested
       and the subject string is "456abc12", a partial match is found for  the
       string  "abc12",  because  all these characters are needed for a subse-
       quent re-match with additional characters.

       What happens when a partial match is identified depends on which of the
       two partial matching options are set.

   PCRE2_PARTIAL_SOFT WITH pcre2_match()

       If  PCRE2_PARTIAL_SOFT  is  set when pcre2_match() identifies a partial
       match, the partial match is remembered, but matching continues as  nor-
       mal,  and  other  alternatives in the pattern are tried. If no complete
       match  can  be  found,  PCRE2_ERROR_PARTIAL  is  returned  instead   of
       PCRE2_ERROR_NOMATCH.

       This  option  is "soft" because it prefers a complete match over a par-
       tial match.  All the various matching items in a pattern behave  as  if
       the  subject string is potentially complete. For example, \z, \Z, and $
       match at the end of the subject, as normal, and for \b and \B  the  end
       of the subject is treated as a non-alphanumeric.

       If  there  is more than one partial match, the first one that was found
       provides the data that is returned. Consider this pattern:

         /123\w+X|dogY/

       If this is matched against the subject string "abc123dog", both  alter-
       natives  fail  to  match,  but the end of the subject is reached during
       matching, so PCRE2_ERROR_PARTIAL is returned. The offsets are set to  3
       and  9, identifying "123dog" as the first partial match that was found.
       (In this example, there are two partial matches, because "dog"  on  its
       own partially matches the second alternative.)

   PCRE2_PARTIAL_HARD WITH pcre2_match()

       If  PCRE2_PARTIAL_HARD is set for pcre2_match(), PCRE2_ERROR_PARTIAL is
       returned as soon as a partial match is  found,  without  continuing  to
       search  for possible complete matches. This option is "hard" because it
       prefers an earlier partial match over a later complete match. For  this
       reason,  the  assumption  is  made that the end of the supplied subject
       string may not be the true end of the available data, and  so,  if  \z,
       \Z,  \b, \B, or $ are encountered at the end of the subject, the result
       is PCRE2_ERROR_PARTIAL, provided that at least  one  character  in  the
       subject has been inspected.

   Comparing hard and soft partial matching

       The  difference  between the two partial matching options can be illus-
       trated by a pattern such as:

         /dog(sbody)?/

       This matches either "dog" or "dogsbody", greedily (that is, it  prefers
       the  longer  string  if  possible). If it is matched against the string
       "dog" with PCRE2_PARTIAL_SOFT, it yields a complete  match  for  "dog".
       However,  if  PCRE2_PARTIAL_HARD is set, the result is PCRE2_ERROR_PAR-
       TIAL. On the other hand, if the pattern is made ungreedy the result  is
       different:

         /dog(sbody)??/

       In  this  case  the  result  is always a complete match because that is
       found first, and matching never  continues  after  finding  a  complete
       match. It might be easier to follow this explanation by thinking of the
       two patterns like this:

         /dog(sbody)?/    is the same as  /dogsbody|dog/
         /dog(sbody)??/   is the same as  /dog|dogsbody/

       The second pattern will never match "dogsbody", because it will  always
       find the shorter match first.


PARTIAL MATCHING USING pcre2_dfa_match()

       The DFA functions move along the subject string character by character,
       without backtracking, searching for  all  possible  matches  simultane-
       ously.  If the end of the subject is reached before the end of the pat-
       tern, there is the possibility of a partial match, again provided  that
       at least one character has been inspected.

       When PCRE2_PARTIAL_SOFT is set, PCRE2_ERROR_PARTIAL is returned only if
       there have been no complete matches. Otherwise,  the  complete  matches
       are  returned.   However, if PCRE2_PARTIAL_HARD is set, a partial match
       takes precedence over any complete matches. The portion of  the  string
       that was matched when the longest partial match was found is set as the
       first matching string.

       Because the DFA functions always search for all possible  matches,  and
       there  is  no  difference between greedy and ungreedy repetition, their
       behaviour is different from  the  standard  functions  when  PCRE2_PAR-
       TIAL_HARD  is  set.  Consider  the  string  "dog"  matched  against the
       ungreedy pattern shown above:

         /dog(sbody)??/

       Whereas the standard function stops as soon as it  finds  the  complete
       match  for  "dog",  the  DFA  function also finds the partial match for
       "dogsbody", and so returns that when PCRE2_PARTIAL_HARD is set.


PARTIAL MATCHING AND WORD BOUNDARIES

       If a pattern ends with one of sequences \b or \B, which test  for  word
       boundaries,  partial matching with PCRE2_PARTIAL_SOFT can give counter-
       intuitive results. Consider this pattern:

         /\bcat\b/

       This matches "cat", provided there is a word boundary at either end. If
       the subject string is "the cat", the comparison of the final "t" with a
       following character cannot take place, so a  partial  match  is  found.
       However,  normal  matching carries on, and \b matches at the end of the
       subject when the last character is a letter, so  a  complete  match  is
       found.   The  result,  therefore,  is  not  PCRE2_ERROR_PARTIAL.  Using
       PCRE2_PARTIAL_HARD in this case does yield PCRE2_ERROR_PARTIAL, because
       then the partial match takes precedence.


EXAMPLE OF PARTIAL MATCHING USING PCRE2TEST

       If  the  partial_soft  (or  ps) modifier is present on a pcre2test data
       line, the PCRE2_PARTIAL_SOFT option is used for the match.  Here  is  a
       run of pcre2test that uses the date example quoted above:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 25jun04\=ps
          0: 25jun04
          1: jun
         data> 25dec3\=ps
         Partial match: 23dec3
         data> 3ju\=ps
         Partial match: 3ju
         data> 3juj\=ps
         No match
         data> j\=ps
         No match

       The  first  data  string  is matched completely, so pcre2test shows the
       matched substrings. The remaining four strings do not  match  the  com-
       plete pattern, but the first two are partial matches. Similar output is
       obtained if DFA matching is used.

       If the partial_hard (or ph) modifier is present  on  a  pcre2test  data
       line, the PCRE2_PARTIAL_HARD option is set for the match.


MULTI-SEGMENT MATCHING WITH pcre2_dfa_match()

       When  a  partial match has been found using a DFA matching function, it
       is possible to continue the match by providing additional subject  data
       and  calling  the function again with the same compiled regular expres-
       sion, this time setting the PCRE2_DFA_RESTART option. You must pass the
       same working space as before, because this is where details of the pre-
       vious partial match are stored. Here is an example using pcre2test:

           re> /^\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d$/
         data> 23ja\=dfa,ps
         Partial match: 23ja
         data> n05\=dfa,dfa_restart
          0: n05

       The first call has "23ja" as the subject, and requests  partial  match-
       ing;  the  second  call  has  "n05"  as  the  subject for the continued
       (restarted) match.  Notice that when the match is  complete,  only  the
       last  part  is  shown;  PCRE2 does not retain the previously partially-
       matched string. It is up to the calling program to do that if it  needs
       to.

       That means that, for an unanchored pattern, if a continued match fails,
       it is not possible to try again at  a  new  starting  point.  All  this
       facility  is  capable  of  doing  is continuing with the previous match
       attempt. In the previous example, if the second set of data  is  "ug23"
       the  result is no match, even though there would be a match for "aug23"
       if the entire string were given at once. Depending on the  application,
       this may or may not be what you want.  The only way to allow for start-
       ing again at the next character is to retain the matched  part  of  the
       subject and try a new complete match.

       You  can  set the PCRE2_PARTIAL_SOFT or PCRE2_PARTIAL_HARD options with
       PCRE2_DFA_RESTART to continue partial matching over multiple  segments.
       This  facility can be used to pass very long subject strings to the DFA
       matching functions.


MULTI-SEGMENT MATCHING WITH pcre2_match()

       Unlike the DFA function, it is not possible  to  restart  the  previous
       match with a new segment of data when using pcre2_match(). Instead, new
       data must be added to the previous subject string, and the entire match
       re-run,  starting from the point where the partial match occurred. Ear-
       lier data can be discarded.

       It is best to use PCRE2_PARTIAL_HARD in this situation, because it does
       not  treat the end of a segment as the end of the subject when matching
       \z, \Z, \b, \B, and $. Consider  an  unanchored  pattern  that  matches
       dates:

           re> /\d?\d(jan|feb|mar|apr|may|jun|jul|aug|sep|oct|nov|dec)\d\d/
         data> The date is 23ja\=ph
         Partial match: 23ja

       At  this stage, an application could discard the text preceding "23ja",
       add on text from the next  segment,  and  call  the  matching  function
       again.  Unlike  the  DFA  matching function, the entire matching string
       must always be available, and the complete matching process occurs  for
       each call, so more memory and more processing time is needed.


ISSUES WITH MULTI-SEGMENT MATCHING

       Certain types of pattern may give problems with multi-segment matching,
       whichever matching function is used.

       1. If the pattern contains a test for the beginning of a line, you need
       to  pass  the  PCRE2_NOTBOL option when the subject string for any call
       does start at the beginning of a line. There  is  also  a  PCRE2_NOTEOL
       option, but in practice when doing multi-segment matching you should be
       using PCRE2_PARTIAL_HARD, which includes the effect of PCRE2_NOTEOL.

       2. If a pattern contains a lookbehind assertion, characters  that  pre-
       cede  the start of the partial match may have been inspected during the
       matching process.  When using pcre2_match(), sufficient characters must
       be  retained  for  the  next  match attempt. You can ensure that enough
       characters are retained by doing the following:

       Before doing any matching, find the length of the longest lookbehind in
       the     pattern    by    calling    pcre2_pattern_info()    with    the
       PCRE2_INFO_MAXLOOKBEHIND option. Note that the resulting  count  is  in
       characters, not code units. After a partial match, moving back from the
       ovector[0] offset in the subject by the number of characters given  for
       the  maximum lookbehind gets you to the earliest character that must be
       retained. In a non-UTF or a 32-bit situation, moving  back  is  just  a
       subtraction,  but in UTF-8 or UTF-16 you have to count characters while
       moving back through the code units.

       Characters before the point you have now reached can be discarded,  and
       after  the  next segment has been added to what is retained, you should
       run the next match with the startoffset argument set so that the  match
       begins at the same point as before.

       For  example, if the pattern "(?<=123)abc" is partially matched against
       the string "xx123ab", the ovector offsets are 5 and 7 ("ab"). The maxi-
       mum  lookbehind  count  is  3, so all characters before offset 2 can be
       discarded. The value of startoffset for the next  match  should  be  3.
       When  pcre2test  displays  a partial match, it indicates the lookbehind
       characters with '<' characters:

           re> "(?<=123)abc"
         data> xx123ab\=ph
         Partial match: 123ab
                        <<<

       3. Because a partial match must always contain at least one  character,
       what  might  be  considered a partial match of an empty string actually
       gives a "no match" result. For example:

           re> /c(?<=abc)x/
         data> ab\=ps
         No match

       If the next segment begins "cx", a match should be found, but this will
       only  happen  if characters from the previous segment are retained. For
       this reason, a "no match" result  should  be  interpreted  as  "partial
       match of an empty string" when the pattern contains lookbehinds.

       4.  Matching  a subject string that is split into multiple segments may
       not always produce exactly the same result as matching over one  single
       long  string,  especially  when PCRE2_PARTIAL_SOFT is used. The section
       "Partial Matching and Word Boundaries" above describes  an  issue  that
       arises  if  the  pattern ends with \b or \B. Another kind of difference
       may occur when there are multiple matching possibilities, because  (for
       PCRE2_PARTIAL_SOFT) a partial match result is given only when there are
       no completed matches. This means that as soon as the shortest match has
       been  found,  continuation to a new subject segment is no longer possi-
       ble. Consider this pcre2test example:

           re> /dog(sbody)?/
         data> dogsb\=ps
          0: dog
         data> do\=ps,dfa
         Partial match: do
         data> gsb\=ps,dfa,dfa_restart
          0: g
         data> dogsbody\=dfa
          0: dogsbody
          1: dog

       The first data line passes the string "dogsb" to  a  standard  matching
       function, setting the PCRE2_PARTIAL_SOFT option. Although the string is
       a partial match for "dogsbody", the result is not  PCRE2_ERROR_PARTIAL,
       because  the  shorter string "dog" is a complete match. Similarly, when
       the subject is presented to a DFA matching function  in  several  parts
       ("do"  and  "gsb"  being  the first two) the match stops when "dog" has
       been found, and it is not possible to continue.  On the other hand,  if
       "dogsbody"  is  presented  as  a single string, a DFA matching function
       finds both matches.

       Because of these problems, it is best to  use  PCRE2_PARTIAL_HARD  when
       matching  multi-segment  data.  The  example above then behaves differ-
       ently:

           re> /dog(sbody)?/
         data> dogsb\=ph
         Partial match: dogsb
         data> do\=ps,dfa
         Partial match: do
         data> gsb\=ph,dfa,dfa_restart
         Partial match: gsb

       5. Patterns that contain alternatives at the top level which do not all
       start  with  the  same  pattern  item  may  not  work  as expected when
       PCRE2_DFA_RESTART is used. For example, consider this pattern:

         1234|3789

       If the first part of the subject is "ABC123", a partial  match  of  the
       first  alternative  is found at offset 3. There is no partial match for
       the second alternative, because such a match does not start at the same
       point  in  the  subject  string. Attempting to continue with the string
       "7890" does not yield a match  because  only  those  alternatives  that
       match  at  one  point in the subject are remembered. The problem arises
       because the start of the second alternative matches  within  the  first
       alternative.  There  is  no  problem with anchored patterns or patterns
       such as:

         1234|ABCD

       where no string can be a partial match for both alternatives.  This  is
       not  a  problem  if  a  standard matching function is used, because the
       entire match has to be rerun each time:

           re> /1234|3789/
         data> ABC123\=ph
         Partial match: 123
         data> 1237890
          0: 3789

       Of course, instead of using PCRE2_DFA_RESTART, the  same  technique  of
       re-running  the  entire  match  can  also be used with the DFA matching
       function. Another possibility is to work with two buffers. If a partial
       match  at  offset  n in the first buffer is followed by "no match" when
       PCRE2_DFA_RESTART is used on the second buffer, you can then try a  new
       match starting at offset n+1 in the first buffer.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 22 December 2014
       Copyright (c) 1997-2014 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2PATTERN(3)            Library Functions Manual            PCRE2PATTERN(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 REGULAR EXPRESSION DETAILS

       The  syntax and semantics of the regular expressions that are supported
       by PCRE2 are described in detail below. There is a quick-reference syn-
       tax  summary  in the pcre2syntax page. PCRE2 tries to match Perl syntax
       and semantics as closely as it can.  PCRE2 also supports some  alterna-
       tive  regular  expression syntax (which does not conflict with the Perl
       syntax) in order to provide some compatibility with regular expressions
       in Python, .NET, and Oniguruma.

       Perl's  regular expressions are described in its own documentation, and
       regular expressions in general are covered in a number of  books,  some
       of  which  have  copious  examples. Jeffrey Friedl's "Mastering Regular
       Expressions", published by  O'Reilly,  covers  regular  expressions  in
       great  detail.  This  description  of  PCRE2's  regular  expressions is
       intended as reference material.

       This document discusses the regular expression patterns that  are  sup-
       ported  by  PCRE2  when  its  main matching function, pcre2_match(), is
       used.   PCRE2   also   has   an    alternative    matching    function,
       pcre2_dfa_match(),  which  matches  using a different algorithm that is
       not Perl-compatible. Some of  the  features  discussed  below  are  not
       available  when  DFA matching is used. The advantages and disadvantages
       of the alternative function, and how it differs from the  normal  func-
       tion, are discussed in the pcre2matching page.


SPECIAL START-OF-PATTERN ITEMS

       A  number  of options that can be passed to pcre2_compile() can also be
       set by special items at the start of a pattern. These are not Perl-com-
       patible,  but  are provided to make these options accessible to pattern
       writers who are not able to change the program that processes the  pat-
       tern.  Any  number  of  these  items  may  appear, but they must all be
       together right at the start of the pattern string, and the letters must
       be in upper case.

   UTF support

       In the 8-bit and 16-bit PCRE2 libraries, characters may be coded either
       as single code units, or as multiple UTF-8 or UTF-16 code units. UTF-32
       can  be  specified  for the 32-bit library, in which case it constrains
       the character values to valid  Unicode  code  points.  To  process  UTF
       strings,  PCRE2  must be built to include Unicode support (which is the
       default). When using UTF strings you must  either  call  the  compiling
       function  with the PCRE2_UTF option, or the pattern must start with the
       special sequence (*UTF), which is equivalent to  setting  the  relevant
       option. How setting a UTF mode affects pattern matching is mentioned in
       several places below. There is  also  a  summary  of  features  in  the
       pcre2unicode page.

       Some applications that allow their users to supply patterns may wish to
       restrict  them  to  non-UTF  data  for   security   reasons.   If   the
       PCRE2_NEVER_UTF  option  is  passed  to  pcre2_compile(), (*UTF) is not
       allowed, and its appearance in a pattern causes an error.

   Unicode property support

       Another special sequence that may appear at the start of a  pattern  is
       (*UCP).   This  has the same effect as setting the PCRE2_UCP option: it
       causes sequences such as \d and \w to use Unicode properties to  deter-
       mine character types, instead of recognizing only characters with codes
       less than 256 via a lookup table.

       Some applications that allow their users to supply patterns may wish to
       restrict  them  for  security reasons. If the PCRE2_NEVER_UCP option is
       passed to pcre2_compile(), (*UCP) is not allowed, and its appearance in
       a pattern causes an error.

   Locking out empty string matching

       Starting a pattern with (*NOTEMPTY) or (*NOTEMPTY_ATSTART) has the same
       effect as passing the PCRE2_NOTEMPTY or  PCRE2_NOTEMPTY_ATSTART  option
       to whichever matching function is subsequently called to match the pat-
       tern. These options lock out the  matching  of  empty  strings,  either
       entirely, or only at the start of the subject.

   Disabling auto-possessification

       If  a pattern starts with (*NO_AUTO_POSSESS), it has the same effect as
       setting the PCRE2_NO_AUTO_POSSESS option. This stops PCRE2 from  making
       quantifiers  possessive  when  what  follows  cannot match the repeated
       item. For example, by default a+b is treated as a++b. For more details,
       see the pcre2api documentation.

   Disabling start-up optimizations

       If  a  pattern  starts  with (*NO_START_OPT), it has the same effect as
       setting the PCRE2_NO_START_OPTIMIZE option. This disables several opti-
       mizations  for  quickly  reaching "no match" results. For more details,
       see the pcre2api documentation.

   Disabling automatic anchoring

       If a pattern starts with (*NO_DOTSTAR_ANCHOR), it has the  same  effect
       as  setting the PCRE2_NO_DOTSTAR_ANCHOR option. This disables optimiza-
       tions that apply to patterns whose top-level branches all start with .*
       (match  any  number of arbitrary characters). For more details, see the
       pcre2api documentation.

   Disabling JIT compilation

       If a pattern that starts with (*NO_JIT) is  successfully  compiled,  an
       attempt  by  the  application  to apply the JIT optimization by calling
       pcre2_jit_compile() is ignored.

   Setting match resource limits

       The pcre2_match() function contains a counter that is incremented every
       time it goes round its main loop. The caller of pcre2_match() can set a
       limit on this counter, which therefore limits the amount  of  computing
       resource used for a match. The maximum depth of nested backtracking can
       also be limited; this indirectly restricts the amount  of  heap  memory
       that  is  used,  but there is also an explicit memory limit that can be
       set.

       These facilities are provided to catch runaway matches  that  are  pro-
       voked  by patterns with huge matching trees. A common example is a pat-
       tern with nested unlimited repeats applied to a long string  that  does
       not  match. When one of these limits is reached, pcre2_match() gives an
       error return. The limits can also be set by items at the start  of  the
       pattern of the form

         (*LIMIT_HEAP=d)
         (*LIMIT_MATCH=d)
         (*LIMIT_DEPTH=d)

       where d is any number of decimal digits. However, the value of the set-
       ting must be less than the value set (or defaulted) by  the  caller  of
       pcre2_match()  for  it  to have any effect. In other words, the pattern
       writer can lower the limits set by the programmer, but not raise  them.
       If  there  is  more  than one setting of one of these limits, the lower
       value is used. The heap limit is specified in kibibytes (units of  1024
       bytes).

       Prior  to  release  10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This
       name is still recognized for backwards compatibility.

       The heap limit applies only when the pcre2_match() or pcre2_dfa_match()
       interpreters are used for matching. It does not apply to JIT. The match
       limit is used (but in a different way) when JIT is being used, or  when
       pcre2_dfa_match() is called, to limit computing resource usage by those
       matching functions. The depth limit is ignored by JIT but  is  relevant
       for  DFA  matching, which uses function recursion for recursions within
       the pattern and for lookaround assertions and atomic  groups.  In  this
       case, the depth limit controls the depth of such recursion.

   Newline conventions

       PCRE2  supports six different conventions for indicating line breaks in
       strings: a single CR (carriage return) character, a  single  LF  (line-
       feed) character, the two-character sequence CRLF, any of the three pre-
       ceding, any Unicode newline sequence,  or  the  NUL  character  (binary
       zero).  The  pcre2api  page  has further discussion about newlines, and
       shows how to set the newline convention when calling pcre2_compile().

       It is also possible to specify a newline convention by starting a  pat-
       tern string with one of the following sequences:

         (*CR)        carriage return
         (*LF)        linefeed
         (*CRLF)      carriage return, followed by linefeed
         (*ANYCRLF)   any of the three above
         (*ANY)       all Unicode newline sequences
         (*NUL)       the NUL character (binary zero)

       These override the default and the options given to the compiling func-
       tion. For example, on a Unix system where LF  is  the  default  newline
       sequence, the pattern

         (*CR)a.b

       changes the convention to CR. That pattern matches "a\nb" because LF is
       no longer a newline. If more than one of these settings is present, the
       last one is used.

       The  newline  convention affects where the circumflex and dollar asser-
       tions are true. It also affects the interpretation of the dot metachar-
       acter  when  PCRE2_DOTALL  is not set, and the behaviour of \N when not
       followed by an opening brace. However, it does not affect what  the  \R
       escape  sequence  matches.  By  default,  this  is  any Unicode newline
       sequence, for Perl compatibility. However, this can be changed; see the
       next section and the description of \R in the section entitled "Newline
       sequences" below. A change of \R setting can be combined with a  change
       of newline convention.

   Specifying what \R matches

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE2_BSR_ANYCRLF  at compile time. This effect can also be achieved by
       starting a pattern with (*BSR_ANYCRLF).  For  completeness,  (*BSR_UNI-
       CODE) is also recognized, corresponding to PCRE2_BSR_UNICODE.


EBCDIC CHARACTER CODES

       PCRE2  can be compiled to run in an environment that uses EBCDIC as its
       character code instead of ASCII or Unicode (typically a mainframe  sys-
       tem).  In  the  sections below, character code values are ASCII or Uni-
       code; in an EBCDIC environment these characters may have different code
       values, and there are no code points greater than 255.


CHARACTERS AND METACHARACTERS

       A  regular  expression  is  a pattern that is matched against a subject
       string from left to right. Most characters stand for  themselves  in  a
       pattern,  and  match  the corresponding characters in the subject. As a
       trivial example, the pattern

         The quick brown fox

       matches a portion of a subject string that is identical to itself. When
       caseless matching is specified (the PCRE2_CASELESS option), letters are
       matched independently of case.

       The power of regular expressions comes from the ability to include wild
       cards, character classes, alternatives, and repetitions in the pattern.
       These are encoded in the pattern by the use of metacharacters, which do
       not  stand  for  themselves but instead are interpreted in some special
       way.

       There are two different sets of metacharacters: those that  are  recog-
       nized  anywhere in the pattern except within square brackets, and those
       that are recognized within square brackets.  Outside  square  brackets,
       the metacharacters are as follows:

         \      general escape character with several uses
         ^      assert start of string (or line, in multiline mode)
         $      assert end of string (or line, in multiline mode)
         .      match any character except newline (by default)
         [      start character class definition
         |      start of alternative branch
         (      start group or control verb
         )      end group or control verb
         *      0 or more quantifier
         +      1 or more quantifier; also "possessive quantifier"
         ?      0 or 1 quantifier; also quantifier minimizer
         {      start min/max quantifier

       Part  of  a  pattern  that is in square brackets is called a "character
       class". In a character class the only metacharacters are:

         \      general escape character
         ^      negate the class, but only if the first character
         -      indicates character range
         [      POSIX character class (if followed by POSIX syntax)
         ]      terminates the character class

       The following sections describe the use of each of the metacharacters.


BACKSLASH

       The backslash character has several uses. Firstly, if it is followed by
       a  character that is not a digit or a letter, it takes away any special
       meaning that character may have. This use of  backslash  as  an  escape
       character applies both inside and outside character classes.

       For  example,  if you want to match a * character, you must write \* in
       the pattern. This escaping action applies whether or not the  following
       character  would  otherwise be interpreted as a metacharacter, so it is
       always safe to precede a non-alphanumeric  with  backslash  to  specify
       that it stands for itself.  In particular, if you want to match a back-
       slash, you write \\.

       In a UTF mode, only ASCII digits and letters have any  special  meaning
       after  a  backslash.  All  other characters (in particular, those whose
       code points are greater than 127) are treated as literals.

       If a pattern is compiled with the  PCRE2_EXTENDED  option,  most  white
       space  in the pattern (other than in a character class), and characters
       between a # outside a character class and the next newline,  inclusive,
       are ignored. An escaping backslash can be used to include a white space
       or # character as part of the pattern.

       If you want to treat all characters in a sequence as literals, you  can
       do so by putting them between \Q and \E. This is different from Perl in
       that $ and @ are handled as literals in  \Q...\E  sequences  in  PCRE2,
       whereas  in Perl, $ and @ cause variable interpolation. Also, Perl does
       "double-quotish backslash interpolation" on any backslashes between  \Q
       and  \E which, its documentation says, "may lead to confusing results".
       PCRE2 treats a backslash between \Q and \E just like any other  charac-
       ter. Note the following examples:

         Pattern            PCRE2 matches   Perl matches

         \Qabc$xyz\E        abc$xyz        abc followed by the
                                             contents of $xyz
         \Qabc\$xyz\E       abc\$xyz       abc\$xyz
         \Qabc\E\$\Qxyz\E   abc$xyz        abc$xyz
         \QA\B\E            A\B            A\B
         \Q\\E              \              \\E

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.  An isolated \E that is not preceded by \Q is ignored.  If  \Q
       is  not followed by \E later in the pattern, the literal interpretation
       continues to the end of the pattern (that is,  \E  is  assumed  at  the
       end).  If  the  isolated \Q is inside a character class, this causes an
       error, because the character class  is  not  terminated  by  a  closing
       square bracket.

   Non-printing characters

       A second use of backslash provides a way of encoding non-printing char-
       acters in patterns in a visible manner. There is no restriction on  the
       appearance  of non-printing characters in a pattern, but when a pattern
       is being prepared by text editing, it is often easier to use one of the
       following  escape  sequences  instead of the binary character it repre-
       sents. In an ASCII or Unicode environment, these escapes  are  as  fol-
       lows:

         \a          alarm, that is, the BEL character (hex 07)
         \cx         "control-x", where x is any printable ASCII character
         \e          escape (hex 1B)
         \f          form feed (hex 0C)
         \n          linefeed (hex 0A)
         \r          carriage return (hex 0D) (but see below)
         \t          tab (hex 09)
         \0dd        character with octal code 0dd
         \ddd        character with octal code ddd, or backreference
         \o{ddd..}   character with octal code ddd..
         \xhh        character with hex code hh
         \x{hhh..}   character with hex code hhh..
         \N{U+hhh..} character with Unicode hex code point hhh..

       By  default, after \x that is not followed by {, from zero to two hexa-
       decimal digits are read (letters can be in upper or  lower  case).  Any
       number of hexadecimal digits may appear between \x{ and }. If a charac-
       ter other than a hexadecimal digit appears between \x{  and  },  or  if
       there is no terminating }, an error occurs.

       Characters whose code points are less than 256 can be defined by either
       of the two syntaxes for \x or by an octal sequence. There is no differ-
       ence in the way they are handled. For example, \xdc is exactly the same
       as \x{dc} or \334.  However, using the braced versions does  make  such
       sequences easier to read.

       Support  is  available  for  some  ECMAScript  (aka  JavaScript) escape
       sequences via two compile-time options. If PCRE2_ALT_BSUX is  set,  the
       sequence  \x followed by { is not recognized. Only if \x is followed by
       two hexadecimal digits is it recognized as a character  escape.  Other-
       wise  it  is interpreted as a literal "x" character. In this mode, sup-
       port for code points greater than 256 is provided by \u, which must  be
       followed  by  four hexadecimal digits; otherwise it is interpreted as a
       literal "u" character.

       PCRE2_EXTRA_ALT_BSUX has the same  effect  as  PCRE2_ALT_BSUX  and,  in
       addition,  \u{hhh..}  is recognized as the character specified by hexa-
       decimal code point.  There may be any  number  of  hexadecimal  digits.
       This syntax is from ECMAScript 6.

       The  \N{U+hhh..}  escape sequence is recognized only when the PCRE2_UTF
       option is set, that is, when PCRE2 is operating in a Unicode mode. Perl
       also  uses  \N{name}  to specify characters by Unicode name; PCRE2 does
       not support this.  Note that when \N is  not  followed  by  an  opening
       brace  (curly  bracket)  it has an entirely different meaning, matching
       any character that is not a newline.

       There are some legacy applications where  the  escape  sequence  \r  is
       expected to match a newline. If the PCRE2_EXTRA_ESCAPED_CR_IS_LF option
       is set, \r in a pattern is converted to \n so  that  it  matches  a  LF
       (linefeed) instead of a CR (carriage return) character.

       The  precise effect of \cx on ASCII characters is as follows: if x is a
       lower case letter, it is converted to upper case. Then  bit  6  of  the
       character (hex 40) is inverted. Thus \cA to \cZ become hex 01 to hex 1A
       (A is 41, Z is 5A), but \c{ becomes hex 3B ({ is 7B), and  \c;  becomes
       hex  7B  (; is 3B). If the code unit following \c has a value less than
       32 or greater than 126, a compile-time error occurs.

       When PCRE2 is compiled in EBCDIC mode, \N{U+hhh..}  is  not  supported.
       \a, \e, \f, \n, \r, and \t generate the appropriate EBCDIC code values.
       The \c escape is processed as specified for Perl in the perlebcdic doc-
       ument.  The  only characters that are allowed after \c are A-Z, a-z, or
       one of @, [, \, ], ^, _, or ?. Any other character provokes a  compile-
       time  error.  The  sequence  \c@ encodes character code 0; after \c the
       letters (in either case) encode characters 1-26 (hex 01 to hex 1A);  [,
       \,  ],  ^,  and  _  encode characters 27-31 (hex 1B to hex 1F), and \c?
       becomes either 255 (hex FF) or 95 (hex 5F).

       Thus, apart from \c?, these escapes generate the  same  character  code
       values  as  they do in an ASCII environment, though the meanings of the
       values mostly differ. For example, \cG always generates code  value  7,
       which is BEL in ASCII but DEL in EBCDIC.

       The  sequence  \c? generates DEL (127, hex 7F) in an ASCII environment,
       but because 127 is not a control character in  EBCDIC,  Perl  makes  it
       generate  the  APC character. Unfortunately, there are several variants
       of EBCDIC. In most of them the APC character has  the  value  255  (hex
       FF),  but  in  the one Perl calls POSIX-BC its value is 95 (hex 5F). If
       certain other characters have POSIX-BC values, PCRE2 makes \c? generate
       95; otherwise it generates 255.

       After  \0  up  to two further octal digits are read. If there are fewer
       than two digits, just  those  that  are  present  are  used.  Thus  the
       sequence \0\x\015 specifies two binary zeros followed by a CR character
       (code value 13). Make sure you supply two digits after the initial zero
       if the pattern character that follows is itself an octal digit.

       The  escape \o must be followed by a sequence of octal digits, enclosed
       in braces. An error occurs if this is not the case. This  escape  is  a
       recent  addition  to Perl; it provides way of specifying character code
       points as octal numbers greater than 0777, and  it  also  allows  octal
       numbers and backreferences to be unambiguously specified.

       For greater clarity and unambiguity, it is best to avoid following \ by
       a digit greater than zero. Instead, use \o{} or \x{} to specify numeri-
       cal character code points, and \g{} to specify backreferences. The fol-
       lowing paragraphs describe the old, ambiguous syntax.

       The handling of a backslash followed by a digit other than 0 is compli-
       cated, and Perl has changed over time, causing PCRE2 also to change.

       Outside a character class, PCRE2 reads the digit and any following dig-
       its as a decimal number. If the number is less than 10, begins with the
       digit  8  or  9,  or  if  there are at least that many previous capture
       groups in the expression, the entire sequence is taken as a  backrefer-
       ence.  A  description  of  how this works is given later, following the
       discussion of parenthesized groups.  Otherwise, up to three octal  dig-
       its are read to form a character code.

       Inside  a character class, PCRE2 handles \8 and \9 as the literal char-
       acters "8" and "9", and otherwise reads up to three octal  digits  fol-
       lowing the backslash, using them to generate a data character. Any sub-
       sequent digits stand for themselves. For example, outside  a  character
       class:

         \040   is another way of writing an ASCII space
         \40    is the same, provided there are fewer than 40
                   previous capture groups
         \7     is always a backreference
         \11    might be a backreference, or another way of
                   writing a tab
         \011   is always a tab
         \0113  is a tab followed by the character "3"
         \113   might be a backreference, otherwise the
                   character with octal code 113
         \377   might be a backreference, otherwise
                   the value 255 (decimal)
         \81    is always a backreference

       Note  that octal values of 100 or greater that are specified using this
       syntax must not be introduced by a leading zero, because no  more  than
       three octal digits are ever read.

   Constraints on character values

       Characters  that  are  specified using octal or hexadecimal numbers are
       limited to certain values, as follows:

         8-bit non-UTF mode    no greater than 0xff
         16-bit non-UTF mode   no greater than 0xffff
         32-bit non-UTF mode   no greater than 0xffffffff
         All UTF modes         no greater than 0x10ffff and a valid code point

       Invalid Unicode code points are all those in the range 0xd800 to 0xdfff
       (the  so-called  "surrogate"  code  points). The check for these can be
       disabled by  the  caller  of  pcre2_compile()  by  setting  the  option
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES.  However, this is possible only in
       UTF-8 and UTF-32 modes, because these values are not  representable  in
       UTF-16.

   Escape sequences in character classes

       All the sequences that define a single character value can be used both
       inside and outside character classes. In addition, inside  a  character
       class, \b is interpreted as the backspace character (hex 08).

       When not followed by an opening brace, \N is not allowed in a character
       class.  \B, \R, and \X are not special inside a character  class.  Like
       other  unrecognized  alphabetic  escape sequences, they cause an error.
       Outside a character class, these sequences have different meanings.

   Unsupported escape sequences

       In Perl, the sequences \F, \l, \L, \u, and \U  are  recognized  by  its
       string  handler and used to modify the case of following characters. By
       default, PCRE2 does not support these  escape  sequences  in  patterns.
       However,  if  either  of  the  PCRE2_ALT_BSUX  or  PCRE2_EXTRA_ALT_BSUX
       options is set, \U matches a "U" character,  and  \u  can  be  used  to
       define a character by code point, as described above.

   Absolute and relative backreferences

       The  sequence  \g  followed  by a signed or unsigned number, optionally
       enclosed in braces, is an absolute or relative backreference.  A  named
       backreference  can  be  coded as \g{name}. Backreferences are discussed
       later, following the discussion of parenthesized groups.

   Absolute and relative subroutine calls

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for referencing a capture group as a  subroutine.
       Details  are  discussed  later.   Note  that  \g{...} (Perl syntax) and
       \g<...> (Oniguruma syntax) are not synonymous. The former is a backref-
       erence; the latter is a subroutine call.

   Generic character types

       Another use of backslash is for specifying generic character types:

         \d     any decimal digit
         \D     any character that is not a decimal digit
         \h     any horizontal white space character
         \H     any character that is not a horizontal white space character
         \N     any character that is not a newline
         \s     any white space character
         \S     any character that is not a white space character
         \v     any vertical white space character
         \V     any character that is not a vertical white space character
         \w     any "word" character
         \W     any "non-word" character

       The  \N  escape  sequence has the same meaning as the "." metacharacter
       when PCRE2_DOTALL is not set, but setting PCRE2_DOTALL does not  change
       the meaning of \N. Note that when \N is followed by an opening brace it
       has a different meaning. See the section entitled "Non-printing charac-
       ters"  above for details. Perl also uses \N{name} to specify characters
       by Unicode name; PCRE2 does not support this.

       Each pair of lower and upper case escape sequences partitions the  com-
       plete  set  of  characters  into two disjoint sets. Any given character
       matches one, and only one, of each pair. The sequences can appear  both
       inside  and outside character classes. They each match one character of
       the appropriate type. If the current matching point is at  the  end  of
       the  subject string, all of them fail, because there is no character to
       match.

       The default \s characters are HT (9), LF (10), VT  (11),  FF  (12),  CR
       (13),  and  space  (32),  which  are  defined as white space in the "C"
       locale. This list may vary if locale-specific matching is taking place.
       For  example, in some locales the "non-breaking space" character (\xA0)
       is recognized as white space, and in others the VT character is not.

       A "word" character is an underscore or any character that is  a  letter
       or  digit.   By  default,  the definition of letters and digits is con-
       trolled by PCRE2's low-valued character tables, and may vary if locale-
       specific matching is taking place (see "Locale support" in the pcre2api
       page). For example, in a French locale such  as  "fr_FR"  in  Unix-like
       systems,  or "french" in Windows, some character codes greater than 127
       are used for accented letters, and these are then matched  by  \w.  The
       use of locales with Unicode is discouraged.

       By  default,  characters  whose  code points are greater than 127 never
       match \d, \s, or \w, and always match \D, \S, and \W, although this may
       be  different  for characters in the range 128-255 when locale-specific
       matching is happening.  These escape sequences  retain  their  original
       meanings  from  before  Unicode support was available, mainly for effi-
       ciency reasons. If the  PCRE2_UCP  option  is  set,  the  behaviour  is
       changed  so  that  Unicode  properties  are used to determine character
       types, as follows:

         \d  any character that matches \p{Nd} (decimal digit)
         \s  any character that matches \p{Z} or \h or \v
         \w  any character that matches \p{L} or \p{N}, plus underscore

       The upper case escapes match the inverse sets of characters. Note  that
       \d  matches  only decimal digits, whereas \w matches any Unicode digit,
       as well as any Unicode letter, and underscore. Note also that PCRE2_UCP
       affects  \b,  and  \B  because  they are defined in terms of \w and \W.
       Matching these sequences is noticeably slower when PCRE2_UCP is set.

       The sequences \h, \H, \v, and \V, in contrast to the  other  sequences,
       which  match  only ASCII characters by default, always match a specific
       list of code points, whether or not PCRE2_UCP is  set.  The  horizontal
       space characters are:

         U+0009     Horizontal tab (HT)
         U+0020     Space
         U+00A0     Non-break space
         U+1680     Ogham space mark
         U+180E     Mongolian vowel separator
         U+2000     En quad
         U+2001     Em quad
         U+2002     En space
         U+2003     Em space
         U+2004     Three-per-em space
         U+2005     Four-per-em space
         U+2006     Six-per-em space
         U+2007     Figure space
         U+2008     Punctuation space
         U+2009     Thin space
         U+200A     Hair space
         U+202F     Narrow no-break space
         U+205F     Medium mathematical space
         U+3000     Ideographic space

       The vertical space characters are:

         U+000A     Linefeed (LF)
         U+000B     Vertical tab (VT)
         U+000C     Form feed (FF)
         U+000D     Carriage return (CR)
         U+0085     Next line (NEL)
         U+2028     Line separator
         U+2029     Paragraph separator

       In  8-bit,  non-UTF-8  mode,  only the characters with code points less
       than 256 are relevant.

   Newline sequences

       Outside a character class, by default, the escape sequence  \R  matches
       any  Unicode newline sequence. In 8-bit non-UTF-8 mode \R is equivalent
       to the following:

         (?>\r\n|\n|\x0b|\f|\r|\x85)

       This is an example of an "atomic group", details  of  which  are  given
       below.  This particular group matches either the two-character sequence
       CR followed by LF, or  one  of  the  single  characters  LF  (linefeed,
       U+000A),  VT  (vertical  tab, U+000B), FF (form feed, U+000C), CR (car-
       riage return, U+000D), or NEL (next line, U+0085). Because this  is  an
       atomic  group,  the  two-character sequence is treated as a single unit
       that cannot be split.

       In other modes, two additional characters whose code points are greater
       than 255 are added: LS (line separator, U+2028) and PS (paragraph sepa-
       rator, U+2029).  Unicode support is not needed for these characters  to
       be recognized.

       It is possible to restrict \R to match only CR, LF, or CRLF (instead of
       the complete set  of  Unicode  line  endings)  by  setting  the  option
       PCRE2_BSR_ANYCRLF  at  compile  time. (BSR is an abbrevation for "back-
       slash R".) This can be made the default when PCRE2 is built; if this is
       the  case,  the other behaviour can be requested via the PCRE2_BSR_UNI-
       CODE option. It is also possible to specify these settings by  starting
       a pattern string with one of the following sequences:

         (*BSR_ANYCRLF)   CR, LF, or CRLF only
         (*BSR_UNICODE)   any Unicode newline sequence

       These override the default and the options given to the compiling func-
       tion.  Note that these special settings, which are not Perl-compatible,
       are  recognized only at the very start of a pattern, and that they must
       be in upper case. If more than one of them is present, the last one  is
       used.  They  can  be  combined with a change of newline convention; for
       example, a pattern can start with:

         (*ANY)(*BSR_ANYCRLF)

       They can also be combined with the (*UTF) or (*UCP) special  sequences.
       Inside  a  character  class,  \R  is  treated as an unrecognized escape
       sequence, and causes an error.

   Unicode character properties

       When PCRE2 is built with Unicode support  (the  default),  three  addi-
       tional  escape sequences that match characters with specific properties
       are available. They can be used in any mode, though in 8-bit and 16-bit
       non-UTF  modes these sequences are of course limited to testing charac-
       ters whose code points are less than U+0100 and U+10000,  respectively.
       In  32-bit non-UTF mode, code points greater than 0x10ffff (the Unicode
       limit) may be encountered. These  are  all  treated  as  being  in  the
       Unknown  script and with an unassigned type. The extra escape sequences
       are:

         \p{xx}   a character with the xx property
         \P{xx}   a character without the xx property
         \X       a Unicode extended grapheme cluster

       The property names represented by xx above are case-sensitive. There is
       support  for Unicode script names, Unicode general category properties,
       "Any", which matches any character (including newline), and  some  spe-
       cial  PCRE2  properties  (described  in  the next section).  Other Perl
       properties such as "InMusicalSymbols" are not supported by PCRE2.  Note
       that  \P{Any}  does  not match any characters, so always causes a match
       failure.

       Sets of Unicode characters are defined as belonging to certain scripts.
       A  character from one of these sets can be matched using a script name.
       For example:

         \p{Greek}
         \P{Han}

       Unassigned characters (and in non-UTF 32-bit mode, characters with code
       points greater than 0x10FFFF) are assigned the "Unknown" script. Others
       that are not part of an identified script are lumped together as  "Com-
       mon". The current list of scripts is:

       Adlam,  Ahom,  Anatolian_Hieroglyphs,  Arabic, Armenian, Avestan, Bali-
       nese, Bamum, Bassa_Vah, Batak, Bengali,  Bhaiksuki,  Bopomofo,  Brahmi,
       Braille,  Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Alba-
       nian, Chakma,  Cham,  Cherokee,  Common,  Coptic,  Cuneiform,  Cypriot,
       Cyrillic,  Deseret,  Devanagari, Dogra, Duployan, Egyptian_Hieroglyphs,
       Elbasan,  Ethiopic,  Georgian,  Glagolitic,  Gothic,  Grantha,   Greek,
       Gujarati,   Gunjala_Gondi,   Gurmukhi,  Han,  Hangul,  Hanifi_Rohingya,
       Hanunoo,  Hatran,  Hebrew,   Hiragana,   Imperial_Aramaic,   Inherited,
       Inscriptional_Pahlavi,  Inscriptional_Parthian,  Javanese, Kaithi, Kan-
       nada, Katakana, Kayah_Li, Kharoshthi, Khmer,  Khojki,  Khudawadi,  Lao,
       Latin,  Lepcha,  Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Maha-
       jani, Makasar, Malayalam, Mandaic, Manichaean, Marchen,  Masaram_Gondi,
       Medefaidrin,     Meetei_Mayek,     Mende_Kikakui,     Meroitic_Cursive,
       Meroitic_Hieroglyphs, Miao, Modi,  Mongolian,  Mro,  Multani,  Myanmar,
       Nabataean,  New_Tai_Lue, Newa, Nko, Nushu, Ogham, Ol_Chiki, Old_Hungar-
       ian, Old_Italic, Old_North_Arabian, Old_Permic,  Old_Persian,  Old_Sog-
       dian,    Old_South_Arabian,    Old_Turkic,   Oriya,   Osage,   Osmanya,
       Pahawh_Hmong,    Palmyrene,    Pau_Cin_Hau,    Phags_Pa,    Phoenician,
       Psalter_Pahlavi,  Rejang,  Runic,  Samaritan, Saurashtra, Sharada, Sha-
       vian, Siddham, SignWriting, Sinhala,  Sogdian,  Sora_Sompeng,  Soyombo,
       Sundanese,  Syloti_Nagri,  Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham,
       Tai_Viet, Takri, Tamil, Tangut, Telugu, Thaana,  Thai,  Tibetan,  Tifi-
       nagh,   Tirhuta,   Ugaritic,   Unknown,   Vai,  Warang_Citi,  Yi,  Zan-
       abazar_Square.

       Each character has exactly one Unicode general category property, spec-
       ified  by a two-letter abbreviation. For compatibility with Perl, nega-
       tion can be specified by including a  circumflex  between  the  opening
       brace  and  the  property  name.  For  example,  \p{^Lu} is the same as
       \P{Lu}.

       If only one letter is specified with \p or \P, it includes all the gen-
       eral  category properties that start with that letter. In this case, in
       the absence of negation, the curly brackets in the escape sequence  are
       optional; these two examples have the same effect:

         \p{L}
         \pL

       The following general category property codes are supported:

         C     Other
         Cc    Control
         Cf    Format
         Cn    Unassigned
         Co    Private use
         Cs    Surrogate

         L     Letter
         Ll    Lower case letter
         Lm    Modifier letter
         Lo    Other letter
         Lt    Title case letter
         Lu    Upper case letter

         M     Mark
         Mc    Spacing mark
         Me    Enclosing mark
         Mn    Non-spacing mark

         N     Number
         Nd    Decimal number
         Nl    Letter number
         No    Other number

         P     Punctuation
         Pc    Connector punctuation
         Pd    Dash punctuation
         Pe    Close punctuation
         Pf    Final punctuation
         Pi    Initial punctuation
         Po    Other punctuation
         Ps    Open punctuation

         S     Symbol
         Sc    Currency symbol
         Sk    Modifier symbol
         Sm    Mathematical symbol
         So    Other symbol

         Z     Separator
         Zl    Line separator
         Zp    Paragraph separator
         Zs    Space separator

       The  special property L& is also supported: it matches a character that
       has the Lu, Ll, or Lt property, in other words, a letter  that  is  not
       classified as a modifier or "other".

       The  Cs  (Surrogate)  property  applies  only  to characters whose code
       points are in the range U+D800 to U+DFFF. These characters are no  dif-
       ferent  to any other character when PCRE2 is not in UTF mode (using the
       16-bit or 32-bit library).  However, they  are  not  valid  in  Unicode
       strings and so cannot be tested by PCRE2 in UTF mode, unless UTF valid-
       ity  checking  has   been   turned   off   (see   the   discussion   of
       PCRE2_NO_UTF_CHECK in the pcre2api page).

       The  long  synonyms  for  property  names  that  Perl supports (such as
       \p{Letter}) are not supported by PCRE2, nor is it permitted  to  prefix
       any of these properties with "Is".

       No character that is in the Unicode table has the Cn (unassigned) prop-
       erty.  Instead, this property is assumed for any code point that is not
       in the Unicode table.

       Specifying  caseless  matching  does not affect these escape sequences.
       For example, \p{Lu} always matches only upper  case  letters.  This  is
       different from the behaviour of current versions of Perl.

       Matching  characters by Unicode property is not fast, because PCRE2 has
       to do a multistage table lookup in order to find  a  character's  prop-
       erty. That is why the traditional escape sequences such as \d and \w do
       not use Unicode properties in PCRE2 by default,  though  you  can  make
       them  do  so by setting the PCRE2_UCP option or by starting the pattern
       with (*UCP).

   Extended grapheme clusters

       The \X escape matches any number of Unicode  characters  that  form  an
       "extended grapheme cluster", and treats the sequence as an atomic group
       (see below).  Unicode supports various kinds of composite character  by
       giving  each  character  a grapheme breaking property, and having rules
       that use these properties to define the boundaries of extended grapheme
       clusters.  The rules are defined in Unicode Standard Annex 29, "Unicode
       Text Segmentation". Unicode 11.0.0 abandoned the use of  some  previous
       properties  that had been used for emojis.  Instead it introduced vari-
       ous emoji-specific properties. PCRE2  uses  only  the  Extended  Picto-
       graphic property.

       \X  always  matches  at least one character. Then it decides whether to
       add additional characters according to the following rules for ending a
       cluster:

       1. End at the end of the subject string.

       2.  Do not end between CR and LF; otherwise end after any control char-
       acter.

       3. Do not break Hangul (a Korean  script)  syllable  sequences.  Hangul
       characters  are of five types: L, V, T, LV, and LVT. An L character may
       be followed by an L, V, LV, or LVT character; an LV or V character  may
       be followed by a V or T character; an LVT or T character may be follwed
       only by a T character.

       4. Do not end before extending  characters  or  spacing  marks  or  the
       "zero-width  joiner"  character.  Characters  with  the "mark" property
       always have the "extend" grapheme breaking property.

       5. Do not end after prepend characters.

       6. Do not break within emoji modifier sequences or emoji zwj sequences.
       That is, do not break between characters with the Extended_Pictographic
       property.  Extend and ZWJ characters are allowed  between  the  charac-
       ters.

       7.  Do  not  break  within  emoji flag sequences. That is, do not break
       between regional indicator (RI) characters if there are an  odd  number
       of RI characters before the break point.

       8. Otherwise, end the cluster.

   PCRE2's additional properties

       As  well as the standard Unicode properties described above, PCRE2 sup-
       ports four more that make it possible  to  convert  traditional  escape
       sequences such as \w and \s to use Unicode properties. PCRE2 uses these
       non-standard, non-Perl properties internally  when  PCRE2_UCP  is  set.
       However, they may also be used explicitly. These properties are:

         Xan   Any alphanumeric character
         Xps   Any POSIX space character
         Xsp   Any Perl space character
         Xwd   Any Perl "word" character

       Xan  matches  characters that have either the L (letter) or the N (num-
       ber) property. Xps matches the characters tab, linefeed, vertical  tab,
       form  feed,  or carriage return, and any other character that has the Z
       (separator) property.  Xsp is the same as Xps;  in  PCRE1  it  used  to
       exclude  vertical  tab,  for  Perl compatibility, but Perl changed. Xwd
       matches the same characters as Xan, plus underscore.

       There is another non-standard property, Xuc, which matches any  charac-
       ter  that  can  be represented by a Universal Character Name in C++ and
       other programming languages. These are the characters $,  @,  `  (grave
       accent),  and  all  characters with Unicode code points greater than or
       equal to U+00A0, except for the surrogates U+D800 to U+DFFF. Note  that
       most  base  (ASCII) characters are excluded. (Universal Character Names
       are of the form \uHHHH or \UHHHHHHHH where H is  a  hexadecimal  digit.
       Note that the Xuc property does not match these sequences but the char-
       acters that they represent.)

   Resetting the match start

       In normal use, the escape sequence \K  causes  any  previously  matched
       characters  not  to  be  included in the final matched sequence that is
       returned. For example, the pattern:

         foo\Kbar

       matches "foobar", but reports that it has matched "bar".  \K  does  not
       interact with anchoring in any way. The pattern:

         ^foo\Kbar

       matches  only  when  the  subject  begins with "foobar" (in single line
       mode), though it again reports the matched string as "bar".  This  fea-
       ture  is similar to a lookbehind assertion (described below).  However,
       in this case, the part of the subject before the real  match  does  not
       have  to be of fixed length, as lookbehind assertions do. The use of \K
       does not interfere with the setting of captured substrings.  For  exam-
       ple, when the pattern

         (foo)\Kbar

       matches "foobar", the first substring is still set to "foo".

       Perl  documents  that  the  use  of  \K  within assertions is "not well
       defined". In PCRE2, \K is acted upon when  it  occurs  inside  positive
       assertions,  but  is  ignored  in negative assertions. Note that when a
       pattern such as (?=ab\K) matches, the reported start of the  match  can
       be  greater  than the end of the match. Using \K in a lookbehind asser-
       tion at the start of a pattern can also lead to odd effects. For  exam-
       ple, consider this pattern:

         (?<=\Kfoo)bar

       If  the  subject  is  "foobar", a call to pcre2_match() with a starting
       offset of 3 succeeds and reports the matching string as "foobar",  that
       is,  the  start  of  the reported match is earlier than where the match
       started.

   Simple assertions

       The final use of backslash is for certain simple assertions. An  asser-
       tion  specifies a condition that has to be met at a particular point in
       a match, without consuming any characters from the subject string.  The
       use  of groups for more complicated assertions is described below.  The
       backslashed assertions are:

         \b     matches at a word boundary
         \B     matches when not at a word boundary
         \A     matches at the start of the subject
         \Z     matches at the end of the subject
                 also matches before a newline at the end of the subject
         \z     matches only at the end of the subject
         \G     matches at the first matching position in the subject

       Inside a character class, \b has a different meaning;  it  matches  the
       backspace  character.  If  any  other  of these assertions appears in a
       character class, an "invalid escape sequence" error is generated.

       A word boundary is a position in the subject string where  the  current
       character  and  the previous character do not both match \w or \W (i.e.
       one matches \w and the other matches \W), or the start or  end  of  the
       string  if  the  first or last character matches \w, respectively. When
       PCRE2 is built with Unicode support, the meanings of \w and \W  can  be
       changed  by  setting  the  PCRE2_UCP option. When this is done, it also
       affects \b and \B. Neither PCRE2 nor Perl  has  a  separate  "start  of
       word"  or "end of word" metasequence. However, whatever follows \b nor-
       mally determines which it is. For example, the fragment \ba matches "a"
       at the start of a word.

       The  \A,  \Z,  and \z assertions differ from the traditional circumflex
       and dollar (described in the next section) in that they only ever match
       at  the  very start and end of the subject string, whatever options are
       set. Thus, they are independent of multiline mode. These  three  asser-
       tions  are  not  affected  by the PCRE2_NOTBOL or PCRE2_NOTEOL options,
       which affect only the behaviour of the circumflex and dollar  metachar-
       acters.  However,  if the startoffset argument of pcre2_match() is non-
       zero, indicating that matching is to start at a point  other  than  the
       beginning  of  the subject, \A can never match.  The difference between
       \Z and \z is that \Z matches before a newline at the end of the  string
       as well as at the very end, whereas \z matches only at the end.

       The  \G assertion is true only when the current matching position is at
       the start point of the matching process, as specified by the  startoff-
       set  argument  of  pcre2_match().  It differs from \A when the value of
       startoffset is non-zero. By calling pcre2_match() multiple  times  with
       appropriate  arguments,  you  can  mimic Perl's /g option, and it is in
       this kind of implementation where \G can be useful.

       Note, however, that PCRE2's implementation of \G,  being  true  at  the
       starting  character  of  the matching process, is subtly different from
       Perl's, which defines it as true at the end of the previous  match.  In
       Perl,  these  can  be  different when the previously matched string was
       empty. Because PCRE2 does just one match at a time, it cannot reproduce
       this behaviour.

       If  all  the alternatives of a pattern begin with \G, the expression is
       anchored to the starting match position, and the "anchored" flag is set
       in the compiled regular expression.


CIRCUMFLEX AND DOLLAR

       The  circumflex  and  dollar  metacharacters are zero-width assertions.
       That is, they test for a particular condition being true  without  con-
       suming any characters from the subject string. These two metacharacters
       are concerned with matching the starts and ends of lines. If  the  new-
       line  convention is set so that only the two-character sequence CRLF is
       recognized as a newline, isolated CR and LF characters are  treated  as
       ordinary data characters, and are not recognized as newlines.

       Outside a character class, in the default matching mode, the circumflex
       character is an assertion that is true only  if  the  current  matching
       point  is  at the start of the subject string. If the startoffset argu-
       ment of pcre2_match() is non-zero, or if PCRE2_NOTBOL is  set,  circum-
       flex  can  never match if the PCRE2_MULTILINE option is unset. Inside a
       character class, circumflex has  an  entirely  different  meaning  (see
       below).

       Circumflex  need  not be the first character of the pattern if a number
       of alternatives are involved, but it should be the first thing in  each
       alternative  in  which  it appears if the pattern is ever to match that
       branch. If all possible alternatives start with a circumflex, that  is,
       if  the  pattern  is constrained to match only at the start of the sub-
       ject, it is said to be an "anchored" pattern.  (There  are  also  other
       constructs that can cause a pattern to be anchored.)

       The  dollar  character is an assertion that is true only if the current
       matching point is at the end of  the  subject  string,  or  immediately
       before  a  newline  at  the  end  of  the  string  (by default), unless
       PCRE2_NOTEOL is set. Note, however, that it does not actually match the
       newline. Dollar need not be the last character of the pattern if a num-
       ber of alternatives are involved, but it should be the last item in any
       branch  in which it appears. Dollar has no special meaning in a charac-
       ter class.

       The meaning of dollar can be changed so that it  matches  only  at  the
       very  end  of the string, by setting the PCRE2_DOLLAR_ENDONLY option at
       compile time. This does not affect the \Z assertion.

       The meanings of the circumflex and dollar metacharacters are changed if
       the  PCRE2_MULTILINE  option  is  set.  When this is the case, a dollar
       character matches before any newlines in the string, as well as at  the
       very  end, and a circumflex matches immediately after internal newlines
       as well as at the start of the subject string. It does not match  after
       a  newline  that ends the string, for compatibility with Perl. However,
       this can be changed by setting the PCRE2_ALT_CIRCUMFLEX option.

       For example, the pattern /^abc$/ matches the subject string  "def\nabc"
       (where  \n  represents a newline) in multiline mode, but not otherwise.
       Consequently, patterns that are anchored in single  line  mode  because
       all  branches  start  with  ^ are not anchored in multiline mode, and a
       match for circumflex is  possible  when  the  startoffset  argument  of
       pcre2_match()  is  non-zero. The PCRE2_DOLLAR_ENDONLY option is ignored
       if PCRE2_MULTILINE is set.

       When the newline convention (see "Newline  conventions"  below)  recog-
       nizes  the two-character sequence CRLF as a newline, this is preferred,
       even if the single characters CR and LF are  also  recognized  as  new-
       lines.  For  example,  if  the newline convention is "any", a multiline
       mode circumflex matches before "xyz" in the string "abc\r\nxyz"  rather
       than  after  CR, even though CR on its own is a valid newline. (It also
       matches at the very start of the string, of course.)

       Note that the sequences \A, \Z, and \z can be used to match  the  start
       and  end of the subject in both modes, and if all branches of a pattern
       start with \A it is always anchored, whether or not PCRE2_MULTILINE  is
       set.


FULL STOP (PERIOD, DOT) AND \N

       Outside a character class, a dot in the pattern matches any one charac-
       ter in the subject string except (by default) a character  that  signi-
       fies the end of a line.

       When  a line ending is defined as a single character, dot never matches
       that character; when the two-character sequence CRLF is used, dot  does
       not  match  CR  if  it  is immediately followed by LF, but otherwise it
       matches all characters (including isolated CRs and LFs). When any  Uni-
       code  line endings are being recognized, dot does not match CR or LF or
       any of the other line ending characters.

       The behaviour of dot with regard to newlines can  be  changed.  If  the
       PCRE2_DOTALL  option  is  set, a dot matches any one character, without
       exception.  If the two-character sequence CRLF is present in  the  sub-
       ject string, it takes two dots to match it.

       The  handling of dot is entirely independent of the handling of circum-
       flex and dollar, the only relationship being  that  they  both  involve
       newlines. Dot has no special meaning in a character class.

       The  escape  sequence  \N when not followed by an opening brace behaves
       like a dot, except that it is not affected by the PCRE2_DOTALL  option.
       In  other words, it matches any character except one that signifies the
       end of a line.

       When \N is followed by an opening brace it has a different meaning. See
       the  section entitled "Non-printing characters" above for details. Perl
       also uses \N{name} to specify characters by Unicode  name;  PCRE2  does
       not support this.


MATCHING A SINGLE CODE UNIT

       Outside  a character class, the escape sequence \C matches any one code
       unit, whether or not a UTF mode is set. In the 8-bit library, one  code
       unit  is  one  byte;  in the 16-bit library it is a 16-bit unit; in the
       32-bit library it is a 32-bit unit. Unlike a  dot,  \C  always  matches
       line-ending  characters.  The  feature  is provided in Perl in order to
       match individual bytes in UTF-8 mode, but it is unclear how it can use-
       fully be used.

       Because  \C  breaks  up characters into individual code units, matching
       one unit with \C in UTF-8 or UTF-16 mode means that  the  rest  of  the
       string  may  start  with  a malformed UTF character. This has undefined
       results, because PCRE2 assumes that it is matching character by charac-
       ter  in  a  valid UTF string (by default it checks the subject string's
       validity at the  start  of  processing  unless  the  PCRE2_NO_UTF_CHECK
       option is used).

       An   application   can   lock   out  the  use  of  \C  by  setting  the
       PCRE2_NEVER_BACKSLASH_C option when compiling a  pattern.  It  is  also
       possible to build PCRE2 with the use of \C permanently disabled.

       PCRE2  does  not allow \C to appear in lookbehind assertions (described
       below) in UTF-8 or UTF-16 modes, because this would make it  impossible
       to  calculate  the  length  of  the lookbehind. Neither the alternative
       matching function pcre2_dfa_match() nor the JIT optimizer support \C in
       these UTF modes.  The former gives a match-time error; the latter fails
       to optimize and so the match is always run using the interpreter.

       In the 32-bit library,  however,  \C  is  always  supported  (when  not
       explicitly  locked  out)  because it always matches a single code unit,
       whether or not UTF-32 is specified.

       In general, the \C escape sequence is best avoided. However, one way of
       using  it  that avoids the problem of malformed UTF-8 or UTF-16 charac-
       ters is to use a lookahead to check the length of the  next  character,
       as  in  this  pattern,  which could be used with a UTF-8 string (ignore
       white space and line breaks):

         (?| (?=[\x00-\x7f])(\C) |
             (?=[\x80-\x{7ff}])(\C)(\C) |
             (?=[\x{800}-\x{ffff}])(\C)(\C)(\C) |
             (?=[\x{10000}-\x{1fffff}])(\C)(\C)(\C)(\C))

       In this example, a group that starts  with  (?|  resets  the  capturing
       parentheses  numbers in each alternative (see "Duplicate Group Numbers"
       below). The assertions at the start of each branch check the next UTF-8
       character  for  values whose encoding uses 1, 2, 3, or 4 bytes, respec-
       tively. The character's individual  bytes  are  then  captured  by  the
       appropriate number of \C groups.


SQUARE BRACKETS AND CHARACTER CLASSES

       An opening square bracket introduces a character class, terminated by a
       closing square bracket. A closing square bracket on its own is not spe-
       cial  by  default.  If a closing square bracket is required as a member
       of the class, it should be the first data character in the class (after
       an  initial  circumflex,  if present) or escaped with a backslash. This
       means that, by default, an empty class cannot be defined.  However,  if
       the  PCRE2_ALLOW_EMPTY_CLASS option is set, a closing square bracket at
       the start does end the (empty) class.

       A character class matches a single character in the subject. A  matched
       character must be in the set of characters defined by the class, unless
       the first character in the class definition is a circumflex,  in  which
       case the subject character must not be in the set defined by the class.
       If a circumflex is actually required as a member of the  class,  ensure
       it is not the first character, or escape it with a backslash.

       For  example, the character class [aeiou] matches any lower case vowel,
       while [^aeiou] matches any character that is not a  lower  case  vowel.
       Note that a circumflex is just a convenient notation for specifying the
       characters that are in the class by enumerating those that are  not.  A
       class  that starts with a circumflex is not an assertion; it still con-
       sumes a character from the subject string, and therefore  it  fails  if
       the current pointer is at the end of the string.

       Characters  in  a class may be specified by their code points using \o,
       \x, or \N{U+hh..} in the usual way. When caseless matching is set,  any
       letters  in a class represent both their upper case and lower case ver-
       sions, so for example, a caseless [aeiou] matches "A" as well  as  "a",
       and  a  caseless [^aeiou] does not match "A", whereas a caseful version
       would.

       Characters that might indicate line breaks are  never  treated  in  any
       special  way  when  matching  character  classes,  whatever line-ending
       sequence is in use,  and  whatever  setting  of  the  PCRE2_DOTALL  and
       PCRE2_MULTILINE  options  is  used. A class such as [^a] always matches
       one of these characters.

       The generic character type escape sequences \d, \D, \h, \H, \p, \P, \s,
       \S,  \v,  \V,  \w,  and \W may appear in a character class, and add the
       characters that they  match  to  the  class.  For  example,  [\dABCDEF]
       matches  any  hexadecimal  digit.  In  UTF  modes, the PCRE2_UCP option
       affects the meanings of \d, \s, \w and their upper case partners,  just
       as  it does when they appear outside a character class, as described in
       the section  entitled  "Generic  character  types"  above.  The  escape
       sequence  \b  has  a  different  meaning  inside  a character class; it
       matches the backspace character. The sequences \B, \R, and \X  are  not
       special  inside  a  character class. Like any other unrecognized escape
       sequences, they cause an error. The same is true for \N when  not  fol-
       lowed by an opening brace.

       The  minus (hyphen) character can be used to specify a range of charac-
       ters in a character  class.  For  example,  [d-m]  matches  any  letter
       between  d  and  m,  inclusive.  If  a minus character is required in a
       class, it must be escaped with a backslash  or  appear  in  a  position
       where  it cannot be interpreted as indicating a range, typically as the
       first or last character in the class, or immediately after a range. For
       example,  [b-d-z] matches letters in the range b to d, a hyphen charac-
       ter, or z.

       Perl treats a hyphen as a literal if it appears before or after a POSIX
       class (see below) or before or after a character type escape such as as
       \d or \H.  However, unless the hyphen is  the  last  character  in  the
       class,  Perl  outputs  a  warning  in its warning mode, as this is most
       likely a user error. As PCRE2 has no facility for warning, an error  is
       given in these cases.

       It is not possible to have the literal character "]" as the end charac-
       ter of a range. A pattern such as [W-]46] is interpreted as a class  of
       two  characters ("W" and "-") followed by a literal string "46]", so it
       would match "W46]" or "-46]". However, if the "]"  is  escaped  with  a
       backslash  it is interpreted as the end of range, so [W-\]46] is inter-
       preted as a class containing a range followed by two other  characters.
       The  octal or hexadecimal representation of "]" can also be used to end
       a range.

       Ranges normally include all code points between the start and end char-
       acters,  inclusive.  They  can  also  be used for code points specified
       numerically, for example [\000-\037]. Ranges can include any characters
       that  are  valid  for  the current mode. In any UTF mode, the so-called
       "surrogate" characters (those whose code points lie between 0xd800  and
       0xdfff  inclusive)  may  not  be  specified  explicitly by default (the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES option disables this  check).  How-
       ever, ranges such as [\x{d7ff}-\x{e000}], which include the surrogates,
       are always permitted.

       There is a special case in EBCDIC environments  for  ranges  whose  end
       points are both specified as literal letters in the same case. For com-
       patibility with Perl, EBCDIC code points within the range that are  not
       letters  are  omitted. For example, [h-k] matches only four characters,
       even though the codes for h and k are 0x88 and 0x92, a range of 11 code
       points.  However,  if  the range is specified numerically, for example,
       [\x88-\x92] or [h-\x92], all code points are included.

       If a range that includes letters is used when caseless matching is set,
       it matches the letters in either case. For example, [W-c] is equivalent
       to [][\\^_`wxyzabc], matched caselessly, and  in  a  non-UTF  mode,  if
       character  tables  for  a French locale are in use, [\xc8-\xcb] matches
       accented E characters in both cases.

       A circumflex can conveniently be used with  the  upper  case  character
       types  to specify a more restricted set of characters than the matching
       lower case type.  For example, the class [^\W_] matches any  letter  or
       digit, but not underscore, whereas [\w] includes underscore. A positive
       character class should be read as "something OR something OR ..." and a
       negative class as "NOT something AND NOT something AND NOT ...".

       The  only  metacharacters  that are recognized in character classes are
       backslash, hyphen (only where it can be  interpreted  as  specifying  a
       range),  circumflex  (only  at the start), opening square bracket (only
       when it can be interpreted as introducing a POSIX class name, or for  a
       special  compatibility  feature  -  see the next two sections), and the
       terminating  closing  square  bracket.  However,  escaping  other  non-
       alphanumeric characters does no harm.


POSIX CHARACTER CLASSES

       Perl supports the POSIX notation for character classes. This uses names
       enclosed by [: and :] within the enclosing square brackets. PCRE2  also
       supports this notation. For example,

         [01[:alpha:]%]

       matches "0", "1", any alphabetic character, or "%". The supported class
       names are:

         alnum    letters and digits
         alpha    letters
         ascii    character codes 0 - 127
         blank    space or tab only
         cntrl    control characters
         digit    decimal digits (same as \d)
         graph    printing characters, excluding space
         lower    lower case letters
         print    printing characters, including space
         punct    printing characters, excluding letters and digits and space
         space    white space (the same as \s from PCRE2 8.34)
         upper    upper case letters
         word     "word" characters (same as \w)
         xdigit   hexadecimal digits

       The default "space" characters are HT (9), LF (10), VT (11),  FF  (12),
       CR  (13),  and space (32). If locale-specific matching is taking place,
       the list of space characters may be different; there may  be  fewer  or
       more of them. "Space" and \s match the same set of characters.

       The  name  "word"  is  a Perl extension, and "blank" is a GNU extension
       from Perl 5.8. Another Perl extension is negation, which  is  indicated
       by a ^ character after the colon. For example,

         [12[:^digit:]]

       matches "1", "2", or any non-digit. PCRE2 (and Perl) also recognize the
       POSIX syntax [.ch.] and [=ch=] where "ch" is a "collating element", but
       these are not supported, and an error is given if they are encountered.

       By default, characters with values greater than 127 do not match any of
       the POSIX character classes, although this may be different for charac-
       ters  in  the range 128-255 when locale-specific matching is happening.
       However, if the PCRE2_UCP option is passed to pcre2_compile(), some  of
       the  classes are changed so that Unicode character properties are used.
       This  is  achieved  by  replacing  certain  POSIX  classes  with  other
       sequences, as follows:

         [:alnum:]  becomes  \p{Xan}
         [:alpha:]  becomes  \p{L}
         [:blank:]  becomes  \h
         [:cntrl:]  becomes  \p{Cc}
         [:digit:]  becomes  \p{Nd}
         [:lower:]  becomes  \p{Ll}
         [:space:]  becomes  \p{Xps}
         [:upper:]  becomes  \p{Lu}
         [:word:]   becomes  \p{Xwd}

       Negated  versions, such as [:^alpha:] use \P instead of \p. Three other
       POSIX classes are handled specially in UCP mode:

       [:graph:] This matches characters that have glyphs that mark  the  page
                 when printed. In Unicode property terms, it matches all char-
                 acters with the L, M, N, P, S, or Cf properties, except for:

                   U+061C           Arabic Letter Mark
                   U+180E           Mongolian Vowel Separator
                   U+2066 - U+2069  Various "isolate"s


       [:print:] This matches the same  characters  as  [:graph:]  plus  space
                 characters  that  are  not controls, that is, characters with
                 the Zs property.

       [:punct:] This matches all characters that have the Unicode P (punctua-
                 tion)  property,  plus those characters with code points less
                 than 256 that have the S (Symbol) property.

       The other POSIX classes are unchanged, and match only  characters  with
       code points less than 256.


COMPATIBILITY FEATURE FOR WORD BOUNDARIES

       In  the POSIX.2 compliant library that was included in 4.4BSD Unix, the
       ugly syntax [[:<:]] and [[:>:]] is used for matching  "start  of  word"
       and "end of word". PCRE2 treats these items as follows:

         [[:<:]]  is converted to  \b(?=\w)
         [[:>:]]  is converted to  \b(?<=\w)

       Only these exact character sequences are recognized. A sequence such as
       [a[:<:]b] provokes error for an unrecognized  POSIX  class  name.  This
       support  is not compatible with Perl. It is provided to help migrations
       from other environments, and is best not used in any new patterns. Note
       that  \b matches at the start and the end of a word (see "Simple asser-
       tions" above), and in a Perl-style pattern the preceding  or  following
       character  normally  shows  which  is  wanted, without the need for the
       assertions that are used above in order to give exactly the  POSIX  be-
       haviour.


VERTICAL BAR

       Vertical  bar characters are used to separate alternative patterns. For
       example, the pattern

         gilbert|sullivan

       matches either "gilbert" or "sullivan". Any number of alternatives  may
       appear,  and  an  empty  alternative  is  permitted (matching the empty
       string). The matching process tries each alternative in turn, from left
       to  right, and the first one that succeeds is used. If the alternatives
       are within a group (defined below), "succeeds" means matching the  rest
       of the main pattern as well as the alternative in the group.


INTERNAL OPTION SETTING

       The  settings  of  the  PCRE2_CASELESS,  PCRE2_MULTILINE, PCRE2_DOTALL,
       PCRE2_EXTENDED, PCRE2_EXTENDED_MORE, and PCRE2_NO_AUTO_CAPTURE  options
       can  be  changed  from  within  the  pattern  by  a sequence of letters
       enclosed between "(?"  and ")". These options are Perl-compatible,  and
       are  described in detail in the pcre2api documentation. The option let-
       ters are:

         i  for PCRE2_CASELESS
         m  for PCRE2_MULTILINE
         n  for PCRE2_NO_AUTO_CAPTURE
         s  for PCRE2_DOTALL
         x  for PCRE2_EXTENDED
         xx for PCRE2_EXTENDED_MORE

       For example, (?im) sets caseless, multiline matching. It is also possi-
       ble  to  unset  these  options by preceding the relevant letters with a
       hyphen, for example (?-im). The two "extended" options are not indepen-
       dent; unsetting either one cancels the effects of both of them.

       A   combined  setting  and  unsetting  such  as  (?im-sx),  which  sets
       PCRE2_CASELESS and PCRE2_MULTILINE  while  unsetting  PCRE2_DOTALL  and
       PCRE2_EXTENDED,  is  also  permitted. Only one hyphen may appear in the
       options string. If a letter appears both before and after  the  hyphen,
       the  option  is unset. An empty options setting "(?)" is allowed. Need-
       less to say, it has no effect.

       If the first character following (? is a circumflex, it causes  all  of
       the  above  options to be unset. Thus, (?^) is equivalent to (?-imnsx).
       Letters may follow the circumflex to  cause  some  options  to  be  re-
       instated, but a hyphen may not appear.

       The  PCRE2-specific  options  PCRE2_DUPNAMES  and PCRE2_UNGREEDY can be
       changed in the same way as the Perl-compatible  options  by  using  the
       characters J and U respectively. However, these are not unset by (?^).

       When  one  of  these  option  changes occurs at top level (that is, not
       inside group parentheses), the change applies to the remainder  of  the
       pattern  that follows. An option change within a group (see below for a
       description of groups) affects only that part of the group that follows
       it, so

         (a(?i)b)c

       matches  abc  and  aBc and no other strings (assuming PCRE2_CASELESS is
       not used).  By this means, options can be made to have  different  set-
       tings in different parts of the pattern. Any changes made in one alter-
       native do carry on into subsequent branches within the same group.  For
       example,

         (a(?i)b|c)

       matches  "ab",  "aB",  "c",  and "C", even though when matching "C" the
       first branch is abandoned before the option setting.  This  is  because
       the  effects  of option settings happen at compile time. There would be
       some very weird behaviour otherwise.

       As a convenient shorthand, if any option settings are required  at  the
       start  of a non-capturing group (see the next section), the option let-
       ters may appear between the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings.

       Note: There are other PCRE2-specific options,  applying  to  the  whole
       pattern,  which  can be set by the application when the compiling func-
       tion is called. In addition, the pattern can  contain  special  leading
       sequences  such  as (*CRLF) to override what the application has set or
       what has been defaulted.  Details are given  in  the  section  entitled
       "Newline sequences" above. There are also the (*UTF) and (*UCP) leading
       sequences that can be used to set UTF and Unicode property modes;  they
       are  equivalent to setting the PCRE2_UTF and PCRE2_UCP options, respec-
       tively. However,  the  application  can  set  the  PCRE2_NEVER_UTF  and
       PCRE2_NEVER_UCP  options,  which  lock  out  the  use of the (*UTF) and
       (*UCP) sequences.


GROUPS

       Groups are delimited by parentheses  (round  brackets),  which  can  be
       nested.  Turning part of a pattern into a group does two things:

       1. It localizes a set of alternatives. For example, the pattern

         cat(aract|erpillar|)

       matches  "cataract",  "caterpillar", or "cat". Without the parentheses,
       it would match "cataract", "erpillar" or an empty string.

       2. It creates a "capture group". This means that, when the  whole  pat-
       tern  matches, the portion of the subject string that matched the group
       is passed back to the caller, separately from the portion that  matched
       the  whole  pattern.   (This  applies  only to the traditional matching
       function; the DFA matching function does not support capturing.)

       Opening parentheses are counted from left to right (starting from 1) to
       obtain  numbers for capture groups. For example, if the string "the red
       king" is matched against the pattern

         the ((red|white) (king|queen))

       the captured substrings are "red king", "red", and "king", and are num-
       bered 1, 2, and 3, respectively.

       The  fact  that  plain  parentheses  fulfil two functions is not always
       helpful.  There are often times when grouping is required without  cap-
       turing.  If an opening parenthesis is followed by a question mark and a
       colon, the group does not do any capturing, and  is  not  counted  when
       computing  the number of any subsequent capture groups. For example, if
       the string "the white queen" is matched against the pattern

         the ((?:red|white) (king|queen))

       the captured substrings are "white queen" and "queen", and are numbered
       1 and 2. The maximum number of capture groups is 65535.

       As  a  convenient shorthand, if any option settings are required at the
       start of a non-capturing group, the option letters may  appear  between
       the "?" and the ":". Thus the two patterns

         (?i:saturday|sunday)
         (?:(?i)saturday|sunday)

       match exactly the same set of strings. Because alternative branches are
       tried from left to right, and options are not reset until  the  end  of
       the  group is reached, an option setting in one branch does affect sub-
       sequent branches, so the above patterns match "SUNDAY" as well as "Sat-
       urday".


DUPLICATE GROUP NUMBERS

       Perl 5.10 introduced a feature whereby each alternative in a group uses
       the same numbers for its capturing parentheses.  Such  a  group  starts
       with  (?|  and  is  itself a non-capturing group. For example, consider
       this pattern:

         (?|(Sat)ur|(Sun))day

       Because the two alternatives are inside a (?| group, both sets of  cap-
       turing  parentheses  are  numbered one. Thus, when the pattern matches,
       you can look at captured substring number  one,  whichever  alternative
       matched.  This  construct  is useful when you want to capture part, but
       not all, of one of a number of alternatives. Inside a (?| group, paren-
       theses  are  numbered as usual, but the number is reset at the start of
       each branch. The numbers of any capturing parentheses that  follow  the
       whole group start after the highest number used in any branch. The fol-
       lowing example is taken from the Perl documentation. The numbers under-
       neath show in which buffer the captured content will be stored.

         # before  ---------------branch-reset----------- after
         / ( a )  (?| x ( y ) z | (p (q) r) | (t) u (v) ) ( z ) /x
         # 1            2         2  3        2     3     4

       A  backreference  to a capture group uses the most recent value that is
       set for the group. The following pattern matches "abcabc" or "defdef":

         /(?|(abc)|(def))\1/

       In contrast, a subroutine call to a capture group always refers to  the
       first  one  in the pattern with the given number. The following pattern
       matches "abcabc" or "defabc":

         /(?|(abc)|(def))(?1)/

       A relative reference such as (?-1) is no different: it is just a conve-
       nient way of computing an absolute group number.

       If a condition test for a group's having matched refers to a non-unique
       number, the test is true if any group with that number has matched.

       An alternative approach to using this "branch reset" feature is to  use
       duplicate named groups, as described in the next section.


NAMED CAPTURE GROUPS

       Identifying capture groups by number is simple, but it can be very hard
       to keep track of the numbers in complicated patterns.  Furthermore,  if
       an  expression  is  modified, the numbers may change. To help with this
       difficulty, PCRE2 supports the naming of capture groups.  This  feature
       was  not  added to Perl until release 5.10. Python had the feature ear-
       lier, and PCRE1 introduced it at release 4.0, using the Python  syntax.
       PCRE2 supports both the Perl and the Python syntax.

       In  PCRE2,  a  capture  group  can  be  named  in  one  of  three ways:
       (?<name>...) or (?'name'...) as in Perl, or (?P<name>...) as in Python.
       Names  may be up to 32 code units long. When PCRE2_UTF is not set, they
       may contain only ASCII alphanumeric  characters  and  underscores,  but
       must start with a non-digit. When PCRE2_UTF is set, the syntax of group
       names is extended to allow any Unicode letter or Unicode decimal digit.
       In other words, group names must match one of these patterns:

         ^[_A-Za-z][_A-Za-z0-9]*\z   when PCRE2_UTF is not set
         ^[_\p{L}][_\p{L}\p{Nd}]*\z  when PCRE2_UTF is set

       References  to  capture groups from other parts of the pattern, such as
       backreferences, recursion, and conditions, can all be made by  name  as
       well as by number.

       Named capture groups are allocated numbers as well as names, exactly as
       if the names were not present. In both PCRE2 and Perl,  capture  groups
       are  primarily  identified  by  numbers; any names are just aliases for
       these numbers. The PCRE2 API provides function calls for extracting the
       complete  name-to-number  translation table from a compiled pattern, as
       well as convenience functions for  extracting  captured  substrings  by
       name.

       Warning:  When  more  than  one  capture  group has the same number, as
       described in the previous section, a name given to one of them  applies
       to all of them. Perl allows identically numbered groups to have differ-
       ent names.  Consider this pattern, where there are two capture  groups,
       both numbered 1:

         (?|(?<AA>aa)|(?<BB>bb))

       Perl  allows  this,  with  both  names AA and BB as aliases of group 1.
       Thus, after a successful match, both names yield the same value (either
       "aa" or "bb").

       In  an attempt to reduce confusion, PCRE2 does not allow the same group
       number to be associated with more than one name. The example above pro-
       vokes  a  compile-time  error. However, there is still scope for confu-
       sion. Consider this pattern:

         (?|(?<AA>aa)|(bb))

       Although the second group number 1 is not explicitly named, the name AA
       is  still an alias for any group 1. Whether the pattern matches "aa" or
       "bb", a reference by name to group AA yields the matched string.

       By default, a name must be unique within a pattern, except that  dupli-
       cate names are permitted for groups with the same number, for example:

         (?|(?<AA>aa)|(?<AA>bb))

       The duplicate name constraint can be disabled by setting the PCRE2_DUP-
       NAMES option at compile time, or by the use of (?J) within the pattern.
       Duplicate  names  can be useful for patterns where only one instance of
       the named capture group can match. Suppose you want to match  the  name
       of  a  weekday,  either as a 3-letter abbreviation or as the full name,
       and in both cases you want to extract the  abbreviation.  This  pattern
       (ignoring the line breaks) does the job:

         (?<DN>Mon|Fri|Sun)(?:day)?|
         (?<DN>Tue)(?:sday)?|
         (?<DN>Wed)(?:nesday)?|
         (?<DN>Thu)(?:rsday)?|
         (?<DN>Sat)(?:urday)?

       There  are five capture groups, but only one is ever set after a match.
       The convenience functions for extracting the data by name  returns  the
       substring  for  the first (and in this example, the only) group of that
       name that matched. This saves searching to find which numbered group it
       was.  (An  alternative  way of solving this problem is to use a "branch
       reset" group, as described in the previous section.)

       If you make a backreference to a non-unique named group from  elsewhere
       in  the pattern, the groups to which the name refers are checked in the
       order in which they appear in the overall pattern. The first  one  that
       is  set  is  used  for the reference. For example, this pattern matches
       both "foofoo" and "barbar" but not "foobar" or "barfoo":

         (?:(?<n>foo)|(?<n>bar))\k<n>


       If you make a subroutine call to a non-unique named group, the one that
       corresponds to the first occurrence of the name is used. In the absence
       of duplicate numbers this is the one with the lowest number.

       If you use a named reference in a condition test (see the section about
       conditions below), either to check whether a capture group has matched,
       or to check for recursion, all groups with the same name are tested. If
       the  condition  is  true  for any one of them, the overall condition is
       true. This is the same behaviour as  testing  by  number.  For  further
       details  of  the  interfaces for handling named capture groups, see the
       pcre2api documentation.


REPETITION

       Repetition is specified by quantifiers, which can  follow  any  of  the
       following items:

         a literal data character
         the dot metacharacter
         the \C escape sequence
         the \R escape sequence
         the \X escape sequence
         an escape such as \d or \pL that matches a single character
         a character class
         a backreference
         a parenthesized group (including most assertions)
         a subroutine call (recursive or otherwise)

       The  general repetition quantifier specifies a minimum and maximum num-
       ber of permitted matches, by giving the two numbers in  curly  brackets
       (braces),  separated  by  a comma. The numbers must be less than 65536,
       and the first must be less than or equal to the second. For example,

         z{2,4}

       matches "zz", "zzz", or "zzzz". A closing brace on its  own  is  not  a
       special  character.  If  the second number is omitted, but the comma is
       present, there is no upper limit; if the second number  and  the  comma
       are  both omitted, the quantifier specifies an exact number of required
       matches. Thus

         [aeiou]{3,}

       matches at least 3 successive vowels, but may match many more, whereas

         \d{8}

       matches exactly 8 digits. An opening curly bracket that  appears  in  a
       position  where a quantifier is not allowed, or one that does not match
       the syntax of a quantifier, is taken as a literal character. For  exam-
       ple, {,6} is not a quantifier, but a literal string of four characters.

       In UTF modes, quantifiers apply to characters rather than to individual
       code units. Thus, for example, \x{100}{2} matches two characters,  each
       of which is represented by a two-byte sequence in a UTF-8 string. Simi-
       larly, \X{3} matches three Unicode extended grapheme clusters, each  of
       which  may  be  several  code  units long (and they may be of different
       lengths).

       The quantifier {0} is permitted, causing the expression to behave as if
       the previous item and the quantifier were not present. This may be use-
       ful for capture groups that are referenced as  subroutines  from  else-
       where  in the pattern (but see also the section entitled "Defining cap-
       ture groups for use by reference only" below). Except for parenthesized
       groups,  items that have a {0} quantifier are omitted from the compiled
       pattern.

       For convenience, the three most common quantifiers have  single-charac-
       ter abbreviations:

         *    is equivalent to {0,}
         +    is equivalent to {1,}
         ?    is equivalent to {0,1}

       It  is  possible  to construct infinite loops by following a group that
       can match no characters with a quantifier that has no upper limit,  for
       example:

         (a?)*

       Earlier  versions  of  Perl  and PCRE1 used to give an error at compile
       time for such patterns. However, because there are cases where this can
       be useful, such patterns are now accepted, but if any repetition of the
       group does in fact match no characters, the loop is forcibly broken.

       By default, quantifiers are "greedy", that is, they match  as  much  as
       possible (up to the maximum number of permitted times), without causing
       the rest of the pattern to fail. The  classic  example  of  where  this
       gives  problems  is  in  trying  to match comments in C programs. These
       appear between /* and */ and within the comment,  individual  *  and  /
       characters  may  appear. An attempt to match C comments by applying the
       pattern

         /\*.*\*/

       to the string

         /* first comment */  not comment  /* second comment */

       fails, because it matches the entire string owing to the greediness  of
       the  .*  item. However, if a quantifier is followed by a question mark,
       it ceases to be greedy, and instead matches the minimum number of times
       possible, so the pattern

         /\*.*?\*/

       does  the  right  thing with the C comments. The meaning of the various
       quantifiers is not otherwise changed,  just  the  preferred  number  of
       matches.   Do  not  confuse this use of question mark with its use as a
       quantifier in its own right. Because it has two uses, it can  sometimes
       appear doubled, as in

         \d??\d

       which matches one digit by preference, but can match two if that is the
       only way the rest of the pattern matches.

       If the PCRE2_UNGREEDY option is set (an option that is not available in
       Perl),  the  quantifiers are not greedy by default, but individual ones
       can be made greedy by following them with a  question  mark.  In  other
       words, it inverts the default behaviour.

       When  a  parenthesized  group is quantified with a minimum repeat count
       that is greater than 1 or  with  a  limited  maximum,  more  memory  is
       required  for  the  compiled  pattern, in proportion to the size of the
       minimum or maximum.

       If a pattern starts with  .*  or  .{0,}  and  the  PCRE2_DOTALL  option
       (equivalent  to  Perl's /s) is set, thus allowing the dot to match new-
       lines, the pattern is implicitly  anchored,  because  whatever  follows
       will  be  tried against every character position in the subject string,
       so there is no point in retrying the  overall  match  at  any  position
       after the first. PCRE2 normally treats such a pattern as though it were
       preceded by \A.

       In cases where it is known that the subject  string  contains  no  new-
       lines,  it  is worth setting PCRE2_DOTALL in order to obtain this opti-
       mization, or alternatively, using ^ to indicate anchoring explicitly.

       However, there are some cases where the optimization  cannot  be  used.
       When  .*   is  inside  capturing  parentheses that are the subject of a
       backreference elsewhere in the pattern, a match at the start  may  fail
       where a later one succeeds. Consider, for example:

         (.*)abc\1

       If  the subject is "xyz123abc123" the match point is the fourth charac-
       ter. For this reason, such a pattern is not implicitly anchored.

       Another case where implicit anchoring is not applied is when the  lead-
       ing  .* is inside an atomic group. Once again, a match at the start may
       fail where a later one succeeds. Consider this pattern:

         (?>.*?a)b

       It matches "ab" in the subject "aab". The use of the backtracking  con-
       trol  verbs  (*PRUNE)  and  (*SKIP) also disable this optimization, and
       there is an option, PCRE2_NO_DOTSTAR_ANCHOR, to do so explicitly.

       When a capture group is repeated, the value captured is  the  substring
       that matched the final iteration. For example, after

         (tweedle[dume]{3}\s*)+

       has matched "tweedledum tweedledee" the value of the captured substring
       is "tweedledee". However, if there are nested capture groups, the  cor-
       responding  captured  values  may have been set in previous iterations.
       For example, after

         (a|(b))+

       matches "aba" the value of the second captured substring is "b".


ATOMIC GROUPING AND POSSESSIVE QUANTIFIERS

       With both maximizing ("greedy") and minimizing ("ungreedy"  or  "lazy")
       repetition,  failure  of what follows normally causes the repeated item
       to be re-evaluated to see if a different number of repeats  allows  the
       rest  of  the pattern to match. Sometimes it is useful to prevent this,
       either to change the nature of the match, or to cause it  fail  earlier
       than  it otherwise might, when the author of the pattern knows there is
       no point in carrying on.

       Consider, for example, the pattern \d+foo when applied to  the  subject
       line

         123456bar

       After matching all 6 digits and then failing to match "foo", the normal
       action of the matcher is to try again with only 5 digits  matching  the
       \d+  item,  and  then  with  4,  and  so on, before ultimately failing.
       "Atomic grouping" (a term taken from Jeffrey  Friedl's  book)  provides
       the means for specifying that once a group has matched, it is not to be
       re-evaluated in this way.

       If we use atomic grouping for the previous example, the  matcher  gives
       up  immediately  on failing to match "foo" the first time. The notation
       is a kind of special parenthesis, starting with (?> as in this example:

         (?>\d+)foo

       Perl 5.28 introduced an experimental alphabetic form starting  with  (*
       which may be easier to remember:

         (*atomic:\d+)foo

       This kind of parenthesized group "locks up" the  part of the pattern it
       contains once it has matched, and a failure further into the pattern is
       prevented  from  backtracking into it. Backtracking past it to previous
       items, however, works as normal.

       An alternative description is that a group of this type matches exactly
       the  string  of  characters  that an identical standalone pattern would
       match, if anchored at the current point in the subject string.

       Atomic groups are not capture groups. Simple cases such  as  the  above
       example  can  be  thought  of  as a maximizing repeat that must swallow
       everything it can.  So, while both \d+ and \d+? are prepared to  adjust
       the  number  of digits they match in order to make the rest of the pat-
       tern match, (?>\d+) can only match an entire sequence of digits.

       Atomic groups in general can of course contain arbitrarily  complicated
       expressions, and can be nested. However, when the contents of an atomic
       group is just a single repeated item, as in the example above,  a  sim-
       pler  notation, called a "possessive quantifier" can be used. This con-
       sists of an additional + character following a quantifier.  Using  this
       notation, the previous example can be rewritten as

         \d++foo

       Note that a possessive quantifier can be used with an entire group, for
       example:

         (abc|xyz){2,3}+

       Possessive  quantifiers  are  always  greedy;  the   setting   of   the
       PCRE2_UNGREEDY  option  is  ignored. They are a convenient notation for
       the simpler forms of atomic group. However, there is no  difference  in
       the meaning of a possessive quantifier and the equivalent atomic group,
       though there may be a performance  difference;  possessive  quantifiers
       should be slightly faster.

       The  possessive  quantifier syntax is an extension to the Perl 5.8 syn-
       tax.  Jeffrey Friedl originated the idea (and the name)  in  the  first
       edition of his book. Mike McCloskey liked it, so implemented it when he
       built Sun's Java package, and PCRE1 copied it from there. It found  its
       way into Perl at release 5.10.

       PCRE2  has  an  optimization  that automatically "possessifies" certain
       simple pattern constructs. For example, the sequence A+B is treated  as
       A++B  because  there is no point in backtracking into a sequence of A's
       when B must follow.  This feature can be disabled by the PCRE2_NO_AUTO-
       POSSESS option, or starting the pattern with (*NO_AUTO_POSSESS).

       When  a  pattern  contains  an unlimited repeat inside a group that can
       itself be repeated an unlimited number of times, the use of  an  atomic
       group  is the only way to avoid some failing matches taking a very long
       time indeed. The pattern

         (\D+|<\d+>)*[!?]

       matches an unlimited number of substrings that either consist  of  non-
       digits,  or  digits  enclosed in <>, followed by either ! or ?. When it
       matches, it runs quickly. However, if it is applied to

         aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa

       it takes a long time before reporting  failure.  This  is  because  the
       string  can be divided between the internal \D+ repeat and the external
       * repeat in a large number of ways, and all  have  to  be  tried.  (The
       example  uses  [!?]  rather than a single character at the end, because
       both PCRE2 and Perl have an optimization that allows for  fast  failure
       when  a single character is used. They remember the last single charac-
       ter that is required for a match, and fail early if it is  not  present
       in  the  string.)  If  the pattern is changed so that it uses an atomic
       group, like this:

         ((?>\D+)|<\d+>)*[!?]

       sequences of non-digits cannot be broken, and failure happens quickly.


BACKREFERENCES

       Outside a character class, a backslash followed by a digit greater than
       0  (and  possibly further digits) is a backreference to a capture group
       earlier (that is, to its left) in the pattern, provided there have been
       that many previous capture groups.

       However,  if the decimal number following the backslash is less than 8,
       it is always taken as a backreference, and  causes  an  error  only  if
       there  are not that many capture groups in the entire pattern. In other
       words, the group that is referenced need not be to the left of the ref-
       erence  for numbers less than 8. A "forward backreference" of this type
       can make sense when a repetition is involved and the group to the right
       has participated in an earlier iteration.

       It  is  not  possible  to have a numerical "forward backreference" to a
       group whose number is 8 or more using this syntax  because  a  sequence
       such  as  \50  is  interpreted as a character defined in octal. See the
       subsection entitled "Non-printing characters" above for further details
       of  the  handling of digits following a backslash. Other forms of back-
       referencing do not suffer from this restriction. In  particular,  there
       is no problem when named capture groups are used (see below).

       Another  way  of  avoiding  the ambiguity inherent in the use of digits
       following a backslash is to use the \g  escape  sequence.  This  escape
       must be followed by a signed or unsigned number, optionally enclosed in
       braces. These examples are all identical:

         (ring), \1
         (ring), \g1
         (ring), \g{1}

       An unsigned number specifies an absolute reference without the  ambigu-
       ity that is present in the older syntax. It is also useful when literal
       digits follow the reference. A signed number is a  relative  reference.
       Consider this example:

         (abc(def)ghi)\g{-1}

       The sequence \g{-1} is a reference to the most recently started capture
       group before \g, that is, is it equivalent to \2 in this example. Simi-
       larly, \g{-2} would be equivalent to \1. The use of relative references
       can be helpful in long patterns, and also in patterns that are  created
       by  joining  together  fragments  that  contain references within them-
       selves.

       The sequence \g{+1} is a reference to the next capture group. This kind
       of  forward  reference can be useful in patterns that repeat. Perl does
       not support the use of + in this way.

       A backreference matches whatever actually  most  recently  matched  the
       capture  group  in  the current subject string, rather than anything at
       all that matches the group (see "Groups as subroutines" below for a way
       of doing that). So the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If caseful matching is in force at  the
       time  of  the backreference, the case of letters is relevant. For exam-
       ple,

         ((?i)rah)\s+\1

       matches "rah rah" and "RAH RAH", but not "RAH  rah",  even  though  the
       original capture group is matched caselessly.

       There  are  several  different  ways of writing backreferences to named
       capture groups. The .NET syntax \k{name} and the Perl  syntax  \k<name>
       or  \k'name'  are  supported,  as  is the Python syntax (?P=name). Perl
       5.10's unified backreference syntax, in which \g can be used  for  both
       numeric  and  named references, is also supported. We could rewrite the
       above example in any of the following ways:

         (?<p1>(?i)rah)\s+\k<p1>
         (?'p1'(?i)rah)\s+\k{p1}
         (?P<p1>(?i)rah)\s+(?P=p1)
         (?<p1>(?i)rah)\s+\g{p1}

       A capture group that is referenced by name may appear  in  the  pattern
       before or after the reference.

       There  may be more than one backreference to the same group. If a group
       has not actually been used in a particular match, backreferences to  it
       always fail by default. For example, the pattern

         (a|(bc))\2

       always  fails  if  it starts to match "a" rather than "bc". However, if
       the PCRE2_MATCH_UNSET_BACKREF option is set at compile time, a backref-
       erence to an unset value matches an empty string.

       Because  there may be many capture groups in a pattern, all digits fol-
       lowing a backslash are taken as part of a potential backreference  num-
       ber.  If  the  pattern continues with a digit character, some delimiter
       must be used to terminate the backreference. If the  PCRE2_EXTENDED  or
       PCRE2_EXTENDED_MORE  option is set, this can be white space. Otherwise,
       the \g{} syntax or an empty comment (see "Comments" below) can be used.

   Recursive backreferences

       A backreference that occurs inside the group to which it  refers  fails
       when  the  group  is  first used, so, for example, (a\1) never matches.
       However, such references can be  useful  inside  repeated  groups.  For
       example, the pattern

         (a|b\1)+

       matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
       ation of the group, the backreference matches the character string cor-
       responding  to  the  previous iteration. In order for this to work, the
       pattern must be such that the first iteration does not  need  to  match
       the  backreference. This can be done using alternation, as in the exam-
       ple above, or by a quantifier with a minimum of zero.

       Backreferences of this type cause the group that they reference  to  be
       treated  as  an atomic group.  Once the whole group has been matched, a
       subsequent matching failure cannot cause backtracking into  the  middle
       of the group.


ASSERTIONS

       An  assertion  is  a  test on the characters following or preceding the
       current matching point that does not consume any characters. The simple
       assertions  coded  as  \b,  \B,  \A,  \G, \Z, \z, ^ and $ are described
       above.

       More complicated assertions are coded as  parenthesized  groups.  There
       are  two  kinds:  those  that look ahead of the current position in the
       subject string, and those that look behind it,  and  in  each  case  an
       assertion  may be positive (must match for the assertion to be true) or
       negative (must not match for the assertion to be  true).  An  assertion
       group is matched in the normal way, and if it is true, matching contin-
       ues after it, but with the matching position in the subject  string  is
       was it was before the assertion was processed.

       A  lookaround  assertion  may  also appear as the condition in a condi-
       tional group (see below). In this case,  the  result  of  matching  the
       assertion determines which branch of the condition is followed.

       Assertion  groups are not capture groups. If an assertion contains cap-
       ture groups within it, these are counted for the purposes of  numbering
       the  capture  groups  in  the  whole  pattern. Within each branch of an
       assertion, locally captured substrings may be referenced in  the  usual
       way.  For  example,  a  sequence such as (.)\g{-1} can be used to check
       that two adjacent characters are the same.

       When a branch within an assertion fails to match, any  substrings  that
       were  captured  are  discarded (as happens with any pattern branch that
       fails to match). A  negative  assertion  is  true  only  when  all  its
       branches fail to match; this means that no captured substrings are ever
       retained after a successful negative assertion. When an assertion  con-
       tains a matching branch, what happens depends on the type of assertion.

       For  a  positive  assertion, internally captured substrings in the suc-
       cessful branch are retained, and matching continues with the next  pat-
       tern  item  after  the  assertion. For a negative assertion, a matching
       branch means that the assertion is not true. If such  an  assertion  is
       being  used as a condition in a conditional group (see below), captured
       substrings are retained,  because  matching  continues  with  the  "no"
       branch of the condition. For other failing negative assertions, control
       passes to the previous backtracking point, thus discarding any captured
       strings within the assertion.

       For  compatibility  with  Perl,  most assertion groups may be repeated;
       though it makes no sense to assert the same thing  several  times,  the
       side effect of capturing may occasionally be useful. However, an asser-
       tion that forms the condition for a conditional group may not be  quan-
       tified. In practice, for other assertions, there only three cases:

       (1)  If  the  quantifier  is  {0}, the assertion is never obeyed during
       matching.  However, it may contain internal  capture  groups  that  are
       called from elsewhere via the subroutine mechanism.

       (2)  If quantifier is {0,n} where n is greater than zero, it is treated
       as if it were {0,1}. At run time, the rest  of  the  pattern  match  is
       tried with and without the assertion, the order depending on the greed-
       iness of the quantifier.

       (3) If the minimum repetition is greater than zero, the  quantifier  is
       ignored.   The  assertion  is  obeyed just once when encountered during
       matching.

   Alphabetic assertion names

       Traditionally, symbolic sequences such as (?= and (?<= have  been  used
       to  specify lookaround assertions. Perl 5.28 introduced some experimen-
       tal alphabetic alternatives which might be easier to remember. They all
       start  with  (* instead of (? and must be written using lower case let-
       ters. PCRE2 supports the following synonyms:

         (*positive_lookahead:  or (*pla: is the same as (?=
         (*negative_lookahead:  or (*nla: is the same as (?!
         (*positive_lookbehind: or (*plb: is the same as (?<=
         (*negative_lookbehind: or (*nlb: is the same as (?<!

       For example, (*pla:foo) is the same assertion as (?=foo). In  the  fol-
       lowing  sections, the various assertions are described using the origi-
       nal symbolic forms.

   Lookahead assertions

       Lookahead assertions start with (?= for positive assertions and (?! for
       negative assertions. For example,

         \w+(?=;)

       matches  a word followed by a semicolon, but does not include the semi-
       colon in the match, and

         foo(?!bar)

       matches any occurrence of "foo" that is not  followed  by  "bar".  Note
       that the apparently similar pattern

         (?!foo)bar

       does  not  find  an  occurrence  of "bar" that is preceded by something
       other than "foo"; it finds any occurrence of "bar" whatsoever,  because
       the assertion (?!foo) is always true when the next three characters are
       "bar". A lookbehind assertion is needed to achieve the other effect.

       If you want to force a matching failure at some point in a pattern, the
       most  convenient  way  to  do  it  is with (?!) because an empty string
       always matches, so an assertion that requires there not to be an  empty
       string must always fail.  The backtracking control verb (*FAIL) or (*F)
       is a synonym for (?!).

   Lookbehind assertions

       Lookbehind assertions start with (?<= for positive assertions and  (?<!
       for negative assertions. For example,

         (?<!foo)bar

       does  find  an  occurrence  of "bar" that is not preceded by "foo". The
       contents of a lookbehind assertion are restricted  such  that  all  the
       strings it matches must have a fixed length. However, if there are sev-
       eral top-level alternatives, they do not all  have  to  have  the  same
       fixed length. Thus

         (?<=bullock|donkey)

       is permitted, but

         (?<!dogs?|cats?)

       causes  an  error at compile time. Branches that match different length
       strings are permitted only at the top level of a lookbehind  assertion.
       This is an extension compared with Perl, which requires all branches to
       match the same length of string. An assertion such as

         (?<=ab(c|de))

       is not permitted, because its single top-level  branch  can  match  two
       different  lengths,  but  it is acceptable to PCRE2 if rewritten to use
       two top-level branches:

         (?<=abc|abde)

       In some cases, the escape sequence \K (see above) can be  used  instead
       of a lookbehind assertion to get round the fixed-length restriction.

       The  implementation  of lookbehind assertions is, for each alternative,
       to temporarily move the current position back by the fixed  length  and
       then try to match. If there are insufficient characters before the cur-
       rent position, the assertion fails.

       In UTF-8 and UTF-16 modes, PCRE2 does not allow the  \C  escape  (which
       matches  a single code unit even in a UTF mode) to appear in lookbehind
       assertions, because it makes it impossible to calculate the  length  of
       the  lookbehind.  The \X and \R escapes, which can match different num-
       bers of code units, are never permitted in lookbehinds.

       "Subroutine" calls (see below) such as (?2) or (?&X) are  permitted  in
       lookbehinds, as long as the called capture group matches a fixed-length
       string. However, recursion, that is, a "subroutine" call into  a  group
       that is already active, is not supported.

       Perl does not support backreferences in lookbehinds. PCRE2 does support
       them,   but   only    if    certain    conditions    are    met.    The
       PCRE2_MATCH_UNSET_BACKREF  option must not be set, there must be no use
       of (?| in the pattern (it creates duplicate group numbers), and if  the
       backreference  is by name, the name must be unique. Of course, the ref-
       erenced group must itself match a fixed length substring. The following
       pattern matches words containing at least two characters that begin and
       end with the same character:

          \b(\w)\w++(?<=\1)

       Possessive quantifiers can  be  used  in  conjunction  with  lookbehind
       assertions to specify efficient matching of fixed-length strings at the
       end of subject strings. Consider a simple pattern such as

         abcd$

       when applied to a long string that does  not  match.  Because  matching
       proceeds  from  left to right, PCRE2 will look for each "a" in the sub-
       ject and then see if what follows matches the rest of the  pattern.  If
       the pattern is specified as

         ^.*abcd$

       the  initial .* matches the entire string at first, but when this fails
       (because there is no following "a"), it backtracks to match all but the
       last  character,  then all but the last two characters, and so on. Once
       again the search for "a" covers the entire string, from right to  left,
       so we are no better off. However, if the pattern is written as

         ^.*+(?<=abcd)

       there can be no backtracking for the .*+ item because of the possessive
       quantifier; it can match only the entire string. The subsequent lookbe-
       hind  assertion  does  a single test on the last four characters. If it
       fails, the match fails immediately. For  long  strings,  this  approach
       makes a significant difference to the processing time.

   Using multiple assertions

       Several assertions (of any sort) may occur in succession. For example,

         (?<=\d{3})(?<!999)foo

       matches  "foo" preceded by three digits that are not "999". Notice that
       each of the assertions is applied independently at the  same  point  in
       the  subject  string.  First  there  is a check that the previous three
       characters are all digits, and then there is  a  check  that  the  same
       three characters are not "999".  This pattern does not match "foo" pre-
       ceded by six characters, the first of which are  digits  and  the  last
       three  of  which  are not "999". For example, it doesn't match "123abc-
       foo". A pattern to do that is

         (?<=\d{3}...)(?<!999)foo

       This time the first assertion looks at the  preceding  six  characters,
       checking that the first three are digits, and then the second assertion
       checks that the preceding three characters are not "999".

       Assertions can be nested in any combination. For example,

         (?<=(?<!foo)bar)baz

       matches an occurrence of "baz" that is preceded by "bar" which in  turn
       is not preceded by "foo", while

         (?<=\d{3}(?!999)...)foo

       is  another pattern that matches "foo" preceded by three digits and any
       three characters that are not "999".


SCRIPT RUNS

       In concept, a script run is a sequence of characters that are all  from
       the  same  Unicode script such as Latin or Greek. However, because some
       scripts are commonly used together, and because  some  diacritical  and
       other  marks  are  used  with  multiple scripts, it is not that simple.
       There is a full description of the rules that PCRE2 uses in the section
       entitled "Script Runs" in the pcre2unicode documentation.

       If  part  of a pattern is enclosed between (*script_run: or (*sr: and a
       closing parenthesis, it fails if the sequence  of  characters  that  it
       matches  are  not  a  script  run. After a failure, normal backtracking
       occurs. Script runs can be used to detect spoofing attacks using  char-
       acters  that  look the same, but are from different scripts. The string
       "paypal.com" is an infamous example, where the letters could be a  mix-
       ture of Latin and Cyrillic. This pattern ensures that the matched char-
       acters in a sequence of non-spaces that follow white space are a script
       run:

         \s+(*sr:\S+)

       To  be  sure  that  they are all from the Latin script (for example), a
       lookahead can be used:

         \s+(?=\p{Latin})(*sr:\S+)

       This works as long as the first character is expected to be a character
       in  that  script,  and  not (for example) punctuation, which is allowed
       with any script. If this is not the case, a more creative lookahead  is
       needed.  For  example, if digits, underscore, and dots are permitted at
       the start:

         \s+(?=[0-9_.]*\p{Latin})(*sr:\S+)


       In many cases, backtracking into a script run pattern fragment  is  not
       desirable.  The  script run can employ an atomic group to prevent this.
       Because this is a common requirement, a shorthand notation is  provided
       by (*atomic_script_run: or (*asr:

         (*asr:...) is the same as (*sr:(?>...))

       Note that the atomic group is inside the script run. Putting it outside
       would not prevent backtracking into the script run pattern.

       Support for script runs is not available if PCRE2 is  compiled  without
       Unicode support. A compile-time error is given if any of the above con-
       structs is encountered. Script runs are not supported by the  alternate
       matching  function,  pcre2_dfa_match() because they use the same mecha-
       nism as capturing parentheses.

       Warning: The (*ACCEPT) control verb (see  below)  should  not  be  used
       within a script run group, because it causes an immediate exit from the
       group, bypassing the script run checking.


CONDITIONAL GROUPS

       It is possible to cause the matching process to obey a pattern fragment
       conditionally or to choose between two alternative fragments, depending
       on the result of an assertion, or whether a specific capture group  has
       already been matched. The two possible forms of conditional group are:

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

       If  the  condition is satisfied, the yes-pattern is used; otherwise the
       no-pattern (if present) is used. An absent no-pattern is equivalent  to
       an  empty string (it always matches). If there are more than two alter-
       natives in the group, a compile-time error  occurs.  Each  of  the  two
       alternatives  may  itself  contain nested groups of any form, including
       conditional groups; the restriction to two alternatives applies only at
       the  level of the condition itself. This pattern fragment is an example
       where the alternatives are complex:

         (?(1) (A|B|C) | (D | (?(2)E|F) | E) )


       There are five kinds of condition: references to capture groups, refer-
       ences  to  recursion,  two pseudo-conditions called DEFINE and VERSION,
       and assertions.

   Checking for a used capture group by number

       If the text between the parentheses consists of a sequence  of  digits,
       the  condition is true if a capture group of that number has previously
       matched. If there is more than one capture group with the  same  number
       (see  the earlier section about duplicate group numbers), the condition
       is true if any of them have matched. An alternative notation is to pre-
       cede the digits with a plus or minus sign. In this case, the group num-
       ber is relative rather than absolute. The most recently opened  capture
       group  can be referenced by (?(-1), the next most recent by (?(-2), and
       so on. Inside loops it can also  make  sense  to  refer  to  subsequent
       groups.  The next capture group can be referenced as (?(+1), and so on.
       (The value zero in any of these forms is not used; it provokes  a  com-
       pile-time error.)

       Consider  the  following  pattern, which contains non-significant white
       space to make it more readable (assume the PCRE2_EXTENDED  option)  and
       to divide it into three parts for ease of discussion:

         ( \( )?    [^()]+    (?(1) \) )

       The  first  part  matches  an optional opening parenthesis, and if that
       character is present, sets it as the first captured substring. The sec-
       ond  part  matches one or more characters that are not parentheses. The
       third part is a conditional group that tests whether or not  the  first
       capture  group  matched. If it did, that is, if subject started with an
       opening parenthesis, the condition is true, and so the  yes-pattern  is
       executed  and  a  closing parenthesis is required. Otherwise, since no-
       pattern is not present, the conditional group matches nothing. In other
       words,  this  pattern matches a sequence of non-parentheses, optionally
       enclosed in parentheses.

       If you were embedding this pattern in a larger one,  you  could  use  a
       relative reference:

         ...other stuff... ( \( )?    [^()]+    (?(-1) \) ) ...

       This  makes  the  fragment independent of the parentheses in the larger
       pattern.

   Checking for a used capture group by name

       Perl uses the syntax (?(<name>)...) or (?('name')...)  to  test  for  a
       used  capture group by name. For compatibility with earlier versions of
       PCRE1, which had this facility before Perl, the syntax (?(name)...)  is
       also  recognized.   Note, however, that undelimited names consisting of
       the letter R followed by digits are ambiguous (see the  following  sec-
       tion). Rewriting the above example to use a named group gives this:

         (?<OPEN> \( )?    [^()]+    (?(<OPEN>) \) )

       If  the  name used in a condition of this kind is a duplicate, the test
       is applied to all groups of the same name, and is true if  any  one  of
       them has matched.

   Checking for pattern recursion

       "Recursion"  in  this sense refers to any subroutine-like call from one
       part of the pattern to another, whether or not it  is  actually  recur-
       sive.  See  the  sections  entitled "Recursive patterns" and "Groups as
       subroutines" below for details of recursion and subroutine calls.

       If a condition is the string (R), and there is no  capture  group  with
       the  name R, the condition is true if matching is currently in a recur-
       sion or subroutine call to the whole pattern or any capture  group.  If
       digits  follow  the letter R, and there is no group with that name, the
       condition is true if the most recent call is  into  a  group  with  the
       given  number,  which must exist somewhere in the overall pattern. This
       is a contrived example that is equivalent to a+b:

         ((?(R1)a+|(?1)b))

       However, in both cases, if there is a capture  group  with  a  matching
       name,  the  condition tests for its being set, as described in the sec-
       tion above, instead of testing for recursion. For example,  creating  a
       group  with  the  name  R1  by adding (?<R1>) to the above pattern com-
       pletely changes its meaning.

       If a name preceded by ampersand follows the letter R, for example:

         (?(R&name)...)

       the condition is true if the most recent recursion is into a  group  of
       that name (which must exist within the pattern).

       This condition does not check the entire recursion stack. It tests only
       the current level. If the name used in a condition of this  kind  is  a
       duplicate,  the  test is applied to all groups of the same name, and is
       true if any one of them is the most recent recursion.

       At "top level", all these recursion test conditions are false.

   Defining capture groups for use by reference only

       If the condition is the string (DEFINE), the condition is always false,
       even  if there is a group with the name DEFINE. In this case, there may
       be only one alternative in the rest of the  conditional  group.  It  is
       always  skipped  if control reaches this point in the pattern; the idea
       of DEFINE is that it can be used to define subroutines that can be ref-
       erenced  from  elsewhere.  (The use of subroutines is described below.)
       For  example,  a  pattern  to   match   an   IPv4   address   such   as
       "192.168.23.245"  could  be  written  like this (ignore white space and
       line breaks):

         (?(DEFINE) (?<byte> 2[0-4]\d | 25[0-5] | 1\d\d | [1-9]?\d) )
         \b (?&byte) (\.(?&byte)){3} \b

       The first part of the pattern is a DEFINE group inside which a  another
       group  named "byte" is defined. This matches an individual component of
       an IPv4 address (a number less than 256). When  matching  takes  place,
       this  part  of  the pattern is skipped because DEFINE acts like a false
       condition. The rest of the pattern uses references to the  named  group
       to  match the four dot-separated components of an IPv4 address, insist-
       ing on a word boundary at each end.

   Checking the PCRE2 version

       Programs that link with a PCRE2 library can check the version by  call-
       ing  pcre2_config()  with  appropriate arguments. Users of applications
       that do not have access to the underlying code cannot do this.  A  spe-
       cial  "condition" called VERSION exists to allow such users to discover
       which version of PCRE2 they are dealing with by using this condition to
       match  a string such as "yesno". VERSION must be followed either by "="
       or ">=" and a version number.  For example:

         (?(VERSION>=10.4)yes|no)

       This pattern matches "yes" if the PCRE2 version is greater or equal  to
       10.4,  or "no" otherwise. The fractional part of the version number may
       not contain more than two digits.

   Assertion conditions

       If the condition is not in any of the  above  formats,  it  must  be  a
       parenthesized  assertion.  This may be a positive or negative lookahead
       or lookbehind assertion. Consider this pattern, again  containing  non-
       significant  white  space,  and with the two alternatives on the second
       line:

         (?(?=[^a-z]*[a-z])
         \d{2}-[a-z]{3}-\d{2}  |  \d{2}-\d{2}-\d{2} )

       The condition  is  a  positive  lookahead  assertion  that  matches  an
       optional  sequence of non-letters followed by a letter. In other words,
       it tests for the presence of at least one letter in the subject.  If  a
       letter  is found, the subject is matched against the first alternative;
       otherwise it is  matched  against  the  second.  This  pattern  matches
       strings  in  one  of the two forms dd-aaa-dd or dd-dd-dd, where aaa are
       letters and dd are digits.

       When an assertion that is a condition contains capture groups, any cap-
       turing  that  occurs  in  a matching branch is retained afterwards, for
       both positive and negative assertions, because matching always  contin-
       ues  after  the  assertion, whether it succeeds or fails. (Compare non-
       conditional assertions, for which captures are retained only for  posi-
       tive assertions that succeed.)


COMMENTS

       There are two ways of including comments in patterns that are processed
       by PCRE2. In both cases, the start of the comment  must  not  be  in  a
       character  class,  nor  in  the middle of any other sequence of related
       characters such as (?: or a group name or number. The  characters  that
       make up a comment play no part in the pattern matching.

       The  sequence (?# marks the start of a comment that continues up to the
       next closing parenthesis. Nested parentheses are not permitted. If  the
       PCRE2_EXTENDED  or  PCRE2_EXTENDED_MORE  option  is set, an unescaped #
       character also introduces a comment, which in this  case  continues  to
       immediately  after  the next newline character or character sequence in
       the pattern. Which characters are interpreted as newlines is controlled
       by  an option passed to the compiling function or by a special sequence
       at the start of the pattern, as described in the section entitled "New-
       line conventions" above. Note that the end of this type of comment is a
       literal newline sequence in the pattern; escape sequences  that  happen
       to represent a newline do not count. For example, consider this pattern
       when PCRE2_EXTENDED is set, and the default newline convention (a  sin-
       gle linefeed character) is in force:

         abc #comment \n still comment

       On  encountering  the # character, pcre2_compile() skips along, looking
       for a newline in the pattern. The sequence \n is still literal at  this
       stage,  so  it does not terminate the comment. Only an actual character
       with the code value 0x0a (the default newline) does so.


RECURSIVE PATTERNS

       Consider the problem of matching a string in parentheses, allowing  for
       unlimited  nested  parentheses.  Without the use of recursion, the best
       that can be done is to use a pattern that  matches  up  to  some  fixed
       depth  of  nesting.  It  is not possible to handle an arbitrary nesting
       depth.

       For some time, Perl has provided a facility that allows regular expres-
       sions  to recurse (amongst other things). It does this by interpolating
       Perl code in the expression at run time, and the code can refer to  the
       expression itself. A Perl pattern using code interpolation to solve the
       parentheses problem can be created like this:

         $re = qr{\( (?: (?>[^()]+) | (?p{$re}) )* \)}x;

       The (?p{...}) item interpolates Perl code at run time, and in this case
       refers recursively to the pattern in which it appears.

       Obviously,  PCRE2  cannot  support  the  interpolation  of  Perl  code.
       Instead, it supports special syntax for recursion of  the  entire  pat-
       tern, and also for individual capture group recursion. After its intro-
       duction in PCRE1 and Python, this kind of  recursion  was  subsequently
       introduced into Perl at release 5.10.

       A  special  item  that consists of (? followed by a number greater than
       zero and a closing parenthesis is a recursive subroutine  call  of  the
       capture  group of the given number, provided that it occurs inside that
       group. (If not,  it  is  a  non-recursive  subroutine  call,  which  is
       described  in  the  next  section.)  The special item (?R) or (?0) is a
       recursive call of the entire regular expression.

       This PCRE2 pattern solves the nested parentheses  problem  (assume  the
       PCRE2_EXTENDED option is set so that white space is ignored):

         \( ( [^()]++ | (?R) )* \)

       First  it matches an opening parenthesis. Then it matches any number of
       substrings which can either be a  sequence  of  non-parentheses,  or  a
       recursive  match  of the pattern itself (that is, a correctly parenthe-
       sized substring).  Finally there is a closing parenthesis. Note the use
       of a possessive quantifier to avoid backtracking into sequences of non-
       parentheses.

       If this were part of a larger pattern, you would not  want  to  recurse
       the entire pattern, so instead you could use this:

         ( \( ( [^()]++ | (?1) )* \) )

       We  have  put the pattern into parentheses, and caused the recursion to
       refer to them instead of the whole pattern.

       In a larger pattern,  keeping  track  of  parenthesis  numbers  can  be
       tricky.  This is made easier by the use of relative references. Instead
       of (?1) in the pattern above you can write (?-2) to refer to the second
       most  recently  opened  parentheses  preceding  the recursion. In other
       words, a negative number counts capturing  parentheses  leftwards  from
       the point at which it is encountered.

       Be  aware  however, that if duplicate capture group numbers are in use,
       relative references refer to the earliest group  with  the  appropriate
       number. Consider, for example:

         (?|(a)|(b)) (c) (?-2)

       The first two capture groups (a) and (b) are both numbered 1, and group
       (c) is number 2. When the reference (?-2) is  encountered,  the  second
       most  recently opened parentheses has the number 1, but it is the first
       such group (the (a) group) to which the recursion refers. This would be
       the  same if an absolute reference (?1) was used. In other words, rela-
       tive references are just a shorthand for computing a group number.

       It is also possible to refer to subsequent capture groups,  by  writing
       references  such  as  (?+2). However, these cannot be recursive because
       the reference is not inside the parentheses that are  referenced.  They
       are  always  non-recursive  subroutine  calls, as described in the next
       section.

       An alternative approach is to use named parentheses.  The  Perl  syntax
       for  this  is  (?&name);  PCRE1's earlier syntax (?P>name) is also sup-
       ported. We could rewrite the above example as follows:

         (?<pn> \( ( [^()]++ | (?&pn) )* \) )

       If there is more than one group with the same name, the earliest one is
       used.

       The example pattern that we have been looking at contains nested unlim-
       ited repeats, and so the use of a possessive  quantifier  for  matching
       strings  of  non-parentheses  is important when applying the pattern to
       strings that do not match. For example, when this pattern is applied to

         (aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa()

       it yields "no match" quickly. However, if a  possessive  quantifier  is
       not  used, the match runs for a very long time indeed because there are
       so many different ways the + and * repeats can carve  up  the  subject,
       and all have to be tested before failure can be reported.

       At  the  end  of a match, the values of capturing parentheses are those
       from the outermost level. If you want to obtain intermediate values,  a
       callout function can be used (see below and the pcre2callout documenta-
       tion). If the pattern above is matched against

         (ab(cd)ef)

       the value for the inner capturing parentheses  (numbered  2)  is  "ef",
       which  is  the last value taken on at the top level. If a capture group
       is not matched at the top level, its final  captured  value  is  unset,
       even  if it was (temporarily) set at a deeper level during the matching
       process.

       Do not confuse the (?R) item with the condition (R),  which  tests  for
       recursion.   Consider  this pattern, which matches text in angle brack-
       ets, allowing for arbitrary nesting. Only digits are allowed in  nested
       brackets  (that is, when recursing), whereas any characters are permit-
       ted at the outer level.

         < (?: (?(R) \d++  | [^<>]*+) | (?R)) * >

       In this pattern, (?(R) is the start of a conditional  group,  with  two
       different  alternatives  for the recursive and non-recursive cases. The
       (?R) item is the actual recursive call.

   Differences in recursion processing between PCRE2 and Perl

       Some former differences between PCRE2 and Perl no longer exist.

       Before release 10.30, recursion processing in PCRE2 differed from  Perl
       in  that  a  recursive  subroutine call was always treated as an atomic
       group. That is, once it had matched some of the subject string, it  was
       never  re-entered,  even if it contained untried alternatives and there
       was a subsequent matching failure. (Historical note:  PCRE  implemented
       recursion before Perl did.)

       Starting  with  release 10.30, recursive subroutine calls are no longer
       treated as atomic. That is, they can be re-entered to try unused alter-
       natives  if  there  is a matching failure later in the pattern. This is
       now compatible with the way Perl works. If you want a  subroutine  call
       to be atomic, you must explicitly enclose it in an atomic group.

       Supporting  backtracking  into  recursions  simplifies certain types of
       recursive  pattern.  For  example,  this  pattern  matches  palindromic
       strings:

         ^((.)(?1)\2|.?)$

       The  second  branch  in the group matches a single central character in
       the palindrome when there are an odd number of characters,  or  nothing
       when  there  are  an even number of characters, but in order to work it
       has to be able to try the second case when  the  rest  of  the  pattern
       match fails. If you want to match typical palindromic phrases, the pat-
       tern has to ignore all non-word characters,  which  can  be  done  like
       this:

         ^\W*+((.)\W*+(?1)\W*+\2|\W*+.?)\W*+$

       If  run  with  the  PCRE2_CASELESS option, this pattern matches phrases
       such as "A man, a plan, a canal: Panama!". Note the use of the  posses-
       sive  quantifier  *+  to  avoid backtracking into sequences of non-word
       characters. Without this, PCRE2 takes a great deal longer (ten times or
       more)  to  match typical phrases, and Perl takes so long that you think
       it has gone into a loop.

       Another way in which PCRE2 and Perl used to differ in  their  recursion
       processing  is  in  the  handling of captured values. Formerly in Perl,
       when a group was called recursively or as a subroutine  (see  the  next
       section), it had no access to any values that were captured outside the
       recursion, whereas in PCRE2 these values can  be  referenced.  Consider
       this pattern:

         ^(.)(\1|a(?2))

       This  pattern matches "bab". The first capturing parentheses match "b",
       then in the second group, when the backreference \1 fails to match "b",
       the second alternative matches "a" and then recurses. In the recursion,
       \1 does now match "b" and so the whole match succeeds. This match  used
       to fail in Perl, but in later versions (I tried 5.024) it now works.


GROUPS AS SUBROUTINES

       If  the syntax for a recursive group call (either by number or by name)
       is used outside the parentheses to which it refers, it operates  a  bit
       like  a  subroutine  in  a programming language. More accurately, PCRE2
       treats the referenced group as an independent subpattern which it tries
       to  match  at  the  current  matching position. The called group may be
       defined before or after the reference.  A  numbered  reference  can  be
       absolute or relative, as in these examples:

         (...(absolute)...)...(?2)...
         (...(relative)...)...(?-1)...
         (...(?+1)...(relative)...

       An earlier example pointed out that the pattern

         (sens|respons)e and \1ibility

       matches  "sense and sensibility" and "response and responsibility", but
       not "sense and responsibility". If instead the pattern

         (sens|respons)e and (?1)ibility

       is used, it does match "sense and responsibility" as well as the  other
       two  strings.  Another  example  is  given  in the discussion of DEFINE
       above.

       Like recursions, subroutine calls used to be  treated  as  atomic,  but
       this  changed  at  PCRE2 release 10.30, so backtracking into subroutine
       calls can now occur. However, any capturing parentheses  that  are  set
       during the subroutine call revert to their previous values afterwards.

       Processing  options such as case-independence are fixed when a group is
       defined, so if it is used as  a  subroutine,  such  options  cannot  be
       changed for different calls. For example, consider this pattern:

         (abc)(?i:(?-1))

       It  matches  "abcabc". It does not match "abcABC" because the change of
       processing option does not affect the called group.

       The behaviour of backtracking control verbs in groups  when  called  as
       subroutines is described in the section entitled "Backtracking verbs in
       subroutines" below.


ONIGURUMA SUBROUTINE SYNTAX

       For compatibility with Oniguruma, the non-Perl syntax \g followed by  a
       name or a number enclosed either in angle brackets or single quotes, is
       an alternative syntax for calling a group  as  a  subroutine,  possibly
       recursively.  Here  are two of the examples used above, rewritten using
       this syntax:

         (?<pn> \( ( (?>[^()]+) | \g<pn> )* \) )
         (sens|respons)e and \g'1'ibility

       PCRE2 supports an extension to Oniguruma: if a number is preceded by  a
       plus or a minus sign it is taken as a relative reference. For example:

         (abc)(?i:\g<-1>)

       Note  that \g{...} (Perl syntax) and \g<...> (Oniguruma syntax) are not
       synonymous. The former is a backreference; the latter is  a  subroutine
       call.


CALLOUTS

       Perl has a feature whereby using the sequence (?{...}) causes arbitrary
       Perl code to be obeyed in the middle of matching a regular  expression.
       This makes it possible, amongst other things, to extract different sub-
       strings that match the same pair of parentheses when there is a repeti-
       tion.

       PCRE2  provides  a  similar feature, but of course it cannot obey arbi-
       trary Perl code. The feature is called "callout". The caller  of  PCRE2
       provides  an  external  function  by putting its entry point in a match
       context using the function pcre2_set_callout(), and then  passing  that
       context  to  pcre2_match() or pcre2_dfa_match(). If no match context is
       passed, or if the callout entry point is set to NULL, callouts are dis-
       abled.

       Within  a  regular expression, (?C<arg>) indicates a point at which the
       external function is to be called. There  are  two  kinds  of  callout:
       those  with a numerical argument and those with a string argument. (?C)
       on its own with no argument is treated as (?C0). A  numerical  argument
       allows  the  application  to  distinguish  between  different callouts.
       String arguments were added for release 10.20 to make it  possible  for
       script  languages that use PCRE2 to embed short scripts within patterns
       in a similar way to Perl.

       During matching, when PCRE2 reaches a callout point, the external func-
       tion  is  called.  It is provided with the number or string argument of
       the callout, the position in the pattern, and one item of data that  is
       also set in the match block. The callout function may cause matching to
       proceed, to backtrack, or to fail.

       By default, PCRE2 implements a  number  of  optimizations  at  matching
       time,  and  one  side-effect is that sometimes callouts are skipped. If
       you need all possible callouts to happen, you need to set options  that
       disable  the relevant optimizations. More details, including a complete
       description of the programming interface to the callout  function,  are
       given in the pcre2callout documentation.

   Callouts with numerical arguments

       If  you  just  want  to  have  a means of identifying different callout
       points, put a number less than 256 after the  letter  C.  For  example,
       this pattern has two callout points:

         (?C1)abc(?C2)def

       If  the PCRE2_AUTO_CALLOUT flag is passed to pcre2_compile(), numerical
       callouts are automatically installed before each item in  the  pattern.
       They  are all numbered 255. If there is a conditional group in the pat-
       tern whose condition is an assertion, an additional callout is inserted
       just  before the condition. An explicit callout may also be set at this
       position, as in this example:

         (?(?C9)(?=a)abc|def)

       Note that this applies only to assertion conditions, not to other types
       of condition.

   Callouts with string arguments

       A  delimited  string may be used instead of a number as a callout argu-
       ment. The starting delimiter must be one of ` ' " ^ % #  $  {  and  the
       ending delimiter is the same as the start, except for {, where the end-
       ing delimiter is }. If  the  ending  delimiter  is  needed  within  the
       string, it must be doubled. For example:

         (?C'ab ''c'' d')xyz(?C{any text})pqr

       The  doubling  is  removed  before  the string is passed to the callout
       function.


BACKTRACKING CONTROL

       There are a number of special  "Backtracking  Control  Verbs"  (to  use
       Perl's  terminology)  that  modify the behaviour of backtracking during
       matching. They are generally of the form (*VERB) or (*VERB:NAME).  Some
       verbs  take  either  form,  possibly  behaving differently depending on
       whether or not a name is present.  The names are  not  required  to  be
       unique within the pattern.

       By  default,  for  compatibility  with  Perl, a name is any sequence of
       characters that does not include a closing parenthesis. The name is not
       processed  in  any  way,  and  it  is not possible to include a closing
       parenthesis  in  the  name.   This  can  be  changed  by  setting   the
       PCRE2_ALT_VERBNAMES  option,  but the result is no longer Perl-compati-
       ble.

       When PCRE2_ALT_VERBNAMES is set, backslash  processing  is  applied  to
       verb  names  and  only  an unescaped closing parenthesis terminates the
       name. However, the only backslash items that are permitted are \Q,  \E,
       and  sequences such as \x{100} that define character code points. Char-
       acter type escapes such as \d are faulted.

       A closing parenthesis can be included in a name either as \) or between
       \Q  and  \E. In addition to backslash processing, if the PCRE2_EXTENDED
       or PCRE2_EXTENDED_MORE option is also set, unescaped whitespace in verb
       names is skipped, and #-comments are recognized, exactly as in the rest
       of the pattern.  PCRE2_EXTENDED and PCRE2_EXTENDED_MORE do  not  affect
       verb names unless PCRE2_ALT_VERBNAMES is also set.

       The  maximum  length of a name is 255 in the 8-bit library and 65535 in
       the 16-bit and 32-bit libraries. If the name is empty, that is, if  the
       closing  parenthesis immediately follows the colon, the effect is as if
       the colon were not there. Any number of these verbs may occur in a pat-
       tern.

       Since  these  verbs  are  specifically related to backtracking, most of
       them can be used only when the pattern is to be matched using the  tra-
       ditional matching function, because that uses a backtracking algorithm.
       With the exception of (*FAIL), which behaves like  a  failing  negative
       assertion, the backtracking control verbs cause an error if encountered
       by the DFA matching function.

       The behaviour of these verbs in repeated  groups,  assertions,  and  in
       capture  groups  called  as subroutines (whether or not recursively) is
       documented below.

   Optimizations that affect backtracking verbs

       PCRE2 contains some optimizations that are used to speed up matching by
       running some checks at the start of each match attempt. For example, it
       may know the minimum length of matching subject, or that  a  particular
       character must be present. When one of these optimizations bypasses the
       running of a match,  any  included  backtracking  verbs  will  not,  of
       course, be processed. You can suppress the start-of-match optimizations
       by setting the PCRE2_NO_START_OPTIMIZE option when  calling  pcre2_com-
       pile(),  or by starting the pattern with (*NO_START_OPT). There is more
       discussion of this option in the section entitled "Compiling a pattern"
       in the pcre2api documentation.

       Experiments  with  Perl  suggest that it too has similar optimizations,
       and like PCRE2, turning them off can change the result of a match.

   Verbs that act immediately

       The following verbs act as soon as they are encountered.

          (*ACCEPT) or (*ACCEPT:NAME)

       This verb causes the match to end successfully, skipping the  remainder
       of  the  pattern.  However,  when  it is inside a capture group that is
       called as a subroutine, only that group is ended successfully. Matching
       then continues at the outer level. If (*ACCEPT) in triggered in a posi-
       tive assertion, the assertion succeeds; in a  negative  assertion,  the
       assertion fails.

       If  (*ACCEPT)  is inside capturing parentheses, the data so far is cap-
       tured. For example:

         A((?:A|B(*ACCEPT)|C)D)

       This matches "AB", "AAD", or "ACD"; when it matches "AB", "B"  is  cap-
       tured by the outer parentheses.

       Warning:  (*ACCEPT)  should  not  be  used  within  a script run group,
       because it causes an immediate  exit  from  the  group,  bypassing  the
       script run checking.

         (*FAIL) or (*FAIL:NAME)

       This  verb causes a matching failure, forcing backtracking to occur. It
       may be abbreviated to (*F). It is equivalent  to  (?!)  but  easier  to
       read. The Perl documentation notes that it is probably useful only when
       combined with (?{}) or (??{}). Those are, of course, Perl features that
       are  not  present  in PCRE2. The nearest equivalent is the callout fea-
       ture, as for example in this pattern:

         a+(?C)(*FAIL)

       A match with the string "aaaa" always fails, but the callout  is  taken
       before each backtrack happens (in this example, 10 times).

       (*ACCEPT:NAME)  and  (*FAIL:NAME)  are treated as (*MARK:NAME)(*ACCEPT)
       and (*MARK:NAME)(*FAIL), respectively.

   Recording which path was taken

       There is one verb whose main purpose  is  to  track  how  a  match  was
       arrived  at,  though  it  also  has a secondary use in conjunction with
       advancing the match starting point (see (*SKIP) below).

         (*MARK:NAME) or (*:NAME)

       A name is always required with this verb. For all the other  backtrack-
       ing control verbs, a NAME argument is optional.

       When  a  match  succeeds, the name of the last-encountered mark name on
       the matching path is passed back to the caller as described in the sec-
       tion entitled "Other information about the match" in the pcre2api docu-
       mentation. This applies to all instances of (*MARK)  and  other  verbs,
       including those inside assertions and atomic groups. However, there are
       differences in those cases when (*MARK) is  used  in  conjunction  with
       (*SKIP) as described below.

       The  mark name that was last encountered on the matching path is passed
       back. A verb without a NAME argument is ignored for this purpose.  Here
       is  an  example of pcre2test output, where the "mark" modifier requests
       the retrieval and outputting of (*MARK) data:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XY
          0: XY
         MK: A
         XZ
          0: XZ
         MK: B

       The (*MARK) name is tagged with "MK:" in this output, and in this exam-
       ple  it indicates which of the two alternatives matched. This is a more
       efficient way of obtaining this information than putting each  alterna-
       tive in its own capturing parentheses.

       If  a  verb  with a name is encountered in a positive assertion that is
       true, the name is recorded and passed back if it  is  the  last-encoun-
       tered. This does not happen for negative assertions or failing positive
       assertions.

       After a partial match or a failed match, the last encountered  name  in
       the entire match process is returned. For example:

           re> /X(*MARK:A)Y|X(*MARK:B)Z/mark
         data> XP
         No match, mark = B

       Note  that  in  this  unanchored  example the mark is retained from the
       match attempt that started at the letter "X" in the subject. Subsequent
       match attempts starting at "P" and then with an empty string do not get
       as far as the (*MARK) item, but nevertheless do not reset it.

       If you are interested in  (*MARK)  values  after  failed  matches,  you
       should  probably  set the PCRE2_NO_START_OPTIMIZE option (see above) to
       ensure that the match is always attempted.

   Verbs that act after backtracking

       The following verbs do nothing when they are encountered. Matching con-
       tinues  with  what follows, but if there is a subsequent match failure,
       causing a backtrack to the verb, a failure is forced.  That  is,  back-
       tracking  cannot  pass  to  the  left of the verb. However, when one of
       these verbs appears inside an atomic group or in a lookaround assertion
       that  is  true,  its effect is confined to that group, because once the
       group has been matched, there is never any backtracking into it.  Back-
       tracking from beyond an assertion or an atomic group ignores the entire
       group, and seeks a preceding backtracking point.

       These verbs differ in exactly what kind of failure  occurs  when  back-
       tracking  reaches  them.  The behaviour described below is what happens
       when the verb is not in a subroutine or an assertion.  Subsequent  sec-
       tions cover these special cases.

         (*COMMIT) or (*COMMIT:NAME)

       This  verb  causes the whole match to fail outright if there is a later
       matching failure that causes backtracking to reach it. Even if the pat-
       tern  is  unanchored,  no further attempts to find a match by advancing
       the starting point take place. If (*COMMIT) is  the  only  backtracking
       verb that is encountered, once it has been passed pcre2_match() is com-
       mitted to finding a match at the current starting point, or not at all.
       For example:

         a+(*COMMIT)b

       This  matches  "xxaab" but not "aacaab". It can be thought of as a kind
       of dynamic anchor, or "I've started, so I must finish."

       The behaviour of (*COMMIT:NAME) is not the same  as  (*MARK:NAME)(*COM-
       MIT).  It is like (*MARK:NAME) in that the name is remembered for pass-
       ing back to the caller. However, (*SKIP:NAME) searches only  for  names
       that are set with (*MARK), ignoring those set by any of the other back-
       tracking verbs.

       If there is more than one backtracking verb in a pattern,  a  different
       one  that  follows  (*COMMIT) may be triggered first, so merely passing
       (*COMMIT) during a match does not always guarantee that a match must be
       at this starting point.

       Note  that  (*COMMIT)  at  the start of a pattern is not the same as an
       anchor, unless PCRE2's start-of-match optimizations are turned off,  as
       shown in this output from pcre2test:

           re> /(*COMMIT)abc/
         data> xyzabc
          0: abc
         data>
         re> /(*COMMIT)abc/no_start_optimize
         data> xyzabc
         No match

       For  the first pattern, PCRE2 knows that any match must start with "a",
       so the optimization skips along the subject to "a" before applying  the
       pattern  to the first set of data. The match attempt then succeeds. The
       second pattern disables the optimization that skips along to the  first
       character.  The  pattern  is  now  applied  starting at "x", and so the
       (*COMMIT) causes the match to fail without trying  any  other  starting
       points.

         (*PRUNE) or (*PRUNE:NAME)

       This  verb causes the match to fail at the current starting position in
       the subject if there is a later matching failure that causes backtrack-
       ing  to  reach it. If the pattern is unanchored, the normal "bumpalong"
       advance to the next starting character then happens.  Backtracking  can
       occur  as  usual to the left of (*PRUNE), before it is reached, or when
       matching to the right of (*PRUNE), but if there  is  no  match  to  the
       right,  backtracking cannot cross (*PRUNE). In simple cases, the use of
       (*PRUNE) is just an alternative to an atomic group or possessive  quan-
       tifier, but there are some uses of (*PRUNE) that cannot be expressed in
       any other way. In an anchored pattern (*PRUNE) has the same  effect  as
       (*COMMIT).

       The behaviour of (*PRUNE:NAME) is not the same as (*MARK:NAME)(*PRUNE).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to  the  caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

         (*SKIP)

       This verb, when given without a name, is like (*PRUNE), except that  if
       the  pattern  is unanchored, the "bumpalong" advance is not to the next
       character, but to the position in the subject where (*SKIP) was encoun-
       tered.  (*SKIP)  signifies that whatever text was matched leading up to
       it cannot be part of a successful match if there is a  later  mismatch.
       Consider:

         a+(*SKIP)b

       If  the  subject  is  "aaaac...",  after  the first match attempt fails
       (starting at the first character in the  string),  the  starting  point
       skips on to start the next attempt at "c". Note that a possessive quan-
       tifer does not have the same effect as this example; although it  would
       suppress  backtracking  during  the  first  match  attempt,  the second
       attempt would start at the second character instead of skipping  on  to
       "c".

         (*SKIP:NAME)

       When  (*SKIP)  has  an associated name, its behaviour is modified. When
       such a (*SKIP) is triggered, the previous path through the  pattern  is
       searched  for the most recent (*MARK) that has the same name. If one is
       found, the "bumpalong" advance is to the subject position  that  corre-
       sponds  to that (*MARK) instead of to where (*SKIP) was encountered. If
       no (*MARK) with a matching name is found, the (*SKIP) is ignored.

       The search for a (*MARK) name uses the normal  backtracking  mechanism,
       which  means  that  it  does  not  see (*MARK) settings that are inside
       atomic groups or assertions, because they are never re-entered by back-
       tracking. Compare the following pcre2test examples:

           re> /a(?>(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: a
          1: a
         data:
           re> /a(?:(*MARK:X))(*SKIP:X)(*F)|(.)/
         data: abc
          0: b
          1: b

       In  the first example, the (*MARK) setting is in an atomic group, so it
       is not seen when (*SKIP:X) triggers, causing the (*SKIP) to be ignored.
       This  allows  the second branch of the pattern to be tried at the first
       character position.  In the second example, the (*MARK) setting is  not
       in  an  atomic group. This allows (*SKIP:X) to find the (*MARK) when it
       backtracks, and this causes a new matching attempt to start at the sec-
       ond  character.  This  time, the (*MARK) is never seen because "a" does
       not match "b", so the matcher immediately jumps to the second branch of
       the pattern.

       Note  that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
       ignores names that are set by other backtracking verbs.

         (*THEN) or (*THEN:NAME)

       This verb causes a skip to the next innermost  alternative  when  back-
       tracking  reaches  it.  That  is,  it  cancels any further backtracking
       within the current alternative. Its name  comes  from  the  observation
       that it can be used for a pattern-based if-then-else block:

         ( COND1 (*THEN) FOO | COND2 (*THEN) BAR | COND3 (*THEN) BAZ ) ...

       If  the COND1 pattern matches, FOO is tried (and possibly further items
       after the end of the group if FOO succeeds); on  failure,  the  matcher
       skips  to  the second alternative and tries COND2, without backtracking
       into COND1. If that succeeds and BAR fails, COND3 is tried.  If  subse-
       quently  BAZ fails, there are no more alternatives, so there is a back-
       track to whatever came before the  entire  group.  If  (*THEN)  is  not
       inside an alternation, it acts like (*PRUNE).

       The  behaviour  of (*THEN:NAME) is not the same as (*MARK:NAME)(*THEN).
       It is like (*MARK:NAME) in that the name is remembered for passing back
       to  the  caller. However, (*SKIP:NAME) searches only for names set with
       (*MARK), ignoring those set by other backtracking verbs.

       A group that does not contain a | character  is  just  a  part  of  the
       enclosing  alternative;  it  is  not a nested alternation with only one
       alternative. The effect of (*THEN) extends beyond such a group  to  the
       enclosing  alternative.   Consider  this  pattern, where A, B, etc. are
       complex pattern fragments that do not contain any | characters at  this
       level:

         A (B(*THEN)C) | D

       If  A and B are matched, but there is a failure in C, matching does not
       backtrack into A; instead it moves to the next alternative, that is, D.
       However,  if  the  group containing (*THEN) is given an alternative, it
       behaves differently:

         A (B(*THEN)C | (*FAIL)) | D

       The effect of (*THEN) is now confined to the inner group. After a fail-
       ure  in  C,  matching moves to (*FAIL), which causes the whole group to
       fail because there are no more  alternatives  to  try.  In  this  case,
       matching does backtrack into A.

       Note  that a conditional group is not considered as having two alterna-
       tives, because only one is ever used. In other words, the  |  character
       in  a  conditional group has a different meaning. Ignoring white space,
       consider:

         ^.*? (?(?=a) a | b(*THEN)c )

       If the subject is "ba", this pattern does not  match.  Because  .*?  is
       ungreedy,  it  initially  matches  zero characters. The condition (?=a)
       then fails, the character "b" is matched,  but  "c"  is  not.  At  this
       point,  matching does not backtrack to .*? as might perhaps be expected
       from the presence of the | character. The conditional group is part  of
       the  single  alternative  that  comprises the whole pattern, and so the
       match fails. (If there was a backtrack into .*?, allowing it  to  match
       "b", the match would succeed.)

       The  verbs just described provide four different "strengths" of control
       when subsequent matching fails. (*THEN) is the weakest, carrying on the
       match  at  the next alternative. (*PRUNE) comes next, failing the match
       at the current starting position, but allowing an advance to  the  next
       character  (for an unanchored pattern). (*SKIP) is similar, except that
       the advance may be more than one character. (*COMMIT) is the strongest,
       causing the entire match to fail.

   More than one backtracking verb

       If  more  than  one  backtracking verb is present in a pattern, the one
       that is backtracked onto first acts. For example,  consider  this  pat-
       tern, where A, B, etc. are complex pattern fragments:

         (A(*COMMIT)B(*THEN)C|ABD)

       If  A matches but B fails, the backtrack to (*COMMIT) causes the entire
       match to fail. However, if A and B match, but C fails, the backtrack to
       (*THEN)  causes  the next alternative (ABD) to be tried. This behaviour
       is consistent, but is not always the same as Perl's. It means  that  if
       two  or  more backtracking verbs appear in succession, all the the last
       of them has no effect. Consider this example:

         ...(*COMMIT)(*PRUNE)...

       If there is a matching failure to the right, backtracking onto (*PRUNE)
       causes  it to be triggered, and its action is taken. There can never be
       a backtrack onto (*COMMIT).

   Backtracking verbs in repeated groups

       PCRE2 sometimes differs from Perl in its handling of backtracking verbs
       in repeated groups. For example, consider:

         /(a(*COMMIT)b)+ac/

       If  the  subject  is  "abac", Perl matches unless its optimizations are
       disabled, but PCRE2 always fails because the (*COMMIT)  in  the  second
       repeat of the group acts.

   Backtracking verbs in assertions

       (*FAIL)  in any assertion has its normal effect: it forces an immediate
       backtrack. The behaviour of the other  backtracking  verbs  depends  on
       whether  or  not the assertion is standalone or acting as the condition
       in a conditional group.

       (*ACCEPT) in a standalone positive assertion causes  the  assertion  to
       succeed  without  any  further  processing; captured strings and a mark
       name (if  set)  are  retained.  In  a  standalone  negative  assertion,
       (*ACCEPT)  causes the assertion to fail without any further processing;
       captured substrings and any mark name are discarded.

       If the assertion is a condition, (*ACCEPT) causes the condition  to  be
       true  for  a  positive assertion and false for a negative one; captured
       substrings are retained in both cases.

       The remaining verbs act only when a later failure causes a backtrack to
       reach  them. This means that their effect is confined to the assertion,
       because lookaround assertions are atomic. A backtrack that occurs after
       an assertion is complete does not jump back into the assertion. Note in
       particular that a (*MARK) name that is  set  in  an  assertion  is  not
       "seen" by an instance of (*SKIP:NAME) latter in the pattern.

       The  effect of (*THEN) is not allowed to escape beyond an assertion. If
       there are no more branches to try, (*THEN) causes a positive  assertion
       to be false, and a negative assertion to be true.

       The  other  backtracking verbs are not treated specially if they appear
       in a standalone positive assertion. In a  conditional  positive  asser-
       tion, backtracking (from within the assertion) into (*COMMIT), (*SKIP),
       or (*PRUNE) causes the condition to be false. However, for both  stand-
       alone and conditional negative assertions, backtracking into (*COMMIT),
       (*SKIP), or (*PRUNE) causes the assertion to be true, without consider-
       ing any further alternative branches.

   Backtracking verbs in subroutines

       These behaviours occur whether or not the group is called recursively.

       (*ACCEPT) in a group called as a subroutine causes the subroutine match
       to succeed without any  further  processing.  Matching  then  continues
       after the subroutine call. Perl documents this behaviour. Perl's treat-
       ment of the other verbs in subroutines is different in some cases.

       (*FAIL) in a group called as a subroutine has  its  normal  effect:  it
       forces an immediate backtrack.

       (*COMMIT),  (*SKIP),  and  (*PRUNE)  cause the subroutine match to fail
       when triggered by being backtracked to in a group called as  a  subrou-
       tine. There is then a backtrack at the outer level.

       (*THEN), when triggered, skips to the next alternative in the innermost
       enclosing group that has alternatives (its normal behaviour).  However,
       if there is no such group within the subroutine's group, the subroutine
       match fails and there is a backtrack at the outer level.


SEE ALSO

       pcre2api(3),   pcre2callout(3),    pcre2matching(3),    pcre2syntax(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 12 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2PERFORM(3)            Library Functions Manual            PCRE2PERFORM(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 PERFORMANCE

       Two  aspects  of performance are discussed below: memory usage and pro-
       cessing time. The way you express your pattern as a regular  expression
       can affect both of them.


COMPILED PATTERN MEMORY USAGE

       Patterns are compiled by PCRE2 into a reasonably efficient interpretive
       code, so that most simple patterns do not use much memory  for  storing
       the compiled version. However, there is one case where the memory usage
       of a compiled pattern can be unexpectedly  large.  If  a  parenthesized
       group  has  a quantifier with a minimum greater than 1 and/or a limited
       maximum, the whole group is repeated in the compiled code. For example,
       the pattern

         (abc|def){2,4}

       is compiled as if it were

         (abc|def)(abc|def)((abc|def)(abc|def)?)?

       (Technical  aside:  It is done this way so that backtrack points within
       each of the repetitions can be independently maintained.)

       For regular expressions whose quantifiers use only small numbers,  this
       is  not  usually a problem. However, if the numbers are large, and par-
       ticularly if such repetitions are nested, the memory usage  can  become
       an embarrassment. For example, the very simple pattern

         ((ab){1,1000}c){1,3}

       uses  over  50KiB  when compiled using the 8-bit library. When PCRE2 is
       compiled with its default internal pointer size of two bytes, the  size
       limit on a compiled pattern is 65535 code units in the 8-bit and 16-bit
       libraries, and this is reached with the above pattern if the outer rep-
       etition  is  increased from 3 to 4. PCRE2 can be compiled to use larger
       internal pointers and thus handle larger compiled patterns, but  it  is
       better to try to rewrite your pattern to use less memory if you can.

       One  way  of reducing the memory usage for such patterns is to make use
       of PCRE2's "subroutine" facility. Re-writing the above pattern as

         ((ab)(?2){0,999}c)(?1){0,2}

       reduces the memory requirements to around 16KiB, and indeed it  remains
       under  20KiB  even with the outer repetition increased to 100. However,
       this kind of pattern is not always exactly equivalent, because any cap-
       tures  within  subroutine calls are lost when the subroutine completes.
       If this is not a problem, this kind of  rewriting  will  allow  you  to
       process  patterns that PCRE2 cannot otherwise handle. The matching per-
       formance of the two different versions of the pattern are  roughly  the
       same.  (This applies from release 10.30 - things were different in ear-
       lier releases.)


STACK AND HEAP USAGE AT RUN TIME

       From release 10.30, the interpretive (non-JIT) version of pcre2_match()
       uses  very  little system stack at run time. In earlier releases recur-
       sive function calls could use a great deal of  stack,  and  this  could
       cause  problems, but this usage has been eliminated. Backtracking posi-
       tions are now explicitly remembered in memory frames controlled by  the
       code.  An  initial  20KiB  vector  of frames is allocated on the system
       stack (enough for about 100 frames for small patterns), but if this  is
       insufficient,  heap  memory  is  used. The amount of heap memory can be
       limited; if the limit is set to zero, only the initial stack vector  is
       used.  Rewriting patterns to be time-efficient, as described below, may
       also reduce the memory requirements.

       In contrast to  pcre2_match(),  pcre2_dfa_match()  does  use  recursive
       function  calls,  but  only  for  processing  atomic groups, lookaround
       assertions, and recursion within the pattern. The original  version  of
       the code used to allocate quite large internal workspace vectors on the
       stack, which caused some problems for  some  patterns  in  environments
       with  small  stacks.  From release 10.32 the code for pcre2_dfa_match()
       has been re-factored to use heap memory  when  necessary  for  internal
       workspace  when  recursing,  though  recursive function calls are still
       used.

       The "match depth" parameter can be used to limit the depth of  function
       recursion,  and  the  "match  heap"  parameter  to limit heap memory in
       pcre2_dfa_match().


PROCESSING TIME

       Certain items in regular expression patterns are processed  more  effi-
       ciently than others. It is more efficient to use a character class like
       [aeiou]  than  a  set  of   single-character   alternatives   such   as
       (a|e|i|o|u).  In  general,  the simplest construction that provides the
       required behaviour is usually the most efficient. Jeffrey Friedl's book
       contains  a  lot  of useful general discussion about optimizing regular
       expressions for efficient performance. This  document  contains  a  few
       observations about PCRE2.

       Using  Unicode  character  properties  (the  \p, \P, and \X escapes) is
       slow, because PCRE2 has to use a multi-stage table lookup  whenever  it
       needs  a  character's  property. If you can find an alternative pattern
       that does not use character properties, it will probably be faster.

       By default, the escape sequences \b, \d, \s,  and  \w,  and  the  POSIX
       character  classes  such  as  [:alpha:]  do not use Unicode properties,
       partly for backwards compatibility, and partly for performance reasons.
       However,  you  can  set  the PCRE2_UCP option or start the pattern with
       (*UCP) if you want Unicode character properties to be  used.  This  can
       double  the  matching  time  for  items  such  as \d, when matched with
       pcre2_match(); the performance loss is less with a DFA  matching  func-
       tion, and in both cases there is not much difference for \b.

       When  a pattern begins with .* not in atomic parentheses, nor in paren-
       theses that are the subject of a backreference,  and  the  PCRE2_DOTALL
       option  is  set,  the pattern is implicitly anchored by PCRE2, since it
       can match only at the start of a subject string.  If  the  pattern  has
       multiple top-level branches, they must all be anchorable. The optimiza-
       tion can be disabled by  the  PCRE2_NO_DOTSTAR_ANCHOR  option,  and  is
       automatically disabled if the pattern contains (*PRUNE) or (*SKIP).

       If  PCRE2_DOTALL  is  not  set,  PCRE2  cannot  make this optimization,
       because the dot metacharacter does not then match a newline, and if the
       subject  string contains newlines, the pattern may match from the char-
       acter immediately following one of them instead of from the very start.
       For example, the pattern

         .*second

       matches  the subject "first\nand second" (where \n stands for a newline
       character), with the match starting at the seventh character. In  order
       to  do  this, PCRE2 has to retry the match starting after every newline
       in the subject.

       If you are using such a pattern with subject strings that do  not  con-
       tain   newlines,   the   best   performance   is  obtained  by  setting
       PCRE2_DOTALL, or starting the pattern with  ^.*  or  ^.*?  to  indicate
       explicit anchoring. That saves PCRE2 from having to scan along the sub-
       ject looking for a newline to restart at.

       Beware of patterns that contain nested indefinite  repeats.  These  can
       take  a  long time to run when applied to a string that does not match.
       Consider the pattern fragment

         ^(a+)*

       This can match "aaaa" in 16 different ways, and this  number  increases
       very  rapidly  as the string gets longer. (The * repeat can match 0, 1,
       2, 3, or 4 times, and for each of those cases other than 0 or 4, the  +
       repeats  can  match  different numbers of times.) When the remainder of
       the pattern is such that the entire match is going to fail,  PCRE2  has
       in  principle  to  try  every  possible variation, and this can take an
       extremely long time, even for relatively short strings.

       An optimization catches some of the more simple cases such as

         (a+)*b

       where a literal character follows. Before  embarking  on  the  standard
       matching  procedure, PCRE2 checks that there is a "b" later in the sub-
       ject string, and if there is not, it fails the match immediately.  How-
       ever,  when  there  is no following literal this optimization cannot be
       used. You can see the difference by comparing the behaviour of

         (a+)*\d

       with the pattern above. The former gives  a  failure  almost  instantly
       when  applied  to  a  whole  line of "a" characters, whereas the latter
       takes an appreciable time with strings longer than about 20 characters.

       In many cases, the solution to this kind of performance issue is to use
       an  atomic group or a possessive quantifier. This can often reduce mem-
       ory requirements as well. As another example, consider this pattern:

         ([^<]|<(?!inet))+

       It matches from wherever it starts until it encounters "<inet"  or  the
       end  of  the  data,  and is the kind of pattern that might be used when
       processing an XML file. Each iteration of the outer parentheses matches
       either  one  character that is not "<" or a "<" that is not followed by
       "inet". However, each time a parenthesis is processed,  a  backtracking
       position  is  passed,  so this formulation uses a memory frame for each
       matched character. For a long string, a lot of memory is required. Con-
       sider  now  this  rewritten  pattern,  which  matches  exactly the same
       strings:

         ([^<]++|<(?!inet))+

       This runs much faster, because sequences of characters that do not con-
       tain "<" are "swallowed" in one item inside the parentheses, and a pos-
       sessive quantifier is used to stop any backtracking into  the  runs  of
       non-"<"  characters.  This  version also uses a lot less memory because
       entry to a new set of parentheses happens only  when  a  "<"  character
       that  is  not  followed by "inet" is encountered (and we assume this is
       relatively rare).

       This example shows that one way of optimizing performance when matching
       long  subject strings is to write repeated parenthesized subpatterns to
       match more than one character whenever possible.

   SETTING RESOURCE LIMITS

       You can set limits on the amount of processing that  takes  place  when
       matching,  and  on  the amount of heap memory that is used. The default
       values of the limits are very large, and unlikely ever to operate. They
       can  be  changed  when  PCRE2  is  built, and they can also be set when
       pcre2_match() or pcre2_dfa_match() is  called.  For  details  of  these
       interfaces,  see  the pcre2build documentation and the section entitled
       "The match context" in the pcre2api documentation.

       The pcre2test test program has a modifier called  "find_limits"  which,
       if  applied  to  a  subject line, causes it to find the smallest limits
       that allow a pattern to match. This is done by repeatedly matching with
       different limits.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 03 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2POSIX(3)              Library Functions Manual              PCRE2POSIX(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

       #include <pcre2posix.h>

       int pcre2_regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int pcre2_regexec(const regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t pcre2_regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void pcre2_regfree(regex_t *preg);


DESCRIPTION

       This  set of functions provides a POSIX-style API for the PCRE2 regular
       expression 8-bit library. There are no POSIX-style wrappers for PCRE2's
       16-bit  and  32-bit  libraries.  See  the  pcre2api documentation for a
       description of PCRE2's native API, which contains much additional func-
       tionality.

       The functions described here are wrapper functions that ultimately call
       the PCRE2 native API. Their prototypes are defined in the  pcre2posix.h
       header  file, and they all have unique names starting with pcre2_. How-
       ever, the pcre2posix.h header also contains macro definitions that con-
       vert  the standard POSIX names such regcomp() into pcre2_regcomp() etc.
       This means that a program can use the usual POSIX names without running
       the  risk of accidentally linking with POSIX functions from a different
       library.

       On Unix-like systems the PCRE2 POSIX library is called  libpcre2-posix,
       so  can  be accessed by adding -lpcre2-posix to the command for linking
       an application. Because the POSIX functions call the native ones, it is
       also necessary to add -lpcre2-8.

       Although  they are not defined as protypes in pcre2posix.h, the library
       does contain functions with the POSIX names regcomp() etc. These simply
       pass  their  arguments to the PCRE2 functions. These functions are pro-
       vided for backwards compatibility with earlier versions  of  PCRE2,  so
       that existing programs do not have to be recompiled.

       Calling  the  header  file  pcre2posix.h avoids any conflict with other
       POSIX libraries. It can, of course, be renamed or aliased  as  regex.h,
       which  is  the  "correct"  name,  if there is no clash. It provides two
       structure types, regex_t for compiled internal  forms,  and  regmatch_t
       for returning captured substrings. It also defines some constants whose
       names start with "REG_"; these are used for setting options and identi-
       fying error codes.


USING THE POSIX FUNCTIONS

       Those  POSIX  option bits that can reasonably be mapped to PCRE2 native
       options have been implemented. In addition, the option REG_EXTENDED  is
       defined  with  the  value  zero. This has no effect, but since programs
       that are written to the POSIX interface often use  it,  this  makes  it
       easier  to  slot in PCRE2 as a replacement library. Other POSIX options
       are not even defined.

       There are also some options that are not defined by POSIX.  These  have
       been  added  at  the  request  of users who want to make use of certain
       PCRE2-specific features via the POSIX calling interface or to  add  BSD
       or GNU functionality.

       When  PCRE2  is  called via these functions, it is only the API that is
       POSIX-like in style. The syntax and semantics of  the  regular  expres-
       sions  themselves  are  still  those of Perl, subject to the setting of
       various PCRE2 options, as described below. "POSIX-like in style"  means
       that  the  API  approximates  to  the POSIX definition; it is not fully
       POSIX-compatible, and in multi-unit encoding  domains  it  is  probably
       even less compatible.

       The  descriptions  below use the actual names of the functions, but, as
       described above, the standard POSIX names (without the  pcre2_  prefix)
       may also be used.


COMPILING A PATTERN

       The  function  pcre2_regcomp()  is  called to compile a pattern into an
       internal form. By default, the pattern is a C string  terminated  by  a
       binary zero (but see REG_PEND below). The preg argument is a pointer to
       a regex_t structure that is used as  a  base  for  storing  information
       about  the compiled regular expression. (It is also used for input when
       REG_PEND is set.)

       The argument cflags is either zero, or contains one or more of the bits
       defined by the following macros:

         REG_DOTALL

       The  PCRE2_DOTALL  option  is set when the regular expression is passed
       for compilation to the native function. Note  that  REG_DOTALL  is  not
       part of the POSIX standard.

         REG_ICASE

       The  PCRE2_CASELESS option is set when the regular expression is passed
       for compilation to the native function.

         REG_NEWLINE

       The PCRE2_MULTILINE option is set when the regular expression is passed
       for  compilation  to the native function. Note that this does not mimic
       the defined POSIX behaviour for REG_NEWLINE  (see  the  following  sec-
       tion).

         REG_NOSPEC

       The  PCRE2_LITERAL  option is set when the regular expression is passed
       for compilation to the native function. This disables all meta  charac-
       ters  in the pattern, causing it to be treated as a literal string. The
       only other options that are  allowed  with  REG_NOSPEC  are  REG_ICASE,
       REG_NOSUB,  REG_PEND,  and REG_UTF. Note that REG_NOSPEC is not part of
       the POSIX standard.

         REG_NOSUB

       When  a  pattern  that  is  compiled  with  this  flag  is  passed   to
       pcre2_regexec()  for  matching,  the  nmatch  and  pmatch arguments are
       ignored, and no captured strings are returned.  Versions  of  the  PCRE
       library  prior  to  10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile
       option, but this no longer happens because it disables the use of back-
       references.

         REG_PEND

       If  this option is set, the reg_endp field in the preg structure (which
       has the type const char *) must be set to point to the character beyond
       the  end  of  the  pattern  before calling pcre2_regcomp(). The pattern
       itself may now contain binary zeros, which are treated as data  charac-
       ters.  Without  REG_PEND,  a binary zero terminates the pattern and the
       re_endp field is ignored. This is a GNU extension to the POSIX standard
       and  should be used with caution in software intended to be portable to
       other systems.

         REG_UCP

       The PCRE2_UCP option is set when the regular expression is  passed  for
       compilation  to  the  native function. This causes PCRE2 to use Unicode
       properties when matchine \d, \w,  etc.,  instead  of  just  recognizing
       ASCII values. Note that REG_UCP is not part of the POSIX standard.

         REG_UNGREEDY

       The  PCRE2_UNGREEDY option is set when the regular expression is passed
       for compilation to the native function. Note that REG_UNGREEDY  is  not
       part of the POSIX standard.

         REG_UTF

       The  PCRE2_UTF  option is set when the regular expression is passed for
       compilation to the native function. This causes the pattern itself  and
       all  data  strings used for matching it to be treated as UTF-8 strings.
       Note that REG_UTF is not part of the POSIX standard.

       In the absence of these flags, no options  are  passed  to  the  native
       function.   This  means  the  the  regex is compiled with PCRE2 default
       semantics. In particular, the way it handles newline characters in  the
       subject  string  is  the Perl way, not the POSIX way. Note that setting
       PCRE2_MULTILINE has only some of the effects specified for REG_NEWLINE.
       It  does not affect the way newlines are matched by the dot metacharac-
       ter (they are not) or by a negative class such as [^a] (they are).

       The yield of pcre2_regcomp() is zero on success,  and  non-zero  other-
       wise.  The preg structure is filled in on success, and one other member
       of the structure (as well as re_endp) is public: re_nsub  contains  the
       number  of  capturing  subpatterns  in  the regular expression. Various
       error codes are defined in the header file.

       NOTE: If the yield of pcre2_regcomp() is non-zero, you must not attempt
       to use the contents of the preg structure. If, for example, you pass it
       to pcre2_regexec(), the result is undefined and your program is  likely
       to crash.


MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of
       things.  It is not possible to get PCRE2 to obey POSIX  semantics,  but
       then PCRE2 was never intended to be a POSIX engine. The following table
       lists the different possibilities for matching  newline  characters  in
       Perl and PCRE2:

                                 Default   Change with

         . matches newline          no     PCRE2_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE2_DOLLAR_ENDONLY
         $ matches \n in middle     no     PCRE2_MULTILINE
         ^ matches \n in middle     no     PCRE2_MULTILINE

       This is the equivalent table for a POSIX-compatible pattern matcher:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       This  behaviour  is not what happens when PCRE2 is called via its POSIX
       API. By default, PCRE2's behaviour is the same as Perl's,  except  that
       there  is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2
       and Perl, there is no way to stop newline from matching [^a].

       Default POSIX newline handling can be obtained by setting  PCRE2_DOTALL
       and  PCRE2_DOLLAR_ENDONLY  when  calling  pcre2_compile() directly, but
       there is no way to make PCRE2 behave exactly  as  for  the  REG_NEWLINE
       action.  When  using  the  POSIX  API,  passing  REG_NEWLINE to PCRE2's
       pcre2_regcomp()  function  causes  PCRE2_MULTILINE  to  be  passed   to
       pcre2_compile(), and REG_DOTALL passes PCRE2_DOTALL. There is no way to
       pass PCRE2_DOLLAR_ENDONLY.


MATCHING A PATTERN

       The function pcre2_regexec() is called to match a compiled pattern preg
       against  a  given string, which is by default terminated by a zero byte
       (but see REG_STARTEND below), subject to the options in eflags.   These
       can be:

         REG_NOTBOL

       The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_NOTEMPTY

       The PCRE2_NOTEMPTY option is set  when  calling  the  underlying  PCRE2
       matching  function.  Note  that  REG_NOTEMPTY  is not part of the POSIX
       standard. However, setting this option can give more POSIX-like  behav-
       iour in some situations.

         REG_NOTEOL

       The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match-
       ing function.

         REG_STARTEND

       When this option  is  set,  the  subject  string  starts  at  string  +
       pmatch[0].rm_so  and  ends  at  string  + pmatch[0].rm_eo, which should
       point to the first character beyond the string.  There  may  be  binary
       zeros  within the subject string, and indeed, using REG_STARTEND is the
       only way to pass a subject string that contains a binary zero.

       Whatever the value of  pmatch[0].rm_so,  the  offsets  of  the  matched
       string  and  any  captured  substrings  are still given relative to the
       start of string itself. (Before PCRE2 release 10.30  these  were  given
       relative  to  string  +  pmatch[0].rm_so,  but  this differs from other
       implementations.)

       This is a BSD extension, compatible with  but  not  specified  by  IEEE
       Standard  1003.2 (POSIX.2), and should be used with caution in software
       intended to be portable to other systems. Note that  a  non-zero  rm_so
       does  not  imply REG_NOTBOL; REG_STARTEND affects only the location and
       length of the string, not how it is matched. Setting  REG_STARTEND  and
       passing  pmatch as NULL are mutually exclusive; the error REG_INVARG is
       returned.

       If the pattern was compiled with the REG_NOSUB flag, no data about  any
       matched  strings  is  returned.  The  nmatch  and  pmatch  arguments of
       pcre2_regexec() are ignored (except possibly  as  input  for  REG_STAR-
       TEND).

       The  value  of  nmatch  may  be  zero, and the value pmatch may be NULL
       (unless REG_STARTEND is set); in both these cases  no  data  about  any
       matched strings is returned.

       Otherwise,  the  portion  of  the string that was matched, and also any
       captured substrings, are returned via the pmatch argument, which points
       to  an  array  of  nmatch structures of type regmatch_t, containing the
       members rm_so and rm_eo. These contain the byte  offset  to  the  first
       character of each substring and the offset to the first character after
       the end of each substring, respectively. The 0th element of the  vector
       relates  to  the  entire portion of string that was matched; subsequent
       elements relate to the capturing subpatterns of the regular expression.
       Unused entries in the array have both structure members set to -1.

       A  successful  match  yields  a  zero  return;  various error codes are
       defined in the header file, of  which  REG_NOMATCH  is  the  "expected"
       failure code.


ERROR MESSAGES

       The  pcre2_regerror()  function  maps  a non-zero errorcode from either
       pcre2_regcomp() or pcre2_regexec() to a printable message. If  preg  is
       not  NULL, the error should have arisen from the use of that structure.
       A message terminated by a binary zero is placed in errbuf. If the  buf-
       fer  is  too  short,  only  the first errbuf_size - 1 characters of the
       error message are used. The yield of the function is the size of buffer
       needed  to hold the whole message, including the terminating zero. This
       value is greater than errbuf_size if the message was truncated.


MEMORY USAGE

       Compiling a regular expression causes memory to be allocated and  asso-
       ciated  with the preg structure. The function pcre2_regfree() frees all
       such memory, after which preg may no  longer  be  used  as  a  compiled
       expression.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 30 January 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2SAMPLE(3)             Library Functions Manual             PCRE2SAMPLE(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 SAMPLE PROGRAM

       A  simple, complete demonstration program to get you started with using
       PCRE2 is supplied in the file pcre2demo.c in the src directory  in  the
       PCRE2 distribution. A listing of this program is given in the pcre2demo
       documentation. If you do not have a copy of the PCRE2 distribution, you
       can save this listing to re-create the contents of pcre2demo.c.

       The  demonstration  program compiles the regular expression that is its
       first argument, and matches it against the subject string in its second
       argument.  No  PCRE2  options are set, and default character tables are
       used. If matching succeeds, the program outputs the portion of the sub-
       ject  that  matched,  together  with  the contents of any captured sub-
       strings.

       If the -g option is given on the command line, the program then goes on
       to check for further matches of the same regular expression in the same
       subject string. The logic is a little bit tricky because of the  possi-
       bility  of  matching an empty string. Comments in the code explain what
       is going on.

       The code in pcre2demo.c is an 8-bit program that uses the  PCRE2  8-bit
       library.  It  handles  strings  and characters that are stored in 8-bit
       code units.  By default, one character corresponds to  one  code  unit,
       but  if  the  pattern starts with "(*UTF)", both it and the subject are
       treated as UTF-8 strings, where characters  may  occupy  multiple  code
       units.

       If  PCRE2  is installed in the standard include and library directories
       for your operating system, you should be able to compile the demonstra-
       tion program using a command like this:

         cc -o pcre2demo pcre2demo.c -lpcre2-8

       If PCRE2 is installed elsewhere, you may need to add additional options
       to the command line. For example, on a Unix-like system that has  PCRE2
       installed  in  /usr/local,  you  can  compile the demonstration program
       using a command like this:

         cc -o pcre2demo -I/usr/local/include pcre2demo.c \
            -L/usr/local/lib -lpcre2-8

       Once you have built the demonstration program, you can run simple tests
       like this:

         ./pcre2demo 'cat|dog' 'the cat sat on the mat'
         ./pcre2demo -g 'cat|dog' 'the dog sat on the cat'

       Note  that  there  is  a  much  more comprehensive test program, called
       pcre2test, which supports many  more  facilities  for  testing  regular
       expressions using all three PCRE2 libraries (8-bit, 16-bit, and 32-bit,
       though not all three need be installed). The pcre2demo program is  pro-
       vided as a relatively simple coding example.

       If you try to run pcre2demo when PCRE2 is not installed in the standard
       library directory, you may get an error like  this  on  some  operating
       systems (e.g. Solaris):

         ld.so.1: pcre2demo: fatal: libpcre2-8.so.0: open failed: No such file
       or directory

       This is caused by the way shared library support works  on  those  sys-
       tems. You need to add

         -R/usr/local/lib

       (for example) to the compile command to get round this problem.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 02 February 2016
       Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------
PCRE2SERIALIZE(3)          Library Functions Manual          PCRE2SERIALIZE(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

SAVING AND RE-USING PRECOMPILED PCRE2 PATTERNS

       int32_t pcre2_serialize_decode(pcre2_code **codes,
         int32_t number_of_codes, const uint32_t *bytes,
         pcre2_general_context *gcontext);

       int32_t pcre2_serialize_encode(pcre2_code **codes,
         int32_t number_of_codes, uint32_t **serialized_bytes,
         PCRE2_SIZE *serialized_size, pcre2_general_context *gcontext);

       void pcre2_serialize_free(uint8_t *bytes);

       int32_t pcre2_serialize_get_number_of_codes(const uint8_t *bytes);

       If  you  are running an application that uses a large number of regular
       expression patterns, it may be useful to store them  in  a  precompiled
       form  instead  of  having to compile them every time the application is
       run. However, if you are using the just-in-time  optimization  feature,
       it is not possible to save and reload the JIT data, because it is posi-
       tion-dependent. The host on which the patterns  are  reloaded  must  be
       running  the  same version of PCRE2, with the same code unit width, and
       must also have the same endianness, pointer width and PCRE2_SIZE  type.
       For  example, patterns compiled on a 32-bit system using PCRE2's 16-bit
       library cannot be reloaded on a 64-bit system, nor can they be reloaded
       using the 8-bit library.

       Note  that  "serialization" in PCRE2 does not convert compiled patterns
       to an abstract format like Java or .NET serialization.  The  serialized
       output  is  really  just  a  bytecode dump, which is why it can only be
       reloaded in the same environment as the one that created it. Hence  the
       restrictions  mentioned  above.   Applications  that are not statically
       linked with a fixed version of PCRE2 must be prepared to recompile pat-
       terns from their sources, in order to be immune to PCRE2 upgrades.


SECURITY CONCERNS

       The facility for saving and restoring compiled patterns is intended for
       use within individual applications.  As  such,  the  data  supplied  to
       pcre2_serialize_decode()  is expected to be trusted data, not data from
       arbitrary external sources.  There  is  only  some  simple  consistency
       checking, not complete validation of what is being re-loaded. Corrupted
       data may cause undefined results. For example, if the length field of a
       pattern in the serialized data is corrupted, the deserializing code may
       read beyond the end of the byte stream that is passed to it.


SAVING COMPILED PATTERNS

       Before compiled patterns can be saved they must be serialized, which in
       PCRE2  means converting the pattern to a stream of bytes. A single byte
       stream may contain any number of compiled patterns, but they  must  all
       use  the same character tables. A single copy of the tables is included
       in the byte stream (its size is 1088 bytes). For more details of  char-
       acter  tables,  see the section on locale support in the pcre2api docu-
       mentation.

       The function pcre2_serialize_encode() creates a serialized byte  stream
       from  a  list of compiled patterns. Its first two arguments specify the
       list, being a pointer to a vector of pointers to compiled patterns, and
       the length of the vector. The third and fourth arguments point to vari-
       ables which are set to point to the created byte stream and its length,
       respectively.  The  final  argument  is a pointer to a general context,
       which can be used to specify custom memory  mangagement  functions.  If
       this  argument  is NULL, malloc() is used to obtain memory for the byte
       stream. The yield of the function is the number of serialized patterns,
       or one of the following negative error codes:

         PCRE2_ERROR_BADDATA      the number of patterns is zero or less
         PCRE2_ERROR_BADMAGIC     mismatch of id bytes in one of the patterns
         PCRE2_ERROR_MEMORY       memory allocation failed
         PCRE2_ERROR_MIXEDTABLES  the patterns do not all use the same tables
         PCRE2_ERROR_NULL         the 1st, 3rd, or 4th argument is NULL

       PCRE2_ERROR_BADMAGIC  means  either that a pattern's code has been cor-
       rupted, or that a slot in the vector does not point to a compiled  pat-
       tern.

       Once a set of patterns has been serialized you can save the data in any
       appropriate manner. Here is sample code that compiles two patterns  and
       writes them to a file. It assumes that the variable fd refers to a file
       that is open for output. The error checking that should be present in a
       real application has been omitted for simplicity.

         int errorcode;
         uint8_t *bytes;
         PCRE2_SIZE erroroffset;
         PCRE2_SIZE bytescount;
         pcre2_code *list_of_codes[2];
         list_of_codes[0] = pcre2_compile("first pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         list_of_codes[1] = pcre2_compile("second pattern",
           PCRE2_ZERO_TERMINATED, 0, &errorcode, &erroroffset, NULL);
         errorcode = pcre2_serialize_encode(list_of_codes, 2, &bytes,
           &bytescount, NULL);
         errorcode = fwrite(bytes, 1, bytescount, fd);

       Note  that  the  serialized data is binary data that may contain any of
       the 256 possible byte  values.  On  systems  that  make  a  distinction
       between binary and non-binary data, be sure that the file is opened for
       binary output.

       Serializing a set of patterns leaves the original  data  untouched,  so
       they  can  still  be used for matching. Their memory must eventually be
       freed in the usual way by calling pcre2_code_free(). When you have fin-
       ished with the byte stream, it too must be freed by calling pcre2_seri-
       alize_free(). If this function is  called  with  a  NULL  argument,  it
       returns immediately without doing anything.


RE-USING PRECOMPILED PATTERNS

       In  order  to  re-use  a  set of saved patterns you must first make the
       serialized byte stream available in main memory (for example, by  read-
       ing  from  a  file).  The  management of this memory block is up to the
       application.  You  can  use  the  pcre2_serialize_get_number_of_codes()
       function  to  find out how many compiled patterns are in the serialized
       data without actually decoding the patterns:

         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes = pcre2_serialize_get_number_of_codes(bytes);

       The pcre2_serialize_decode() function reads a byte stream and recreates
       the compiled patterns in new memory blocks, setting pointers to them in
       a vector. The first two arguments are a pointer to  a  suitable  vector
       and  its  length,  and  the third argument points to a byte stream. The
       final argument is a pointer to a general context, which can be used  to
       specify  custom  memory mangagement functions for the decoded patterns.
       If this argument is NULL, malloc() and free() are used. After deserial-
       ization, the byte stream is no longer needed and can be discarded.

         int32_t number_of_codes;
         pcre2_code *list_of_codes[2];
         uint8_t *bytes = <serialized data>;
         int32_t number_of_codes =
           pcre2_serialize_decode(list_of_codes, 2, bytes, NULL);

       If  the  vector  is  not  large enough for all the patterns in the byte
       stream, it is filled  with  those  that  fit,  and  the  remainder  are
       ignored.  The  yield of the function is the number of decoded patterns,
       or one of the following negative error codes:

         PCRE2_ERROR_BADDATA    second argument is zero or less
         PCRE2_ERROR_BADMAGIC   mismatch of id bytes in the data
         PCRE2_ERROR_BADMODE    mismatch of code unit size or PCRE2 version
         PCRE2_ERROR_BADSERIALIZEDDATA  other sanity check failure
         PCRE2_ERROR_MEMORY     memory allocation failed
         PCRE2_ERROR_NULL       first or third argument is NULL

       PCRE2_ERROR_BADMAGIC may mean that the data is corrupt, or that it  was
       compiled on a system with different endianness.

       Decoded patterns can be used for matching in the usual way, and must be
       freed by calling pcre2_code_free(). However, be aware that there  is  a
       potential  race  issue  if  you  are  using multiple patterns that were
       decoded from a single byte stream in  a  multithreaded  application.  A
       single copy of the character tables is used by all the decoded patterns
       and a reference count is used to arrange for its memory to be automati-
       cally  freed when the last pattern is freed, but there is no locking on
       this reference count. Therefore, if you want to call  pcre2_code_free()
       for  these  patterns  in  different  threads, you must arrange your own
       locking, and ensure that pcre2_code_free()  cannot  be  called  by  two
       threads at the same time.

       If  a pattern was processed by pcre2_jit_compile() before being serial-
       ized, the JIT data is discarded and so is no longer available  after  a
       save/restore  cycle.  You can, however, process a restored pattern with
       pcre2_jit_compile() if you wish.


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 27 June 2018
       Copyright (c) 1997-2018 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2SYNTAX(3)             Library Functions Manual             PCRE2SYNTAX(3)



NAME
       PCRE2 - Perl-compatible regular expressions (revised API)

PCRE2 REGULAR EXPRESSION SYNTAX SUMMARY

       The  full syntax and semantics of the regular expressions that are sup-
       ported by PCRE2 are described in the pcre2pattern  documentation.  This
       document contains a quick-reference summary of the syntax.


QUOTING

         \x         where x is non-alphanumeric is a literal x
         \Q...\E    treat enclosed characters as literal


ESCAPED CHARACTERS

       This  table  applies to ASCII and Unicode environments. An unrecognized
       escape sequence causes an error.

         \a         alarm, that is, the BEL character (hex 07)
         \cx        "control-x", where x is any ASCII printing character
         \e         escape (hex 1B)
         \f         form feed (hex 0C)
         \n         newline (hex 0A)
         \r         carriage return (hex 0D)
         \t         tab (hex 09)
         \0dd       character with octal code 0dd
         \ddd       character with octal code ddd, or backreference
         \o{ddd..}  character with octal code ddd..
         \N{U+hh..} character with Unicode code point hh.. (Unicode mode only)
         \xhh       character with hex code hh
         \x{hh..}   character with hex code hh..

       If PCRE2_ALT_BSUX or PCRE2_EXTRA_ALT_BSUX is set ("ALT_BSUX mode"), the
       following are also recognized:

         \U         the character "U"
         \uhhhh     character with hex code hhhh
         \u{hh..}   character with hex code hh.. but only for EXTRA_ALT_BSUX

       When  \x  is not followed by {, from zero to two hexadecimal digits are
       read, but in ALT_BSUX mode \x must be followed by two hexadecimal  dig-
       its  to  be  recognized as a hexadecimal escape; otherwise it matches a
       literal "x".  Likewise, if \u (in ALT_BSUX mode)  is  not  followed  by
       four  hexadecimal  digits or (in EXTRA_ALT_BSUX mode) a sequence of hex
       digits in curly brackets, it matches a literal "u".

       Note that \0dd is always an octal code. The treatment of backslash fol-
       lowed  by  a non-zero digit is complicated; for details see the section
       "Non-printing characters"  in  the  pcre2pattern  documentation,  where
       details  of  escape  processing  in EBCDIC environments are also given.
       \N{U+hh..} is synonymous with \x{hh..} in PCRE2 but is not supported in
       EBCDIC  environments.  Note  that  \N  not followed by an opening curly
       bracket has a different meaning (see below).


CHARACTER TYPES

         .          any character except newline;
                      in dotall mode, any character whatsoever
         \C         one code unit, even in UTF mode (best avoided)
         \d         a decimal digit
         \D         a character that is not a decimal digit
         \h         a horizontal white space character
         \H         a character that is not a horizontal white space character
         \N         a character that is not a newline
         \p{xx}     a character with the xx property
         \P{xx}     a character without the xx property
         \R         a newline sequence
         \s         a white space character
         \S         a character that is not a white space character
         \v         a vertical white space character
         \V         a character that is not a vertical white space character
         \w         a "word" character
         \W         a "non-word" character
         \X         a Unicode extended grapheme cluster

       \C is dangerous because it may leave the current matching point in  the
       middle of a UTF-8 or UTF-16 character. The application can lock out the
       use of \C by setting the PCRE2_NEVER_BACKSLASH_C  option.  It  is  also
       possible to build PCRE2 with the use of \C permanently disabled.

       By  default,  \d, \s, and \w match only ASCII characters, even in UTF-8
       mode or in the 16-bit and 32-bit libraries. However, if locale-specific
       matching  is  happening,  \s and \w may also match characters with code
       points in the range 128-255. If the PCRE2_UCP option is set, the behav-
       iour of these escape sequences is changed to use Unicode properties and
       they match many more characters.


GENERAL CATEGORY PROPERTIES FOR \p and \P

         C          Other
         Cc         Control
         Cf         Format
         Cn         Unassigned
         Co         Private use
         Cs         Surrogate

         L          Letter
         Ll         Lower case letter
         Lm         Modifier letter
         Lo         Other letter
         Lt         Title case letter
         Lu         Upper case letter
         L&         Ll, Lu, or Lt

         M          Mark
         Mc         Spacing mark
         Me         Enclosing mark
         Mn         Non-spacing mark

         N          Number
         Nd         Decimal number
         Nl         Letter number
         No         Other number

         P          Punctuation
         Pc         Connector punctuation
         Pd         Dash punctuation
         Pe         Close punctuation
         Pf         Final punctuation
         Pi         Initial punctuation
         Po         Other punctuation
         Ps         Open punctuation

         S          Symbol
         Sc         Currency symbol
         Sk         Modifier symbol
         Sm         Mathematical symbol
         So         Other symbol

         Z          Separator
         Zl         Line separator
         Zp         Paragraph separator
         Zs         Space separator


PCRE2 SPECIAL CATEGORY PROPERTIES FOR \p and \P

         Xan        Alphanumeric: union of properties L and N
         Xps        POSIX space: property Z or tab, NL, VT, FF, CR
         Xsp        Perl space: property Z or tab, NL, VT, FF, CR
         Xuc        Univerally-named character: one that can be
                      represented by a Universal Character Name
         Xwd        Perl word: property Xan or underscore

       Perl and POSIX space are now the same. Perl added VT to its space char-
       acter set at release 5.18.


SCRIPT NAMES FOR \p AND \P

       Adlam,  Ahom,  Anatolian_Hieroglyphs,  Arabic, Armenian, Avestan, Bali-
       nese, Bamum, Bassa_Vah, Batak, Bengali,  Bhaiksuki,  Bopomofo,  Brahmi,
       Braille,  Buginese, Buhid, Canadian_Aboriginal, Carian, Caucasian_Alba-
       nian, Chakma,  Cham,  Cherokee,  Common,  Coptic,  Cuneiform,  Cypriot,
       Cyrillic,  Deseret,  Devanagari, Dogra, Duployan, Egyptian_Hieroglyphs,
       Elbasan,  Ethiopic,  Georgian,  Glagolitic,  Gothic,  Grantha,   Greek,
       Gujarati,   Gunjala_Gondi,   Gurmukhi,  Han,  Hangul,  Hanifi_Rohingya,
       Hanunoo,  Hatran,  Hebrew,   Hiragana,   Imperial_Aramaic,   Inherited,
       Inscriptional_Pahlavi,  Inscriptional_Parthian,  Javanese, Kaithi, Kan-
       nada, Katakana, Kayah_Li, Kharoshthi, Khmer,  Khojki,  Khudawadi,  Lao,
       Latin,  Lepcha,  Limbu, Linear_A, Linear_B, Lisu, Lycian, Lydian, Maha-
       jani, Makasar, Malayalam, Mandaic, Manichaean, Marchen,  Masaram_Gondi,
       Medefaidrin,     Meetei_Mayek,     Mende_Kikakui,     Meroitic_Cursive,
       Meroitic_Hieroglyphs, Miao, Modi,  Mongolian,  Mro,  Multani,  Myanmar,
       Nabataean,  New_Tai_Lue, Newa, Nko, Nushu, Ogham, Ol_Chiki, Old_Hungar-
       ian, Old_Italic, Old_North_Arabian, Old_Permic,  Old_Persian,  Old_Sog-
       dian,    Old_South_Arabian,    Old_Turkic,   Oriya,   Osage,   Osmanya,
       Pahawh_Hmong,    Palmyrene,    Pau_Cin_Hau,    Phags_Pa,    Phoenician,
       Psalter_Pahlavi,  Rejang,  Runic,  Samaritan, Saurashtra, Sharada, Sha-
       vian, Siddham, SignWriting, Sinhala,  Sogdian,  Sora_Sompeng,  Soyombo,
       Sundanese,  Syloti_Nagri,  Syriac, Tagalog, Tagbanwa, Tai_Le, Tai_Tham,
       Tai_Viet, Takri, Tamil, Tangut, Telugu, Thaana,  Thai,  Tibetan,  Tifi-
       nagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi, Zanabazar_Square.


CHARACTER CLASSES

         [...]       positive character class
         [^...]      negative character class
         [x-y]       range (can be used for hex characters)
         [[:xxx:]]   positive POSIX named set
         [[:^xxx:]]  negative POSIX named set

         alnum       alphanumeric
         alpha       alphabetic
         ascii       0-127
         blank       space or tab
         cntrl       control character
         digit       decimal digit
         graph       printing, excluding space
         lower       lower case letter
         print       printing, including space
         punct       printing, excluding alphanumeric
         space       white space
         upper       upper case letter
         word        same as \w
         xdigit      hexadecimal digit

       In  PCRE2, POSIX character set names recognize only ASCII characters by
       default, but some of them use Unicode properties if PCRE2_UCP  is  set.
       You can use \Q...\E inside a character class.


QUANTIFIERS

         ?           0 or 1, greedy
         ?+          0 or 1, possessive
         ??          0 or 1, lazy
         *           0 or more, greedy
         *+          0 or more, possessive
         *?          0 or more, lazy
         +           1 or more, greedy
         ++          1 or more, possessive
         +?          1 or more, lazy
         {n}         exactly n
         {n,m}       at least n, no more than m, greedy
         {n,m}+      at least n, no more than m, possessive
         {n,m}?      at least n, no more than m, lazy
         {n,}        n or more, greedy
         {n,}+       n or more, possessive
         {n,}?       n or more, lazy


ANCHORS AND SIMPLE ASSERTIONS

         \b          word boundary
         \B          not a word boundary
         ^           start of subject
                       also after an internal newline in multiline mode
                       (after any newline if PCRE2_ALT_CIRCUMFLEX is set)
         \A          start of subject
         $           end of subject
                       also before newline at end of subject
                       also before internal newline in multiline mode
         \Z          end of subject
                       also before newline at end of subject
         \z          end of subject
         \G          first matching position in subject


REPORTED MATCH POINT SETTING

         \K          set reported start of match

       \K is honoured in positive assertions, but ignored in negative ones.


ALTERNATION

         expr|expr|expr...


CAPTURING

         (...)           capture group
         (?<name>...)    named capture group (Perl)
         (?'name'...)    named capture group (Perl)
         (?P<name>...)   named capture group (Python)
         (?:...)         non-capture group
         (?|...)         non-capture group; reset group numbers for
                          capture groups in each alternative

       In  non-UTF  modes, names may contain underscores and ASCII letters and
       digits; in UTF modes, any Unicode letters and  Unicode  decimal  digits
       are permitted. In both cases, a name must not start with a digit.


ATOMIC GROUPS

         (?>...)         atomic non-capture group
         (*atomic:...)   atomic non-capture group


COMMENT

         (?#....)        comment (not nestable)


OPTION SETTING
       Changes  of these options within a group are automatically cancelled at
       the end of the group.

         (?i)            caseless
         (?J)            allow duplicate names
         (?m)            multiline
         (?n)            no auto capture
         (?s)            single line (dotall)
         (?U)            default ungreedy (lazy)
         (?x)            extended: ignore white space except in classes
         (?xx)           as (?x) but also ignore space and tab in classes
         (?-...)         unset option(s)
         (?^)            unset imnsx options

       Unsetting x or xx unsets both. Several options may be set at once,  and
       a mixture of setting and unsetting such as (?i-x) is allowed, but there
       may be only one hyphen. Setting (but no unsetting) is allowed after (?^
       for example (?^in). An option setting may appear at the start of a non-
       capture group, for example (?i:...).

       The following are recognized only at the very start  of  a  pattern  or
       after  one  of the newline or \R options with similar syntax. More than
       one of them may appear. For the first three, d is a decimal number.

         (*LIMIT_DEPTH=d) set the backtracking limit to d
         (*LIMIT_HEAP=d)  set the heap size limit to d * 1024 bytes
         (*LIMIT_MATCH=d) set the match limit to d
         (*NOTEMPTY)      set PCRE2_NOTEMPTY when matching
         (*NOTEMPTY_ATSTART) set PCRE2_NOTEMPTY_ATSTART when matching
         (*NO_AUTO_POSSESS) no auto-possessification (PCRE2_NO_AUTO_POSSESS)
         (*NO_DOTSTAR_ANCHOR) no .* anchoring (PCRE2_NO_DOTSTAR_ANCHOR)
         (*NO_JIT)       disable JIT optimization
         (*NO_START_OPT) no start-match optimization (PCRE2_NO_START_OPTIMIZE)
         (*UTF)          set appropriate UTF mode for the library in use
         (*UCP)          set PCRE2_UCP (use Unicode properties for \d etc)

       Note that LIMIT_DEPTH, LIMIT_HEAP, and LIMIT_MATCH can only reduce  the
       value   of   the   limits   set  by  the  caller  of  pcre2_match()  or
       pcre2_dfa_match(), not increase them. LIMIT_RECURSION  is  an  obsolete
       synonym for LIMIT_DEPTH. The application can lock out the use of (*UTF)
       and (*UCP) by setting the PCRE2_NEVER_UTF or  PCRE2_NEVER_UCP  options,
       respectively, at compile time.


NEWLINE CONVENTION

       These  are  recognized  only  at the very start of the pattern or after
       option settings with a similar syntax.

         (*CR)           carriage return only
         (*LF)           linefeed only
         (*CRLF)         carriage return followed by linefeed
         (*ANYCRLF)      all three of the above
         (*ANY)          any Unicode newline sequence
         (*NUL)          the NUL character (binary zero)


WHAT \R MATCHES

       These are recognized only at the very start of  the  pattern  or  after
       option setting with a similar syntax.

         (*BSR_ANYCRLF)  CR, LF, or CRLF
         (*BSR_UNICODE)  any Unicode newline sequence


LOOKAHEAD AND LOOKBEHIND ASSERTIONS

         (?=...)                     )
         (*pla:...)                  ) positive lookahead
         (*positive_lookahead:...)   )

         (?!...)                     )
         (*nla:...)                  ) negative lookahead
         (*negative_lookahead:...)   )

         (?<=...)                    )
         (*plb:...)                  ) positive lookbehind
         (*positive_lookbehind:...)  )

         (?<!...)                    )
         (*nlb:...)                  ) negative lookbehind
         (*negative_lookbehind:...)  )

       Each top-level branch of a lookbehind must be of a fixed length.


SCRIPT RUNS

         (*script_run:...)           ) script run, can be backtracked into
         (*sr:...)                   )

         (*atomic_script_run:...)    ) atomic script run
         (*asr:...)                  )


BACKREFERENCES

         \n              reference by number (can be ambiguous)
         \gn             reference by number
         \g{n}           reference by number
         \g+n            relative reference by number (PCRE2 extension)
         \g-n            relative reference by number
         \g{+n}          relative reference by number (PCRE2 extension)
         \g{-n}          relative reference by number
         \k<name>        reference by name (Perl)
         \k'name'        reference by name (Perl)
         \g{name}        reference by name (Perl)
         \k{name}        reference by name (.NET)
         (?P=name)       reference by name (Python)


SUBROUTINE REFERENCES (POSSIBLY RECURSIVE)

         (?R)            recurse whole pattern
         (?n)            call subroutine by absolute number
         (?+n)           call subroutine by relative number
         (?-n)           call subroutine by relative number
         (?&name)        call subroutine by name (Perl)
         (?P>name)       call subroutine by name (Python)
         \g<name>        call subroutine by name (Oniguruma)
         \g'name'        call subroutine by name (Oniguruma)
         \g<n>           call subroutine by absolute number (Oniguruma)
         \g'n'           call subroutine by absolute number (Oniguruma)
         \g<+n>          call subroutine by relative number (PCRE2 extension)
         \g'+n'          call subroutine by relative number (PCRE2 extension)
         \g<-n>          call subroutine by relative number (PCRE2 extension)
         \g'-n'          call subroutine by relative number (PCRE2 extension)


CONDITIONAL PATTERNS

         (?(condition)yes-pattern)
         (?(condition)yes-pattern|no-pattern)

         (?(n)               absolute reference condition
         (?(+n)              relative reference condition
         (?(-n)              relative reference condition
         (?(<name>)          named reference condition (Perl)
         (?('name')          named reference condition (Perl)
         (?(name)            named reference condition (PCRE2, deprecated)
         (?(R)               overall recursion condition
         (?(Rn)              specific numbered group recursion condition
         (?(R&name)          specific named group recursion condition
         (?(DEFINE)          define groups for reference
         (?(VERSION[>]=n.m)  test PCRE2 version
         (?(assert)          assertion condition

       Note  the  ambiguity of (?(R) and (?(Rn) which might be named reference
       conditions or recursion tests. Such a condition  is  interpreted  as  a
       reference condition if the relevant named group exists.


BACKTRACKING CONTROL

       All  backtracking  control  verbs  may be in the form (*VERB:NAME). For
       (*MARK) the name is mandatory, for the others it is  optional.  (*SKIP)
       changes  its  behaviour if :NAME is present. The others just set a name
       for passing back to the caller, but this is not a name that (*SKIP) can
       see. The following act immediately they are reached:

         (*ACCEPT)       force successful match
         (*FAIL)         force backtrack; synonym (*F)
         (*MARK:NAME)    set name to be passed back; synonym (*:NAME)

       The  following  act only when a subsequent match failure causes a back-
       track to reach them. They all force a match failure, but they differ in
       what happens afterwards. Those that advance the start-of-match point do
       so only if the pattern is not anchored.

         (*COMMIT)       overall failure, no advance of starting point
         (*PRUNE)        advance to next starting character
         (*SKIP)         advance to current matching position
         (*SKIP:NAME)    advance to position corresponding to an earlier
                         (*MARK:NAME); if not found, the (*SKIP) is ignored
         (*THEN)         local failure, backtrack to next alternation

       The effect of one of these verbs in a group called as a  subroutine  is
       confined to the subroutine call.


CALLOUTS

         (?C)            callout (assumed number 0)
         (?Cn)           callout with numerical data n
         (?C"text")      callout with string data

       The allowed string delimiters are ` ' " ^ % # $ (which are the same for
       the start and the end), and the starting delimiter { matched  with  the
       ending  delimiter  }. To encode the ending delimiter within the string,
       double it.


SEE ALSO

       pcre2pattern(3),   pcre2api(3),   pcre2callout(3),    pcre2matching(3),
       pcre2(3).


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 11 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
------------------------------------------------------------------------------
 
 
PCRE2UNICODE(3)            Library Functions Manual            PCRE2UNICODE(3)



NAME
       PCRE - Perl-compatible regular expressions (revised API)

UNICODE AND UTF SUPPORT

       When PCRE2 is built with Unicode support (which is the default), it has
       knowledge of Unicode character properties and can process text  strings
       in  UTF-8, UTF-16, or UTF-32 format (depending on the code unit width).
       However, by default, PCRE2 assumes that one code unit is one character.
       To  process  a  pattern  as a UTF string, where a character may require
       more than one  code  unit,  you  must  call  pcre2_compile()  with  the
       PCRE2_UTF  option  flag,  or  the  pattern must start with the sequence
       (*UTF). When either of these is the case, both the pattern and any sub-
       ject  strings  that  are  matched against it are treated as UTF strings
       instead of strings of individual one-code-unit  characters.  There  are
       also  some  other  changes  to the way characters are handled, as docu-
       mented below.

       If you do not need Unicode support you can build PCRE2 without  it,  in
       which case the library will be smaller.


UNICODE PROPERTY SUPPORT

       When  PCRE2 is built with Unicode support, the escape sequences \p{..},
       \P{..}, and \X can be used. This is not dependent on the PCRE2_UTF set-
       ting.   The  Unicode  properties  that can be tested are limited to the
       general category properties such as Lu for an upper case letter  or  Nd
       for  a  decimal number, the Unicode script names such as Arabic or Han,
       and the derived properties Any and L&. Full  lists  are  given  in  the
       pcre2pattern  and  pcre2syntax  documentation. Only the short names for
       properties are supported. For example, \p{L} matches a letter. Its Perl
       synonym,  \p{Letter},  is  not  supported.   Furthermore, in Perl, many
       properties may optionally be prefixed by "Is", for  compatibility  with
       Perl 5.6. PCRE2 does not support this.


WIDE CHARACTERS AND UTF MODES

       Code points less than 256 can be specified in patterns by either braced
       or unbraced hexadecimal escape sequences (for example, \x{b3} or \xb3).
       Larger  values have to use braced sequences. Unbraced octal code points
       up to \777 are also recognized; larger ones can be coded using \o{...}.

       The escape sequence \N{U+<hex digits>} is recognized as another way  of
       specifying  a  Unicode character by code point in a UTF mode. It is not
       allowed in non-UTF modes.

       In UTF modes, repeat quantifiers apply to complete UTF characters,  not
       to individual code units.

       In  UTF  modes, the dot metacharacter matches one UTF character instead
       of a single code unit.

       In UTF modes, capture group names are not restricted to ASCII, and  may
       contain any Unicode letters and decimal digits, as well as underscore.

       The escape sequence \C can be used to match a single code unit in a UTF
       mode, but its use can lead to some strange effects because it breaks up
       multi-unit  characters  (see  the description of \C in the pcre2pattern
       documentation). For this reason, there is a build-time option that dis-
       ables  support  for  \C completely. There is also a less draconian com-
       pile-time option for locking out the use of \C when a pattern  is  com-
       piled.

       The  use  of  \C  is not supported by the alternative matching function
       pcre2_dfa_match() when in UTF-8 or UTF-16 mode, that is, when a charac-
       ter  may  consist  of  more  than one code unit. The use of \C in these
       modes provokes a match-time error. Also, the JIT optimization does  not
       support \C in these modes. If JIT optimization is requested for a UTF-8
       or UTF-16 pattern that contains \C, it will not succeed,  and  so  when
       pcre2_match() is called, the matching will be carried out by the normal
       interpretive function.

       The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly test
       characters  of  any  code  value,  but, by default, the characters that
       PCRE2 recognizes as digits, spaces, or word characters remain the  same
       set  as  in  non-UTF  mode,  all  with  code points less than 256. This
       remains true even when PCRE2  is  built  to  include  Unicode  support,
       because  to do otherwise would slow down matching in many common cases.
       Note that this also applies to \b and \B, because they are  defined  in
       terms  of  \w  and  \W.  If you want to test for a wider sense of, say,
       "digit", you can use explicit Unicode property tests  such  as  \p{Nd}.
       Alternatively,  if you set the PCRE2_UCP option, the way that the char-
       acter escapes work is changed so that Unicode properties  are  used  to
       determine which characters match. There are more details in the section
       on generic character types in the pcre2pattern documentation.

       Similarly, characters that match the POSIX named character classes  are
       all low-valued characters, unless the PCRE2_UCP option is set.

       However,  the  special  horizontal  and  vertical  white space matching
       escapes (\h, \H, \v, and \V) do match all the appropriate Unicode char-
       acters, whether or not PCRE2_UCP is set.


CASE-EQUIVALENCE IN UTF MODES

       Case-insensitive matching in a UTF mode makes use of Unicode properties
       except for characters whose code points are less than 128 and that have
       at most two case-equivalent values. For these, a direct table lookup is
       used for speed. A few Unicode characters such as Greek sigma have  more
       than two code points that are case-equivalent, and these are treated as
       such.


SCRIPT RUNS

       The pattern constructs (*script_run:...) and  (*atomic_script_run:...),
       with  synonyms (*sr:...) and (*asr:...), verify that the string matched
       within the parentheses is a script run. In concept, a script run  is  a
       sequence  of characters that are all from the same Unicode script. How-
       ever, because some scripts are commonly used together, and because some
       diacritical  and  other marks are used with multiple scripts, it is not
       that simple.

       Every Unicode character has a Script property, mostly with a value cor-
       responding  to the name of a script, such as Latin, Greek, or Cyrillic.
       There are also three special values:

       "Unknown" is used for code points that have not been assigned, and also
       for  the surrogate code points. In the PCRE2 32-bit library, characters
       whose code points are greater  than  the  Unicode  maximum  (U+10FFFF),
       which  are  accessible  only  in non-UTF mode, are assigned the Unknown
       script.

       "Common" is used for characters that are used with many scripts.  These
       include  punctuation,  emoji,  mathematical, musical, and currency sym-
       bols, and the ASCII digits 0 to 9.

       "Inherited" is used for characters such as diacritical marks that  mod-
       ify a previous character. These are considered to take on the script of
       the character that they modify.

       Some Inherited characters are used with many scripts, but many of  them
       are  only  normally  used  with a small number of scripts. For example,
       U+102E0 (Coptic Epact thousands mark) is used only with Arabic and Cop-
       tic.  In  order  to  make it possible to check this, a Unicode property
       called Script Extension exists. Its value is a  list  of  scripts  that
       apply  to  the character. For the majority of characters, the list con-
       tains just one script, the same one as the  Script  property.  However,
       for  characters  such  as U+102E0 more than one Script is listed. There
       are also some Common characters that have a single,  non-Common  script
       in their Script Extension list.

       The next section describes the basic rules for deciding whether a given
       string of characters is a script run. Note,  however,  that  there  are
       some  special cases involving the Chinese Han script, and an additional
       constraint for decimal digits. These are  covered  in  subsequent  sec-
       tions.

   Basic script run rules

       A string that is less than two characters long is a script run. This is
       the only case in which an Unknown character can be  part  of  a  script
       run.  Longer strings are checked using only the Script Extensions prop-
       erty, not the basic Script property.

       If a character's Script Extension property is the single value  "Inher-
       ited", it is always accepted as part of a script run. This is also true
       for the property "Common", subject to the checking  of  decimal  digits
       described below. All the remaining characters in a script run must have
       at least one script in common in their Script Extension lists. In  set-
       theoretic terminology, the intersection of all the sets of scripts must
       not be empty.

       A simple example is an Internet name such as "google.com". The  letters
       are all in the Latin script, and the dot is Common, so this string is a
       script run.  However, the Cyrillic letter "o" looks exactly the same as
       the  Latin "o"; a string that looks the same, but with Cyrillic "o"s is
       not a script run.

       More interesting examples involve characters with more than one  script
       in their Script Extension. Consider the following characters:

         U+060C  Arabic comma
         U+06D4  Arabic full stop

       The  first  has the Script Extension list Arabic, Hanifi Rohingya, Syr-
       iac, and Thaana; the second has just Arabic and Hanifi  Rohingya.  Both
       of  them  could  appear  in  script  runs  of  either  Arabic or Hanifi
       Rohingya. The first could also appear in Syriac or Thaana script  runs,
       but the second could not.

   The Chinese Han script

       The  Chinese  Han  script  is  commonly  used in conjunction with other
       scripts for writing certain languages. Japanese uses the  Hiragana  and
       Katakana  scripts  together  with Han; Korean uses Hangul and Han; Tai-
       wanese Mandarin uses Bopomofo and Han.  These  three  combinations  are
       treated  as special cases when checking script runs and are, in effect,
       "virtual scripts". Thus, a script run may contain a  mixture  of  Hira-
       gana,  Katakana,  and Han, or a mixture of Hangul and Han, or a mixture
       of Bopomofo and Han, but not, for example,  a  mixture  of  Hangul  and
       Bopomofo  and  Han. PCRE2 (like Perl) follows Unicode's Technical Stan-
       dard     39     ("Unicode     Security     Mechanisms",     http://uni-
       code.org/reports/tr39/) in allowing such mixtures.

   Decimal digits

       Unicode  contains  many sets of 10 decimal digits in different scripts,
       and some scripts (including the Common script) contain  more  than  one
       set.  Some  of these decimal digits them are visually indistinguishable
       from the common ASCII  digits.  In  addition  to  the  script  checking
       described above, if a script run contains any decimal digits, they must
       all come from the same set of 10 adjacent characters.


VALIDITY OF UTF STRINGS

       When the PCRE2_UTF option is set, the strings passed  as  patterns  and
       subjects are (by default) checked for validity on entry to the relevant
       functions.  If an invalid UTF string is passed, an negative error  code
       is  returned.  The  code  unit offset to the offending character can be
       extracted from the match data block by  calling  pcre2_get_startchar(),
       which is used for this purpose after a UTF error.

       UTF-16 and UTF-32 strings can indicate their endianness by special code
       knows as a byte-order mark (BOM). The PCRE2  functions  do  not  handle
       this, expecting strings to be in host byte order.

       A UTF string is checked before any other processing takes place. In the
       case of pcre2_match()  and  pcre2_dfa_match()  calls  with  a  non-zero
       starting  offset, the check is applied only to that part of the subject
       that could be inspected during matching, and there is a check that  the
       starting  offset points to the first code unit of a character or to the
       end of the subject. If there are no lookbehind assertions in  the  pat-
       tern,  the check starts at the starting offset. Otherwise, it starts at
       the length of the longest lookbehind before the starting offset, or  at
       the  start  of the subject if there are not that many characters before
       the starting offset. Note that the sequences \b and \B are  one-charac-
       ter lookbehinds.

       In  addition  to checking the format of the string, there is a check to
       ensure that all code points lie in the range U+0 to U+10FFFF, excluding
       the  surrogate  area. The so-called "non-character" code points are not
       excluded because Unicode corrigendum #9 makes it clear that they should
       not be.

       Characters  in  the "Surrogate Area" of Unicode are reserved for use by
       UTF-16, where they are used in pairs to encode code points with  values
       greater  than  0xFFFF. The code points that are encoded by UTF-16 pairs
       are available independently in the  UTF-8  and  UTF-32  encodings.  (In
       other  words,  the  whole  surrogate  thing is a fudge for UTF-16 which
       unfortunately messes up UTF-8 and UTF-32.)

       In some situations, you may already know that your strings  are  valid,
       and  therefore  want  to  skip these checks in order to improve perfor-
       mance, for example in the case of a long subject string that  is  being
       scanned  repeatedly.   If you set the PCRE2_NO_UTF_CHECK option at com-
       pile time or at match time, PCRE2 assumes that the pattern  or  subject
       it is given (respectively) contains only valid UTF code unit sequences.

       Passing  PCRE2_NO_UTF_CHECK  to pcre2_compile() just disables the check
       for the pattern; it does not also apply to subject strings. If you want
       to  disable the check for a subject string you must pass this option to
       pcre2_match() or pcre2_dfa_match().

       If you pass an invalid UTF string when PCRE2_NO_UTF_CHECK is  set,  the
       result is undefined and your program may crash or loop indefinitely.

       Note  that  setting PCRE2_NO_UTF_CHECK at compile time does not disable
       the error that is given if an escape sequence for  an  invalid  Unicode
       code  point  is encountered in the pattern. If you want to allow escape
       sequences such as \x{d800} (a surrogate code point)  you  can  set  the
       PCRE2_EXTRA_ALLOW_SURROGATE_ESCAPES extra option. However, this is pos-
       sible only in UTF-8 and UTF-32 modes, because these values are not rep-
       resentable in UTF-16.

   Errors in UTF-8 strings

       The following negative error codes are given for invalid UTF-8 strings:

         PCRE2_ERROR_UTF8_ERR1
         PCRE2_ERROR_UTF8_ERR2
         PCRE2_ERROR_UTF8_ERR3
         PCRE2_ERROR_UTF8_ERR4
         PCRE2_ERROR_UTF8_ERR5

       The  string  ends  with a truncated UTF-8 character; the code specifies
       how many bytes are missing (1 to 5). Although RFC 3629 restricts  UTF-8
       characters  to  be  no longer than 4 bytes, the encoding scheme (origi-
       nally defined by RFC 2279) allows for  up  to  6  bytes,  and  this  is
       checked first; hence the possibility of 4 or 5 missing bytes.

         PCRE2_ERROR_UTF8_ERR6
         PCRE2_ERROR_UTF8_ERR7
         PCRE2_ERROR_UTF8_ERR8
         PCRE2_ERROR_UTF8_ERR9
         PCRE2_ERROR_UTF8_ERR10

       The two most significant bits of the 2nd, 3rd, 4th, 5th, or 6th byte of
       the character do not have the binary value 0b10 (that  is,  either  the
       most significant bit is 0, or the next bit is 1).

         PCRE2_ERROR_UTF8_ERR11
         PCRE2_ERROR_UTF8_ERR12

       A  character that is valid by the RFC 2279 rules is either 5 or 6 bytes
       long; these code points are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR13

       A 4-byte character has a value greater than 0x10fff; these code  points
       are excluded by RFC 3629.

         PCRE2_ERROR_UTF8_ERR14

       A  3-byte  character  has  a  value in the range 0xd800 to 0xdfff; this
       range of code points are reserved by RFC 3629 for use with UTF-16,  and
       so are excluded from UTF-8.

         PCRE2_ERROR_UTF8_ERR15
         PCRE2_ERROR_UTF8_ERR16
         PCRE2_ERROR_UTF8_ERR17
         PCRE2_ERROR_UTF8_ERR18
         PCRE2_ERROR_UTF8_ERR19

       A  2-, 3-, 4-, 5-, or 6-byte character is "overlong", that is, it codes
       for a value that can be represented by fewer bytes, which  is  invalid.
       For  example,  the two bytes 0xc0, 0xae give the value 0x2e, whose cor-
       rect coding uses just one byte.

         PCRE2_ERROR_UTF8_ERR20

       The two most significant bits of the first byte of a character have the
       binary  value 0b10 (that is, the most significant bit is 1 and the sec-
       ond is 0). Such a byte can only validly occur as the second  or  subse-
       quent byte of a multi-byte character.

         PCRE2_ERROR_UTF8_ERR21

       The  first byte of a character has the value 0xfe or 0xff. These values
       can never occur in a valid UTF-8 string.

   Errors in UTF-16 strings

       The following  negative  error  codes  are  given  for  invalid  UTF-16
       strings:

         PCRE2_ERROR_UTF16_ERR1  Missing low surrogate at end of string
         PCRE2_ERROR_UTF16_ERR2  Invalid low surrogate follows high surrogate
         PCRE2_ERROR_UTF16_ERR3  Isolated low surrogate


   Errors in UTF-32 strings

       The  following  negative  error  codes  are  given  for  invalid UTF-32
       strings:

         PCRE2_ERROR_UTF32_ERR1  Surrogate character (0xd800 to 0xdfff)
         PCRE2_ERROR_UTF32_ERR2  Code point is greater than 0x10ffff


AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.


REVISION

       Last updated: 03 February 2019
       Copyright (c) 1997-2019 University of Cambridge.
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