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-----------------------------------------------------------------------------
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. Some features that appeared in Python and the origi-
nal PCRE before they appeared in Perl are also available using the
Python syntax. There is also some support for one or two .NET and Onig-
uruma 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 callout feature
pcre2compat discussion of Perl compatibility
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
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: 01 April 2017
Copyright (c) 1997-2017 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. This document contains a description of
all its functions. See the pcre2 document 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_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_offset_limit(pcre2_match_context *mcontext,
PCRE2_SIZE 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 no longer has any effect (it always returns zero).
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. They are described in the pcre2posix documen-
tation. 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 contains def-
initions 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 the memory used for extracted strings.
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. The default default is LF, which is the Unix stan-
dard. 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.
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);
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
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_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), or PCRE2_NEWLINE_ANY (any Unicode newline sequence).
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 option, the newline
convention affects the recognition of white space and the end of inter-
nal comments starting with #. The value is saved with the compiled pat-
tern 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 ajusts 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 compiled. The
limit applies to parentheses of all kinds, not just capturing parenthe-
ses.
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. 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 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_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. For example, if the pattern /abc/ is matched
against "123abc" with an offset limit less than 3, the result is
PCRE2_ERROR_NO_MATCH. A match can never be found if the startoffset
argument of pcre2_match() or pcre2_dfa_match() is greater than the off-
set limit.
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. See also the PCRE2_FIRSTLINE option, which
requires a match to start within the first line of 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, whichever limit comes
first is used.
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 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 unlim-
ited 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 is not rele-
vant to pcre2_dfa_match(), which ignores it.
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, 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.
This 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 internal recursive function calls that
implement lookaround assertions and pattern recursions. This is, there-
fore, an indirect limit on the amount of system stack that is used. A
recursive pattern such as /(.)(?1)/, when matched to a very long string
using pcre2_dfa_match(), can use a great deal of stack.
The default value for the depth limit can be set when PCRE2 is built;
the default default is 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_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 and lookarounds in pcre2_dfa_match(). Further details are
given with pcre2_set_depth_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 64K 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
The default should normally correspond to the standard sequence for
your operating system.
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. 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.
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.
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.
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 running a match, you must not free a compiled pattern (or a sub-
ject string) until after all operations on the match data block have
taken place.
The options argument for pcre2_compile() contains various bit settings
that affect the compilation. It should be zero if no options 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.
Other, less frequently required compile-time parameters (for example,
the newline setting) can be provided in a compile 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.
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.
The pcre2_get_error_message() function (see "Obtaining a textual error
message" below) provides a textual message for each error code. Compi-
lation errors have positive error codes; UTF formatting error codes are
negative. For an invalid UTF-8 or UTF-16 string, the offset 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 */
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).
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
option is set, unescaped whitespace in verb names is skipped and #-com-
ments are recognized in this mode, exactly as in the rest of the pat-
tern.
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.
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, independent
of the setting of this option.
PCRE2_DUPNAMES
If this bit is set, names used to identify capturing subpatterns need
not be unique. This can be helpful for certain types of pattern when it
is known that only one instance of the named subpattern can ever be
matched. There are more details of named subpatterns 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"). This effect
can also be achieved by appropriate constructs in the pattern itself,
which is the only way to do it in Perl.
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 subpatterns, nor within numerical quan-
tifiers such as {1,3}. Ignorable white space is permitted between an
item and a following quantifier and between a quantifier and a follow-
ing + that indicates possessiveness.
PCRE2_EXTENDED also causes characters 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. PCRE2_EXTENDED is equivalent to Perl's /x option, and it can be
changed within a pattern by a (?x) option setting.
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_FIRSTLINE
If this option is set, an unanchored pattern is required to match
before or at the first newline in the subject string, though the
matched text may continue over the newline. See also PCRE2_USE_OFF-
SET_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_MATCH_UNSET_BACKREF
If this option is set, a back reference to an unset subpattern 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).
There is no equivalent of this option in Perl. Note that, if this
option is set, references to capturing groups (back references 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 capturing group that is the subject of a back refer-
ence, or if the pattern contains (*PRUNE) or (*SKIP). When the opti-
mization 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 character, the
matching code searches the subject for that character, 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 valid, 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 pat-
tern 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 validity checking of the subject string.
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 this option changes the behaviour of PCRE2 are given in
the pcre2unicode page.
COMPILATION ERROR CODES
There are nearly 100 positive error codes that pcre2_compile() may
return (via errorcode) if it finds an error in the pattern. There are
also some negative 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. The pcre2_get_error_message()
function (see "Obtaining a textual error message" below) can be called
to obtain a textual error message from any error code.
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
Return a copy 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.
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 capturing group that is the subject
of a back reference
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 back reference in the pattern. The
third argument should point to an uint32_t variable. Named subpatterns
acquire numbers as well as names, and these count towards the highest
back reference. Back references such as \4 or \g{12} match the cap-
tured characters of the given group, but in addition, the check that a
capturing group is set in a conditional subpattern such as (?(3)a|b) is
also a back reference. Zero is returned if there are no back refer-
ences.
PCRE2_INFO_BSR
The output is a uint32_t 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_ANY-
CRLF means that \R matches only CR, LF, or CRLF.
PCRE2_INFO_CAPTURECOUNT
Return the highest capturing subpattern number in the pattern. In pat-
terns where (?| is not used, this is also the total number of capturing
subpatterns. 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 an unsigned 32-bit integer. If no such
value has been set, the call to pcre2_pattern_info() returns the error
PCRE2_ERROR_UNSET.
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 an 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 charac-
ter 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 in the
situation 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 an size_t
variable. The frame size depends on the number of capturing parentheses
in the pattern. Each additional capturing group adds two PCRE2_SIZE
variables.
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_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 data unit that must exist in
any matched string, other than at its start, if such a value has been
recorded. The third argument should point to an uint32_t variable. If
there is no such value, 0 is returned.
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 an unsigned 32-bit integer. If no such value
has been set, the call to pcre2_pattern_info() returns the error
PCRE2_ERROR_UNSET.
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 an
unsigned 32-bit 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
registers 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 pro-
cessed. Otherwise, if there are no lookbehinds in the pattern, \A might
match incorrectly at the start of a new 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
groups with the same number, as described in the section on duplicate
subpattern numbers in the pcre2pattern page, the groups may be given
the same name, but there is only one entry in the table. Different
names for groups of the same number are not permitted.
Duplicate names for subpatterns with different numbers are permitted,
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 subpatterns may have lower numbers.
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 subpatterns, 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 subpatterns 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
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 functions
whose names begin with pcre2_serialize_ are used for this purpose. They
are described in the pcre2serialize documentation.
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 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.
When a match data block itself is no longer needed, it should be freed
by calling pcre2_match_data_free().
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 */
match_data, /* 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 zeroes.
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,
an single attempt to match at the given offset is made. This can only
succeed 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_ENDANCHORED,
PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
PCRE2_NO_JIT, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PAR-
TIAL_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_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 "capturing subpattern" or "capturing group" is used for a frag-
ment of a pattern that picks out a substring. PCRE2 supports several
other kinds of parenthesized subpattern that do not cause substrings to
be captured. The pcre2_pattern_info() function can be used to find out
how many capturing subpatterns 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 capturing subpattern group is matched repeatedly within a single
match operation, 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 capturing subpattern number n+1 to match some part
of the subject when subpattern 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 subpatterns 1 and 3 are matched, but
2 is not. When this happens, both values in the offset pairs corre-
sponding to unused subpatterns are set to PCRE2_UNSET.
Offset values that correspond to unused subpatterns 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)?)? subpatterns 2 and 3
are not matched. The return from the function is 2, because the high-
est used capturing subpattern number is 1. The offsets for for the sec-
ond and third capturing subpatterns (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.
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 avail-
able, and pcre2_get_mark() can be called. It returns a pointer to the
zero-terminated name, which is within the compiled pattern. Otherwise
NULL is returned. The length of the (*MARK) name (excluding the termi-
nating zero) is stored in the code unit that preceeds the name. You
should use this instead of relying on the terminating zero if the
(*MARK) name might contain a binary zero.
After a successful match, the (*MARK) name that is returned is the last
one encountered on the matching path through the pattern. After a "no
match" or a partial match, the last encountered (*MARK) name is
returned. For example, consider this pattern:
^(*MARK:A)((*MARK:B)a|b)c
When it matches "bc", the returned mark 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
mark is B.
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_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.
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 subpattern 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
subpatterns, 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
capturing 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 capturing subpattern number n+1 matches some part of the subject,
but subpattern 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_sub-
string_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 subpattern 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 subpattern number, PCRE2_ERROR_NOSUBSTRING if there
is no subpattern of that name, or PCRE2_ERROR_NOUNIQUESUBSTRING if
there is more than one subpattern of 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 first named string 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 subpat-
terns with the same number, as described in the section on duplicate
subpattern numbers in the pcre2pattern page, you cannot use names to
distinguish the different subpatterns, because names are not included
in the compiled code. The matching process uses only numbers. For this
reason, the use of different names for subpatterns of 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 *outputbufferP,
PCRE2_SIZE *outlengthptr);
This function calls pcre2_match() and then makes a copy of the subject
string in outputbuffer, replacing the part that was matched with the
replacement string, whose length is supplied in rlength. This can be
given as PCRE2_ZERO_TERMINATED for a zero-terminated string. 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.
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.
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 capturing groups or (*MARK) items
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 the name of the last (*MARK) encountered
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$1+", the result
is "=+babcb+=".
The facility for inserting a (*MARK) name can be used to perform simple
simultaneous substitutions, as this pcre2test example shows:
/(*:pear)apple|(*:orange)lemon/g,replace=${*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.
PCRE2_SUBSTITUTE_GLOBAL causes the function to iterate over the subject
string, replacing every matching substring. If this is not set, only
the first matching substring is replaced. If any matched substring has
zero length, after the substitution has happened, an attempt to find a
non-empty match at the same position is performed. If this is not suc-
cessful, the current position 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 current position 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 capturing 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 capturing 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 captured 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.
The second effect of setting PCRE2_SUBSTITUTE_EXTENDED is to add more
flexibility to 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 replacements
that were made. 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-
MISSING_BRACE (closing curly bracket not found), PCRE2_BADSUBSTITUTION
(syntax error in extended group substitution), and PCRE2_BADSUBPATTERN
(the pattern match ended before it started, 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).
DUPLICATE SUBPATTERN 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
subpatterns are not required to be unique. Duplicate names are always
allowed for subpatterns with the same number, created by using the (?|
feature. Indeed, if such subpatterns are named, they are required to
use the same names.
Normally, patterns with duplicate names are such that in any one match,
only one of the named subpatterns 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 */
match_data, /* 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_ENDAN-
CHORED, 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 capturing groups that may exist in the pattern, because DFA match-
ing does not support group capture.
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, and the meanings of some
other errors are slightly different:
PCRE2_ERROR_UNAVAILABLE
The ovector is not big enough to include a slot for the given substring
number.
PCRE2_ERROR_UNSET
There is a slot in the ovector for this substring, but there were
insufficient matches to fill it.
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 auch 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 back reference.
PCRE2_ERROR_DFA_UCOND
This return is given if pcre2_dfa_match() encounters a condition item
that uses a back reference for the condition, or a test for recursion
in a specific 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 recursive subpattern 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: 04 April 2017
Copyright (c) 1997-2017 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.
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 vectors 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 vectors 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. 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. Finally, 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).
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 conventional 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 64K code units. This is sufficient 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 setting 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 has no effect on the
pcre2_dfa_match() matching function, but it does also limit JIT match-
ing (though the counting is done differently).
In some environments it is desirable to limit the depth of nested back-
tracking in order to restrict the maximum amount of heap memory that is
used. A second limit controls this; it 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 also be overridden at run
time. As well as applying to pcre2_match(), this limit also controls
the depth of recursive function calls in pcre2_dfa_match(). These are
used for lookaround assertions, atomic groups, and recursion within
patterns.
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 starting size of the buffer is controlled by a
parameter whose default value is 20K. 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
parameter size. If a longer line is encountered, pcre2grep automati-
cally expands the buffer, up to a specified maximum size, whose default
is 1M 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.
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 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 lonter 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: 31 March 2017
Copyright (c) 1997-2017 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).
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 back reference to the capturing group in which it
appears. It is also disabled if the pattern contains (*PRUNE) or
(*SKIP). However, 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 set in the match context, it is called. This applies to
both normal and DFA matching. The first argument to the callout func-
tion 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 call-
out block structure contains the following fields:
uint32_t version;
uint32_t callout_number;
uint32_t capture_top;
uint32_t capture_last;
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 1; the three callout string fields were added for
this version. If you are writing an application that might use an ear-
lier 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 certain 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 undefined and should not be
used in any way. Substrings that have not been captured (but whose num-
bers are less than capture_top) 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, but it
holds no useful information at callout time 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 subpattern, and before an alternation bar or a
closing 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.
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: 29 March 2017
Copyright (c) 1997-2017 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 5.24, 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. Capturing subpatterns that occur inside negative lookaround asser-
tions 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: \l, \u, \L,
\U, and \N when followed by a character name or Unicode value. (\N on
its own, matching a non-newline character, is supported.) In fact these
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 the PCRE2_ALT_BSUX option
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 does support the \Q...\E escape for quoting substrings. Char-
acters in between are treated as literals. 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). 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$xyz
The \Q...\E sequence is recognized both inside and outside character
classes.
7. Fairly obviously, PCRE2 does not support the (?{code}) and
(??{code}) constructions. However, there is support PCRE2's "callout"
feature, which allows an external function to be called during pattern
matching. See the pcre2callout documentation for details.
8. Subroutine calls (whether recursive or not) were treated as atomic
groups up to PCRE2 release 10.23, but from release 10.30 this changed,
and backtracking into subroutine calls is now supported, as in Perl.
9. If any of the backtracking control verbs are used in a subpattern
that is called as a subroutine (whether or not recursively), their
effect is confined to that subpattern; it does not extend to the sur-
rounding pattern. This is not always the case in Perl. In particular,
if (*THEN) is present in a group that is called as a subroutine, its
action is limited to that group, even if the group does not contain any
| characters. Note that such subpatterns are processed as anchored at
the point where they are tested.
10. If a pattern contains more than one backtracking control verb, the
first one that is backtracked onto acts. For example, in the pattern
A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a failure
in C triggers (*PRUNE). Perl's behaviour is more complex; in many cases
it is the same as PCRE2, but there are cases where it differs.
11. Most backtracking verbs in assertions have their normal actions.
They are not confined to the assertion.
12. There are some differences that are concerned with the settings of
captured strings when part of a pattern is repeated. For example,
matching "aba" against the pattern /^(a(b)?)+$/ in Perl leaves $2
unset, but in PCRE2 it is set to "b".
13. PCRE2's handling of duplicate subpattern numbers and duplicate sub-
pattern 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 translate between numbers and names. In particular, a pattern
such as (?|(?<a>A)|(?<b>B), where the two capturing parentheses 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 parentheses matched, because both names map to cap-
turing subpattern 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 subpattern. 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 5.24:
(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, back references to groups of fixed length are
supported in lookbehinds, provided that there is no possibility of ref-
erencing 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,
PCRE2_NOTEMPTY_ATSTART, and PCRE2_NO_AUTO_CAPTURE 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.
(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) at the start
of a pattern that set overall options that cannot be changed within the
pattern.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 03 April 2017
Copyright (c) 1997-2017 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_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART,
PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. The
PCRE2_ANCHORED option is 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 32K 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. (For the technically minded:
the address space is allocated by mmap or VirtualAlloc.) A maximum
stack size of 512K to 1M 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. There are three cases for the values of the other
two options:
(1) If callback is NULL and data is NULL, an internal 32K 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 32K 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 1M address space, and use only a single memory page (usually
4K) if that is enough. However, we can still grow up to 1M 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 1M? Is that 1M 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(). The return values are also
the same, plus PCRE2_ERROR_JIT_BADOPTION if a matching mode (partial or
complete) is requested that was not compiled. Unsupported option bits
(for example, PCRE2_ANCHORED) are ignored, as is the PCRE2_NO_JIT
option.
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: 31 March 2017
Copyright (c) 1997-2017 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 64K 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 build-
ing 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. However, 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 subpatterns, but there
can be no more than 65535 capturing subpatterns. There is, however, 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; the default default is 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 subpattern is 32 code units, and
the maximum number of named subpatterns 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: 30 March 2017
Copyright (c) 1997-2017 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 back references.
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 by the alternative matching algorithm. They are as fol-
lows:
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, back references within the pat-
tern are not supported, and cause errors if encountered.
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. 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. It causes an
error if encountered.
6. 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.
7. 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.
8. 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 and back references are not supported.
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: 29 September 2014
Copyright (c) 1997-2014 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 patterns that are supported 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 advan-
tages and disadvantages of the alternative function, and how it differs
from the normal function, 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 and backtracking depth 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, and this restricts the amount of heap memory that is
used.
These facilities are provided to catch runaway matches that are pro-
voked by patterns with huge matching trees (a typical 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_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.
Prior to release 10.30, LIMIT_DEPTH was called LIMIT_RECURSION. This
name is still recognized for backwards compatibility.
The match limit is used (but in a different way) when JIT is being
used, but it is not relevant, and is ignored, when matching with
pcre2_dfa_match(). However, the depth limit is relevant for DFA match-
ing, which uses function recursion for recursions within the pattern.
In this case, the depth limit controls the amount of system stack that
is used.
Newline conventions
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 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 five sequences:
(*CR) carriage return
(*LF) linefeed
(*CRLF) carriage return, followed by linefeed
(*ANYCRLF) any of the three above
(*ANY) all Unicode newline sequences
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. 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
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 subpattern
) end subpattern
? extends the meaning of (
also 0 or 1 quantifier
also quantifier minimizer
* 0 or more quantifier
+ 1 or more quantifier
also "possessive quantifier"
{ 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 (only 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 number 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 numbers and letters have any special meaning
after a backslash. All other characters (in particular, those whose
codepoints 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 remove the special meaning from a sequence of charac-
ters, you can do so by putting them between \Q and \E. This is differ-
ent from Perl in that $ and @ are handled as literals in \Q...\E
sequences in PCRE2, whereas in Perl, $ and @ cause variable interpola-
tion. 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
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 than the binary character it represents. In
an ASCII or Unicode environment, these escapes are as follows:
\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)
\t tab (hex 09)
\0dd character with octal code 0dd
\ddd character with octal code ddd, or back reference
\o{ddd..} character with octal code ddd..
\xhh character with hex code hh
\x{hhh..} character with hex code hhh.. (default mode)
\uhhhh character with hex code hhhh (when PCRE2_ALT_BSUX is set)
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, \a, \e, \f, \n, \r, and \t gen-
erate the appropriate EBCDIC code values. The \c escape is processed as
specified for Perl in the perlebcdic document. 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 back references 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 charac-
ter numbers, and \g{} to specify back references. The following para-
graphs 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 capturing
left parentheses in the expression, the entire sequence is taken as a
back reference. A description of how this works is given later, follow-
ing the discussion of parenthesized subpatterns. Otherwise, up to
three octal digits 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 capturing subpatterns
\7 is always a back reference
\11 might be a back reference, 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 back reference, otherwise the
character with octal code 113
\377 might be a back reference, otherwise
the value 255 (decimal)
\81 is always a back reference
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.
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.
If the PCRE2_ALT_BSUX option is set, the interpretation of \x is as
just described only when it is followed by two hexadecimal digits. Oth-
erwise, it matches a literal "x" character. In this mode, support for
code points greater than 256 is provided by \u, which must be followed
by four hexadecimal digits; otherwise it matches a literal "u" charac-
ter.
Characters whose value is less than 256 can be defined by either of the
two syntaxes for \x (or by \u in PCRE2_ALT_BSUX mode). There is no dif-
ference in the way they are handled. For example, \xdc is exactly the
same as \x{dc} (or \u00dc in PCRE2_ALT_BSUX mode).
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 codepoint
Invalid Unicode codepoints are the range 0xd800 to 0xdfff (the so-
called "surrogate" codepoints), and 0xffef.
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).
\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 \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. However, if the
PCRE2_ALT_BSUX option 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 back references
The sequence \g followed by a signed or unsigned number, optionally
enclosed in braces, is an absolute or relative back reference. A named
back reference can be coded as \g{name}. Back references are discussed
later, following the discussion of parenthesized subpatterns.
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 subpattern as a "subroutine".
Details are discussed later. Note that \g{...} (Perl syntax) and
\g<...> (Oniguruma syntax) are not synonymous. The former is a back
reference; 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
\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
There is also the single sequence \N, which matches a non-newline char-
acter. This is the same as the "." metacharacter when PCRE2_DOTALL is
not set. Perl also uses \N to match characters by 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 codepoints 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. In 8-bit non-UTF-8 mode, these sequences are of course
limited to testing characters whose codepoints are less than 256, but
they do work in this mode. In 32-bit non-UTF mode, codepoints greater
than 0x10ffff (the Unicode limit) may be encountered. These are all
treated as being in the Common 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 limited to the Unicode
script names, the general category properties, "Any", which matches any
character (including newline), and some special PCRE2 properties
(described in the next section). Other Perl properties such as "InMu-
sicalSymbols" 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}
Those that are not part of an identified script are lumped together as
"Common". The current list of scripts is:
Ahom, Anatolian_Hieroglyphs, Arabic, Armenian, Avestan, Balinese,
Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese,
Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham,
Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
Devanagari, Duployan, Egyptian_Hieroglyphs, Elbasan, Ethiopic, Geor-
gian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han,
Hangul, 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, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui,
Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro,
Multani, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki,
Old_Hungarian, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian,
Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene,
Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic,
Samaritan, Saurashtra, Sharada, Shavian, Siddham, SignWriting, Sinhala,
Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai,
Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.
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 in the range
U+D800 to U+DFFF. Such characters are not valid in Unicode strings and
so cannot be tested by PCRE2, unless UTF validity checking has been
turned off (see the discussion of PCRE2_NO_UTF_CHECK in the pcre2api
page). Perl does not support the Cs property.
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. \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. Characters
with the "mark" property always have the "extend" grapheme breaking
property.
5. Do not end after prepend characters.
6. 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
The escape sequence \K causes any previously matched characters not to
be included in the final matched sequence. For example, the pattern:
foo\Kbar
matches "foobar", but reports that it has matched "bar". This feature
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 example,
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.
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 subpatterns 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. In a
UTF mode, 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" metase-
quence. However, whatever follows \b normally 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 match, as specified by the startoffset 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 interpretation of \G, as the start of the
current match, is subtly different from Perl's, which defines it as 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 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. Perl also uses
\N to match characters by 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 Subpattern Num-
bers" 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,
respectively. 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.
When caseless matching is set, any letters in a class represent both
their upper case and lower case versions, 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 character 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 hexadeci-
mal 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, \N, \R, and \X are not special inside a character
class. Like any other unrecognized escape sequences, they cause an
error.
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 a character type escape such as as \d, but gives 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.
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 subpattern (defined below), "succeeds" means matching the
rest of the main pattern as well as the alternative in the subpattern.
INTERNAL OPTION SETTING
The settings of the PCRE2_CASELESS, PCRE2_MULTILINE, PCRE2_DOTALL, and
PCRE2_EXTENDED options (which are Perl-compatible) can be changed from
within the pattern by a sequence of Perl option letters enclosed
between "(?" and ")". The option letters are
i for PCRE2_CASELESS
m for PCRE2_MULTILINE
s for PCRE2_DOTALL
x for PCRE2_EXTENDED
For example, (?im) sets caseless, multiline matching. It is also possi-
ble to unset these options by preceding the letter with a hyphen, and a
combined setting and unsetting such as (?im-sx), which sets PCRE2_CASE-
LESS and PCRE2_MULTILINE while unsetting PCRE2_DOTALL and
PCRE2_EXTENDED, is also permitted. If a letter appears both before and
after the hyphen, the option is unset. An empty options setting "(?)"
is allowed. Needless to say, it has no effect.
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.
When one of these option changes occurs at top level (that is, not
inside subpattern parentheses), the change applies to the remainder of
the pattern that follows. An option change within a subpattern (see
below for a description of subpatterns) affects only that part of the
subpattern 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 subpattern.
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 subpattern (see the next section), 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.
Note: There are other PCRE2-specific options that can be set by the
application when the compiling function is called. The pattern can con-
tain 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, respectively. 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.
SUBPATTERNS
Subpatterns are delimited by parentheses (round brackets), which can be
nested. Turning part of a pattern into a subpattern 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 sets up the subpattern as a capturing subpattern. This means
that, when the whole pattern matches, the portion of the subject string
that matched the subpattern 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 the capturing subpatterns. 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 a grouping subpattern is required
without a capturing requirement. If an opening parenthesis is followed
by a question mark and a colon, the subpattern does not do any captur-
ing, and is not counted when computing the number of any subsequent
capturing subpatterns. 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 capturing subpatterns is 65535.
As a convenient shorthand, if any option settings are required at the
start of a non-capturing subpattern, 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 subpattern is reached, an option setting in one branch does affect
subsequent branches, so the above patterns match "SUNDAY" as well as
"Saturday".
DUPLICATE SUBPATTERN NUMBERS
Perl 5.10 introduced a feature whereby each alternative in a subpattern
uses the same numbers for its capturing parentheses. Such a subpattern
starts with (?| and is itself a non-capturing subpattern. 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
subpattern 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 back reference to a numbered subpattern uses the most recent value
that is set for that number by any subpattern. The following pattern
matches "abcabc" or "defdef":
/(?|(abc)|(def))\1/
In contrast, a subroutine call to a numbered subpattern 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 subpattern's having matched refers to a non-
unique number, the test is true if any of the subpatterns of that num-
ber have matched.
An alternative approach to using this "branch reset" feature is to use
duplicate named subpatterns, as described in the next section.
NAMED SUBPATTERNS
Identifying capturing parentheses by number is simple, but it can be
very hard to keep track of the numbers in complicated regular expres-
sions. Furthermore, if an expression is modified, the numbers may
change. To help with this difficulty, PCRE2 supports the naming of sub-
patterns. This feature was not added to Perl until release 5.10. Python
had the feature earlier, and PCRE1 introduced it at release 4.0, using
the Python syntax. PCRE2 supports both the Perl and the Python syntax.
Perl allows identically numbered subpatterns to have different names,
but PCRE2 does not.
In PCRE2, a subpattern can be named in one of three ways: (?<name>...)
or (?'name'...) as in Perl, or (?P<name>...) as in Python. References
to capturing parentheses from other parts of the pattern, such as back
references, recursion, and conditions, can be made by name as well as
by number.
Names consist of up to 32 alphanumeric characters and underscores, but
must start with a non-digit. Named capturing parentheses are still
allocated numbers as well as names, exactly as if the names were not
present. The PCRE2 API provides function calls for extracting the name-
to-number translation table from a compiled pattern. There are also
convenience functions for extracting a captured substring by name.
By default, a name must be unique within a pattern, but it is possible
to relax this constraint by setting the PCRE2_DUPNAMES option at com-
pile time. (Duplicate names are also always permitted for subpatterns
with the same number, set up as described in the previous section.)
Duplicate names can be useful for patterns where only one instance of
the named parentheses 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 capturing substrings, but only one is ever set after a
match. (An alternative way of solving this problem is to use a "branch
reset" subpattern, as described in the previous section.)
The convenience functions for extracting the data by name returns the
substring for the first (and in this example, the only) subpattern of
that name that matched. This saves searching to find which numbered
subpattern it was.
If you make a back reference to a non-unique named subpattern from
elsewhere in the pattern, the subpatterns 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 pat-
tern 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 subpattern, the one
that corresponds to the first occurrence of the name is used. In the
absence of duplicate numbers (see the previous section) 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 subpattern has matched, or
to check for recursion, all subpatterns 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 subpatterns, see the
pcre2api documentation.
Warning: You cannot use different names to distinguish between two sub-
patterns with the same number because PCRE2 uses only the numbers when
matching. For this reason, an error is given at compile time if differ-
ent names are given to subpatterns with the same number. However, you
can always give the same name to subpatterns with the same number, even
when PCRE2_DUPNAMES is not set.
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 \X escape sequence
the \R escape sequence
an escape such as \d or \pL that matches a single character
a character class
a back reference
a parenthesized subpattern (including most assertions)
a subroutine call to a subpattern (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 subpatterns that are referenced as subroutines from elsewhere
in the pattern (but see also the section entitled "Defining subpatterns
for use by reference only" below). Items other than subpatterns 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 subpattern
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
subpattern does in fact match no characters, the loop is forcibly bro-
ken.
By default, the 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.
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 pat-
tern
/\*.*?\*/
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 subpattern 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 back
reference 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 capturing subpattern is repeated, the value captured is the sub-
string 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 capturing subpatterns,
the corresponding captured values may have been set in previous itera-
tions. 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 subpattern 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
This kind of parenthesis "locks up" the part of the pattern it con-
tains 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 subpattern 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 grouping subpatterns are not capturing subpatterns. 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 pre-
pared to adjust the number of digits they match in order to make the
rest of the pattern match, (?>\d+) can only match an entire sequence of
digits.
Atomic groups in general can of course contain arbitrarily complicated
subpatterns, and can be nested. However, when the subpattern for an
atomic group is just a single repeated item, as in the example above, a
simpler notation, called a "possessive quantifier" can be used. This
consists 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 ultimately
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 subpattern 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.
BACK REFERENCES
Outside a character class, a backslash followed by a digit greater than
0 (and possibly further digits) is a back reference to a capturing sub-
pattern earlier (that is, to its left) in the pattern, provided there
have been that many previous capturing left parentheses.
However, if the decimal number following the backslash is less than 8,
it is always taken as a back reference, and causes an error only if
there are not that many capturing left parentheses in the entire pat-
tern. In other words, the parentheses that are referenced need not be
to the left of the reference for numbers less than 8. A "forward back
reference" of this type can make sense when a repetition is involved
and the subpattern to the right has participated in an earlier itera-
tion.
It is not possible to have a numerical "forward back reference" to a
subpattern 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. There is no
such problem when named parentheses are used. A back reference to any
subpattern is possible using named parentheses (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 captur-
ing subpattern before \g, that is, is it equivalent to \2 in this exam-
ple. Similarly, \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 themselves.
The sequence \g{+1} is a reference to the next capturing subpattern.
This kind of forward reference can be useful it patterns that repeat.
Perl does not support the use of + in this way.
A back reference matches whatever actually matched the capturing sub-
pattern in the current subject string, rather than anything matching
the subpattern itself (see "Subpatterns 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 back reference, 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 capturing subpattern is matched caselessly.
There are several different ways of writing back references to named
subpatterns. 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 back reference 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 subpattern that is referenced by name may appear in the pattern
before or after the reference.
There may be more than one back reference to the same subpattern. If a
subpattern has not actually been used in a particular match, any back
references 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 back
reference to an unset value matches an empty string.
Because there may be many capturing parentheses in a pattern, all dig-
its following a backslash are taken as part of a potential back refer-
ence number. If the pattern continues with a digit character, some
delimiter must be used to terminate the back reference. If the
PCRE2_EXTENDED option is set, this can be white space. Otherwise, the
\g{ syntax or an empty comment (see "Comments" below) can be used.
Recursive back references
A back reference that occurs inside the parentheses to which it refers
fails when the subpattern is first used, so, for example, (a\1) never
matches. However, such references can be useful inside repeated sub-
patterns. For example, the pattern
(a|b\1)+
matches any number of "a"s and also "aba", "ababbaa" etc. At each iter-
ation of the subpattern, the back reference matches the character
string corresponding 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 back reference. This can be done using alternation, as in
the example above, or by a quantifier with a minimum of zero.
Back references 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 subpatterns. 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 succeed for matching to continue) or negative
(must not succeed for matching to continue). An assertion subpattern is
matched in the normal way, except that, when matching continues after-
wards, the matching position in the subject string is as it was at the
start of the assertion.
Assertion subpatterns are not capturing subpatterns. If an assertion
contains capturing subpatterns within it, these are counted for the
purposes of numbering the capturing subpatterns in the whole pattern.
However, substring capturing is carried out only for positive asser-
tions that succeed, that is, one of their branches matches, so matching
continues after the assertion. If all branches of a positive assertion
fail to match, nothing is captured, and control is passed to the previ-
ous backtracking point.
No capturing is done for a negative assertion unless it is being used
as a condition in a conditional subpattern (see the discussion below).
Matching continues after a non-conditional negative assertion only if
all its branches fail to match.
For compatibility with Perl, most assertion subpatterns may be
repeated; though it makes no sense to assert the same thing several
times, the side effect of capturing parentheses may occasionally be
useful. However, an assertion that forms the condition for a condi-
tional subpattern may not be quantified. In practice, for other asser-
tions, there only three cases:
(1) If the quantifier is {0}, the assertion is never obeyed during
matching. However, it may contain internal capturing parenthesized
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.
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 subpattern 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 back references in lookbehinds. PCRE2 does sup-
port 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 subpattern numbers), and if
the back reference is by name, the name must be unique. Of course, the
referenced subpattern must itself be of fixed length. 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".
CONDITIONAL SUBPATTERNS
It is possible to cause the matching process to obey a subpattern con-
ditionally or to choose between two alternative subpatterns, depending
on the result of an assertion, or whether a specific capturing subpat-
tern has already been matched. The two possible forms of conditional
subpattern 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. If there are more than two alterna-
tives in the subpattern, a compile-time error occurs. Each of the two
alternatives may itself contain nested subpatterns of any form, includ-
ing conditional subpatterns; the restriction to two alternatives
applies only at the level of the condition. 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 subpatterns, refer-
ences to recursion, two pseudo-conditions called DEFINE and VERSION,
and assertions.
Checking for a used subpattern by number
If the text between the parentheses consists of a sequence of digits,
the condition is true if a capturing subpattern of that number has pre-
viously matched. If there is more than one capturing subpattern with
the same number (see the earlier section about duplicate subpattern
numbers), the condition is true if any of them have matched. An alter-
native notation is to precede the digits with a plus or minus sign. In
this case, the subpattern number is relative rather than absolute. The
most recently opened parentheses 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 parentheses to be opened can be
referenced as (?(+1), and so on. (The value zero in any of these forms
is not used; it provokes a compile-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 subpattern that tests whether or not the
first set of parentheses matched. If they 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. Other-
wise, since no-pattern is not present, the subpattern 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 subpattern by name
Perl uses the syntax (?(<name>)...) or (?('name')...) to test for a
used subpattern 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 subpattern gives this:
(?<OPEN> \( )? [^()]+ (?(<OPEN>) \) )
If the name used in a condition of this kind is a duplicate, the test
is applied to all subpatterns 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 "Subpatterns
as subroutines" below for details of recursion and subpattern calls.
If a condition is the string (R), and there is no subpattern with the
name R, the condition is true if matching is currently in a recursion
or subroutine call to the whole pattern or any subpattern. If digits
follow the letter R, and there is no subpattern with that name, the
condition is true if the most recent call is into a subpattern 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 subpattern with a matching name,
the condition tests for its being set, as described in the section
above, instead of testing for recursion. For example, creating a group
with the name R1 by adding (?<R1>) to the above pattern completely
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 subpattern
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 subpatterns 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 subpatterns 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 subpattern. It is always skipped if con-
trol reaches this point in the pattern; the idea of DEFINE is that it
can be used to define subroutines that can be referenced from else-
where. (The use of subroutines is described below.) For example, a pat-
tern 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 an
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 capturing subpatterns,
any capturing that occurs in a matching branch is retained afterwards,
for both positive and negative assertions, because matching always con-
tinues after the assertion, whether it succeeds or fails. (Compare non-
conditional assertions, when captures are retained only for positive
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 subpattern 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 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 charac-
ters 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 "Newline 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 single
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 subpattern recursion. After its introduc-
tion 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
subpattern of the given number, provided that it occurs inside that
subpattern. (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 subpattern numbers are in use, rel-
ative references refer to the earliest subpattern with the appropriate
number. Consider, for example:
(?|(a)|(b)) (c) (?-2)
The first two capturing 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, relative references are just a shorthand for computing a group
number.
It is also possible to refer to subsequently opened parentheses, by
writing references such as (?+2). However, these cannot be recursive
because the reference is not inside the parentheses that are refer-
enced. 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 subpattern 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 capturing sub-
pattern 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.
If there are more than 15 capturing parentheses in a pattern, PCRE2 has
to obtain extra memory from the heap to store data during a recursion.
If no memory can be obtained, the match fails with the
PCRE2_ERROR_NOMEMORY error.
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 subpattern, 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 subpattern 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 subpattern was called recursively or as a subpattern (see the
next section), it had no access to any values that were captured out-
side 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 back reference \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.
SUBPATTERNS AS SUBROUTINES
If the syntax for a recursive subpattern call (either by number or by
name) is used outside the parentheses to which it refers, it operates
like a subroutine in a programming language. The called subpattern 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 subpat-
tern 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 subpattern.
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 referencing a subpattern as a subroutine,
possibly recursively. Here are two of the examples used above, rewrit-
ten 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 back reference; 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.
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
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 does 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
subpatterns called as subroutines (whether or not recursively) is docu-
mented 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,
sometimes leading to anomalous results.
Verbs that act immediately
The following verbs act as soon as they are encountered. They may not
be followed by a name.
(*ACCEPT)
This verb causes the match to end successfully, skipping the remainder
of the pattern. However, when it is inside a subpattern that is called
as a subroutine, only that subpattern 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.
(*FAIL) or (*F)
This verb causes a matching failure, forcing backtracking to occur. 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 feature, as for example in this pat-
tern:
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).
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. There may be as many
instances of (*MARK) as you like in a pattern, and their names do not
have to be unique.
When a match succeeds, the name of the last-encountered (*MARK:NAME),
(*PRUNE:NAME), or (*THEN:NAME) on the matching path is passed back to
the caller as described in the section entitled "Other information
about the match" in the pcre2api documentation. 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 no subsequent match, causing
a backtrack to the verb, a failure is forced. That is, backtracking
cannot pass to the left of the verb. However, when one of these verbs
appears inside an atomic group or in an 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. In this situation,
backtracking has to jump to the left of the entire atomic group or
assertion.
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)
This verb, which may not be followed by a name, causes the whole match
to fail outright if there is a later matching failure that causes back-
tracking to reach it. Even if the pattern 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 committed 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 name of the
most recently passed (*MARK) in the path is passed back when (*COMMIT)
forces a match failure.
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 (*PRUNE) or (*THEN).
(*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. 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 it
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 corresponds to that
(*MARK) instead of to where (*SKIP) was encountered. If no (*MARK) with
a matching name is found, the (*SKIP) is ignored.
Note that (*SKIP:NAME) searches only for names set by (*MARK:NAME). It
ignores names that are set by (*PRUNE:NAME) or (*THEN:NAME).
(*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 the 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
(*PRUNE) and (*THEN).
A subpattern 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 subpattern 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 subpattern 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 subpattern. After a
failure in C, matching moves to (*FAIL), which causes the whole subpat-
tern to fail because there are no more alternatives to try. In this
case, matching does now backtrack into A.
Note that a conditional subpattern is not considered as having two
alternatives, because only one is ever used. In other words, the |
character in a conditional subpattern 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 subpattern 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 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, but PCRE2 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 subpattern.
(*ACCEPT) in a standalone positive assertion causes the assertion to
succeed without any further processing; captured strings are retained.
In a standalone negative assertion, (*ACCEPT) causes the assertion to
fail without any further processing; captured substrings 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 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 into (*COMMIT), (*SKIP), or (*PRUNE) causes the con-
dition to be false. However, for both standalone and conditional nega-
tive assertions, backtracking into (*COMMIT), (*SKIP), or (*PRUNE)
causes the assertion to be true, without considering any further alter-
native branches.
Backtracking verbs in subroutines
These behaviours occur whether or not the subpattern is called recur-
sively. Perl's treatment of subroutines is different in some cases.
(*FAIL) in a subpattern called as a subroutine has its normal effect:
it forces an immediate backtrack.
(*ACCEPT) in a subpattern called as a subroutine causes the subroutine
match to succeed without any further processing. Matching then contin-
ues after the subroutine call.
(*COMMIT), (*SKIP), and (*PRUNE) in a subpattern called as a subroutine
cause the subroutine match to fail.
(*THEN) skips to the next alternative in the innermost enclosing group
within the subpattern that has alternatives. If there is no such group
within the subpattern, (*THEN) causes the subroutine match to fail.
SEE ALSO
pcre2api(3), pcre2callout(3), pcre2matching(3), pcre2syntax(3),
pcre2(3).
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 03 April 2017
Copyright (c) 1997-2017 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
subpattern has a quantifier with a minimum greater than 1 and/or a lim-
ited maximum, the whole subpattern 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 50K bytes 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 64K code units in the 8-bit and
16-bit libraries, and this is reached with the above pattern if the
outer repetition 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 16K, and indeed it remains
under 20K 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 10K vector of frames is allocated on the system stack
(enough for about 50 frames for small patterns), but if this is insuf-
ficient, heap memory 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. Too much nested recursion
may cause stack issues. The "match depth" parameter can be used to
limit the depth of function recursion 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: 31 March 2017
Copyright (c) 1997-2017 University of Cambridge.
------------------------------------------------------------------------------
PCRE2POSIX(3) Library Functions Manual PCRE2POSIX(3)
NAME
PCRE2 - Perl-compatible regular expressions (revised API)
SYNOPSIS
#include <pcre2posix.h>
int regcomp(regex_t *preg, const char *pattern,
int cflags);
int regexec(const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags);
size_t regerror(int errcode, const regex_t *preg,
char *errbuf, size_t errbuf_size);
void regfree(regex_t *preg);
DESCRIPTION
This set of functions provides a POSIX-style API for the PCRE2 regular
expression 8-bit library. See the pcre2api documentation for a descrip-
tion of PCRE2's native API, which contains much additional functional-
ity. There are no POSIX-style wrappers for PCRE2's 16-bit and 32-bit
libraries.
The functions described here are just wrapper functions that ultimately
call the PCRE2 native API. Their prototypes are defined in the
pcre2posix.h header file, and on Unix systems the library itself is
called libpcre2-posix.a, so can be accessed by adding -lpcre2-posix to
the command for linking an application that uses them. Because the
POSIX functions call the native ones, it is also necessary to add
-lpcre2-8.
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.
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 header for these functions is supplied as pcre2posix.h to avoid any
potential clash with other POSIX libraries. It can, of course, be
renamed or aliased as regex.h, which is the "correct" name. It provides
two structure types, regex_t for compiled internal forms, and reg-
match_t for returning captured substrings. It also defines some con-
stants whose names start with "REG_"; these are used for setting
options and identifying error codes.
COMPILING A PATTERN
The function regcomp() is called to compile a pattern into an internal
form. The pattern is a C string terminated by a binary zero, and is
passed in the argument pattern. 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.
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_NOSUB
When a pattern that is compiled with this flag is passed to regexec()
for matching, the nmatch and pmatch arguments are ignored, and no cap-
tured 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_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 regcomp() is zero on success, and non-zero otherwise. The
preg structure is filled in on success, and one member of the structure
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 regcomp() is non-zero, you must not attempt to
use the contents of the preg structure. If, for example, you pass it to
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 reg-
comp() function causes PCRE2_MULTILINE to be passed to pcre2_compile(),
and REG_DOTALL passes PCRE2_DOTALL. There is no way to pass PCRE2_DOL-
LAR_ENDONLY.
MATCHING A PATTERN
The function 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
The string is considered to start at string + pmatch[0].rm_so and to
have a terminating NUL located at string + pmatch[0].rm_eo (there need
not actually be a NUL at that location), regardless of the value of
nmatch. 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
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
regexec() are ignored (except possibly as input for REG_STARTEND).
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 regerror() function maps a non-zero errorcode from either regcomp()
or 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 buffer 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 regfree() frees all such
memory, after which preg may no longer be used as a compiled expres-
sion.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 31 January 2016
Copyright (c) 1997-2016 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.
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, that is,
converted 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 character tables, see the sec-
tion on locale support in the pcre2api documentation.
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().
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: 21 March 2017
Copyright (c) 1997-2017 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.
\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..
\U "U" if PCRE2_ALT_BSUX is set (otherwise is an error)
\uhhhh character with hex code hhhh (if PCRE2_ALT_BSUX is set)
\xhh character with hex code hh
\x{hhh..} character with hex code hhh..
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.
When \x is not followed by {, from zero to two hexadecimal digits are
read, but if PCRE2_ALT_BSUX is set, \x must be followed by two hexadec-
imal digits to be recognized as a hexadecimal escape; otherwise it
matches a literal "x". Likewise, if \u (in ALT_BSUX mode) is not fol-
lowed by four hexadecimal digits, it matches a literal "u".
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
Ahom, Anatolian_Hieroglyphs, Arabic, Armenian, Avestan, Balinese,
Bamum, Bassa_Vah, Batak, Bengali, Bopomofo, Brahmi, Braille, Buginese,
Buhid, Canadian_Aboriginal, Carian, Caucasian_Albanian, Chakma, Cham,
Cherokee, Common, Coptic, Cuneiform, Cypriot, Cyrillic, Deseret,
Devanagari, Duployan, Egyptian_Hieroglyphs, Elbasan, Ethiopic, Geor-
gian, Glagolitic, Gothic, Grantha, Greek, Gujarati, Gurmukhi, Han,
Hangul, 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, Malayalam, Mandaic, Manichaean, Meetei_Mayek, Mende_Kikakui,
Meroitic_Cursive, Meroitic_Hieroglyphs, Miao, Modi, Mongolian, Mro,
Multani, Myanmar, Nabataean, New_Tai_Lue, Nko, Ogham, Ol_Chiki,
Old_Hungarian, Old_Italic, Old_North_Arabian, Old_Permic, Old_Persian,
Old_South_Arabian, Old_Turkic, Oriya, Osmanya, Pahawh_Hmong, Palmyrene,
Pau_Cin_Hau, Phags_Pa, Phoenician, Psalter_Pahlavi, Rejang, Runic,
Samaritan, Saurashtra, Sharada, Shavian, Siddham, SignWriting, Sinhala,
Sora_Sompeng, Sundanese, Syloti_Nagri, Syriac, Tagalog, Tagbanwa,
Tai_Le, Tai_Tham, Tai_Viet, Takri, Tamil, Telugu, Thaana, Thai,
Tibetan, Tifinagh, Tirhuta, Ugaritic, Vai, Warang_Citi, Yi.
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
MATCH POINT RESET
\K reset start of match
\K is honoured in positive assertions, but ignored in negative ones.
ALTERNATION
expr|expr|expr...
CAPTURING
(...) capturing group
(?<name>...) named capturing group (Perl)
(?'name'...) named capturing group (Perl)
(?P<name>...) named capturing group (Python)
(?:...) non-capturing group
(?|...) non-capturing group; reset group numbers for
capturing groups in each alternative
ATOMIC GROUPS
(?>...) atomic, non-capturing group
COMMENT
(?#....) comment (not nestable)
OPTION SETTING
(?i) caseless
(?J) allow duplicate names
(?m) multiline
(?s) single line (dotall)
(?U) default ungreedy (lazy)
(?x) extended (ignore white space)
(?-...) unset option(s)
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.
(*LIMIT_MATCH=d) set the match limit to d (decimal number)
(*LIMIT_DEPTH=d) set the backtracking limit to d (decimal number)
(*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_MATCH and LIMIT_DEPTH 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 com-
pile 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
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
(?=...) positive look ahead
(?!...) negative look ahead
(?<=...) positive look behind
(?<!...) negative look behind
Each top-level branch of a look behind must be of a fixed length.
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 subpattern by absolute number
(?+n) call subpattern by relative number
(?-n) call subpattern by relative number
(?&name) call subpattern by name (Perl)
(?P>name) call subpattern by name (Python)
\g<name> call subpattern by name (Oniguruma)
\g'name' call subpattern by name (Oniguruma)
\g<n> call subpattern by absolute number (Oniguruma)
\g'n' call subpattern by absolute number (Oniguruma)
\g<+n> call subpattern by relative number (PCRE2 extension)
\g'+n' call subpattern by relative number (PCRE2 extension)
\g<-n> call subpattern by relative number (PCRE2 extension)
\g'-n' call subpattern 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 subpattern 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
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
(*PRUNE:NAME) equivalent to (*MARK:NAME)(*PRUNE)
(*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
(*THEN:NAME) equivalent to (*MARK:NAME)(*THEN)
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: 31 March 2017
Copyright (c) 1997-2017 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.
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. 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 let-
ter. Its Perl synonym, \p{Letter}, is not supported. Furthermore, in
Perl, many properties may optionally be prefixed by "Is", for compati-
bility with Perl 5.6. PCRE does not support this.
WIDE CHARACTERS AND UTF MODES
Codepoints 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{...}.
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.
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).
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-insensitive matching in UTF mode makes use of Unicode properties.
A few Unicode characters such as Greek sigma have more than two code-
points that are case-equivalent, and these are treated as such.
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.
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 July 2016
Copyright (c) 1997-2016 University of Cambridge.
------------------------------------------------------------------------------
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