----------------------------------------------------------------------------- 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 UTF. Alternatively, you can set the PCRE2_NEVER_UTF option at compile time. This causes an compile time error if a pattern contains a UTF-setting sequence. 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. 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. 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 pcre2stack discussion of stack usage 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: 18 November 2014 Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------ PCRE2API(3) Library Functions Manual PCRE2API(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) #include 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); pcre2_code_free(pcre2_code *code); pcre2_match_data_create(uint32_t ovecsize, pcre2_general_context *gcontext); 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_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_match_limit(pcre2_match_context *mcontext, uint32_t value); 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); 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, unsigned int 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, unsigned int 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, unsigned int 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 AUXILIARY FUNCTIONS 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_config(uint32_t what, void *where); 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(). 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. Just-in-time 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. There is also a direct interface for JIT matching, which gives improved performance. 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 lookbehind 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. 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()). 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. (1) 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. An application can compile all its pat- terns at the start, before forking off multiple threads that use them. However, if the just-in-time optimization feature is being used, it needs separate memory stack areas for each thread. See the pcre2jit documentation for more details. (2) The next 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. (3) The matching functions need a block of memory for working space and for storing the results of a match. This includes details of what was matched, as well as additional information such as the name of a (*MARK) setting. Each thread must provide its own version of this mem- ory. 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 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 An external function for stack checking 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_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). When a pattern is compiled with the PCRE2_EXTENDED option, the value of this parameter affects the recognition of white space and the end of internal comments starting with #. The value is saved with the compiled pattern for subsequent use by the JIT compiler and by the two inter- preted 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. 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 change the default values of any of the following match-time parameters: A callout function The limit for calling match() The limit for calling match() recursively A match context is also required if you are using custom memory manage- ment. 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, which PCRE2 will call at specified points during a matching operation. Details are given in the pcre2call- out documentation. 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. Internally, pcre2_match() uses a function called match(), which it calls repeatedly (sometimes recursively). The limit set by match_limit is imposed on the number of times this function is called during a match, which 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 relevant 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. If the limit is exceeded, pcre2_match() returns PCRE2_ERROR_MATCHLIMIT. A value for the match limit may also be sup- plied 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_recursion_limit(pcre2_match_context *mcontext, uint32_t value); The recursion_limit parameter is similar to match_limit, but instead of limiting the total number of times that match() is called, it limits the depth of recursion. The recursion depth is a smaller number than the total number of calls, because not all calls to match() are recur- sive. This limit is of use only if it is set smaller than match_limit. Limiting the recursion depth limits the amount of system stack that can be used, or, when PCRE2 has been compiled to use memory on the heap instead of the stack, the amount of heap memory that can be used. This limit is not relevant, and is ignored, when matching is done using JIT compiled code or by the pcre2_dfa_match() function. The default value for recursion_limit can be set when PCRE2 is built; the default default is the same value as the default for match_limit. If the limit is exceeded, pcre2_match() returns PCRE2_ERROR_RECURSION- LIMIT. A value for the recursion limit may also be supplied by an item at the start of a pattern of the form (*LIMIT_RECURSION=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_recursion_memory_management( pcre2_match_context *mcontext, void *(*private_malloc)(PCRE2_SIZE, void *), void (*private_free)(void *, void *), void *memory_data); This function sets up two additional custom memory management functions for use by pcre2_match() when PCRE2 is compiled to use the heap for remembering backtracking data, instead of recursive function calls that use the system stack. There is a discussion about PCRE2's stack usage in the pcre2stack documentation. See the pcre2build documentation for details of how to build PCRE2. Using the heap for recursion is a non-standard way of building PCRE2, for use in environments that have limited stacks. Because of the greater use of memory management, pcre2_match() runs more slowly. Func- tions that are different to the general custom memory functions are provided so that special-purpose external code can be used for this case, because the memory blocks are all the same size. The blocks are retained by pcre2_match() until it is about to exit so that they can be re-used when possible during the match. In the absence of these func- tions, the normal custom memory management functions are used, if sup- plied, otherwise the system functions. 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_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 limit for the number of internal matching function calls in a pcre2_match() execu- tion. Further details are given with pcre2_match() below. 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_RECURSIONLIMIT The output is a uint32_t integer that gives the default limit for the depth of recursion when calling the internal matching function in a pcre2_match() execution. Further details are given with pcre2_match() below. PCRE2_CONFIG_STACKRECURSE The output is a uint32_t integer that is set to one if internal recur- sion when running pcre2_match() is implemented by recursive function calls that use the system stack to remember their state. This is the usual way that PCRE2 is compiled. The output is zero if PCRE2 was com- piled to use blocks of data on the heap instead of recursive function calls. 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, "7.0.0") 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 12 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); pcre2_code_free(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. The caller must free the memory by calling pcre2_code_free() when it is no longer needed. 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 options argument contains various bit settings that affect the com- pilation. 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 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, if compilation of a pattern fails, pcre2_compile() returns NULL, having set these variables to an error code and an offset (number of code units) within the pattern, respectively. The pcre2_get_error_message() function provides a textual message for each error code. Compilation errors are positive numbers, but UTF formatting errors are negative numbers. 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_AUTO_CALLOUT If this bit is set, pcre2_compile() automatically inserts callout items, all with number 255, before each pattern item. For discussion 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_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. 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. If there are no new- lines in a subject string, or no occurrences of ^ or $ in a pattern, setting PCRE2_MULTILINE has no effect. 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 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 may be useful in applications that process patterns from external sources. The combination 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. 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_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. 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_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 over 80 positive error codes that pcre2_compile() may return 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 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 UTF 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 all 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 information about a compiled pattern. The first argument is a pointer to the compiled 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 numbers: 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 option settings at the start of the pattern itself. In other words, they are the options that will be in force when matching starts. For example, if the pattern /(?im)abc(?-i)d/ is compiled with the PCRE2_EXTENDED option, the result is PCRE2_CASELESS, PCRE2_MULTILINE, and PCRE2_EXTENDED. A pattern is automatically anchored by PCRE2 if all of its top-level alternatives begin with one of the following: ^ unless PCRE2_MULTILINE is set \A always \G always .* if PCRE2_DOTALL is set and there are no back references to the subpattern in which .* appears For such patterns, 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. Zero is returned if there are no back references. 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 number of capturing subpatterns in the pattern. The third argument should point to an uint32_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 character value can be retrieved using PCRE2_INFO_FIRSTCODEUNIT. If there is no fixed first value, and if either (a) the pattern was compiled with the PCRE2_MULTILINE option, and every branch starts with "^", or (b) every branch of the pattern starts with ".*" and PCRE2_DOTALL is not set (if it were set, the pattern would be anchored), 2 is returned, indicating that the pattern matches only at the start of a subject string or after any newline within the string. Otherwise 0 is returned. 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_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_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. 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 fol- lows 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 can match an empty string, otherwise 0. The third argument should point to an uint32_t variable. 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): (? (?(\d\d)?\d\d) - (?\d\d) - (?\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 a uint32_t with one of the following 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 specifies the default character sequence that will be recognized as meaning "newline" while matching. PCRE2_INFO_RECURSIONLIMIT If the pattern set a recursion limit by including an item of the form (*LIMIT_RECURSION=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_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 does not include the size of the pcre2_code structure that is returned by pcre_compile(). The value that is used when pcre2_compile() is getting memory in which to place the compiled data is the value returned by this option plus the size of the pcre2_code structure. Pro- cessing a pattern with the JIT compiler does not alter the value returned by this option. THE MATCH DATA BLOCK pcre2_match_data_create(uint32_t ovecsize, pcre2_general_context *gcontext); 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 successful and unsuccessful matches 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 know 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 another 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. For pcre2_match_data_create_from_pattern(), the first argument is a pointer to a compiled pattern. In this case the ovector is created to be exactly the right size to hold all the substrings a pattern might capture. The second argument of both these functions is a pointer to a general 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. A match data block can be used many times, with the same or different compiled patterns. When it is no longer needed, it should be freed by calling pcre2_match_data_free(). 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. 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, one 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. 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_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, PCRE2_NO_UTF_CHECK, PCRE2_PARTIAL_HARD, and PCRE2_PARTIAL_SOFT. Their action is described below. Setting PCRE2_ANCHORED at match time is not supported by the just-in- time (JIT) compiler. If it is set, JIT matching is disabled and the normal interpretive code in pcre2_match() is run. 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_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_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. The entire string is checked 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. The value of startoff- set is also checked, to ensure that it points to the start of a charac- ter or to the end of the subject. 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 all the matches in a single subject string. NOTE: 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. During matching, the newline choice affects the behaviour 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 escape sequences. Implicit matches such as [^X] do not count, nor does \s, even though it includes CR and LF in the characters 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. The overall matched string and any captured substrings are returned to the caller via a vector of PCRE2_SIZE values. This is called the ovec- tor, and is contained within the match data block. You can obtain direct access to the ovector by calling pcre2_get_ovector_pointer() to find its address, and pcre2_get_ovector_count() to find the number of pairs of values it contains. Alternatively, you can use the auxiliary functions for accessing captured substrings by number or by name (see below). 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. The first pair of offsets (that is, ovector[0] and ovector[1]) identi- fies the portion of the subject string that was matched by the entire pattern. The next pair is used for the first capturing subpattern, and so on. The value returned by pcre2_match() is one more than the high- est numbered pair that has been set. For example, if two substrings have been captured, the returned value is 3. If there are no capturing subpatterns, the return value from a successful match is 1, indicating that just the first pair of offsets has been set. If a capturing subpattern is matched repeatedly within a single match operation, it is the last portion of the string 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). However, if the pattern contains back references and the ovector is not big enough to remember the related substrings, PCRE2 has to get additional memory for use during matching. Thus it is usually advisable to set up a match data block containing an ovector of reason- able size. 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. When a (*MARK) name is to be passed back, pcre2_get_mark() returns a pointer to the zero-terminated name, which is within the compiled pat- tern. Otherwise NULL is returned. A (*MARK) name may be available after a failed match or a partial match, as well as after a successful one. The code unit offset of the character at which a successful match started is returned by pcre2_get_startchar(). For a non-partial match, this can be different to the value of ovector[0] if the pattern con- tains 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. The startchar field is also used to return the offset of an invalid UTF character when UTF checking fails. Details are given in the pcre2uni- code 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 pcre2_get_error_message(). 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 check- ing 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 pattern that was compiled by the 8-bit library is passed to a 16-bit or 32-bit library function, or vice versa. 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() to return a distinctive error code. See the pcre2callout documentation for details. 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_BADOPTION This error is returned when a pattern that was successfully studied using JIT is being matched, but the matching mode (partial or complete match) does not correspond to any JIT compilation mode. When the JIT fast path function is used, this error may be also given for invalid options. See the pcre2jit documentation for more details. 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 limit was reached. PCRE2_ERROR_NOMEMORY If a pattern contains back references, but the ovector is not big enough to remember the referenced substrings, PCRE2 gets a block of memory at the start of matching to use for this purpose. There are some other special cases where extra memory is needed during matching. This error is given when memory cannot be obtained. 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. PCRE2_ERROR_RECURSIONLIMIT The internal recursion limit was reached. EXTRACTING CAPTURED SUBSTRINGS BY NUMBER int pcre2_substring_length_bynumber(pcre2_match_data *match_data, unsigned int number, PCRE2_SIZE *length); int pcre2_substring_copy_bynumber(pcre2_match_data *match_data, unsigned int number, PCRE2_UCHAR *buffer, PCRE2_SIZE *bufflen); int pcre2_substring_get_bynumber(pcre2_match_data *match_data, unsigned int 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. The functions in this section identify substrings by number. The next section describes similar functions for extracting substrings by name. 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. 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. The pcre2_substring_copy_bynumber() function copies one 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 arguments of these func- tions are a pointer to the match data block and a capturing group num- ber. A group number of zero extracts the substring that matched the entire pattern, and higher values extract the captured substrings. 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 these functions is zero for success, or one of these error codes: 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 No substring with the given number was captured. This could be because there is no capturing group of that number in the pattern, or because the group with that number did not participate in the match, or because the ovector was too small to capture that group. 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. 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. 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(?\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, or use one of the functions described above. For convenience, there are also "byname" functions that correspond to the "bynumber" functions, the only differ- ence being that the second argument is a name instead of a number. How- ever, if PCRE2_DUPNAMES is set and there are duplicate names, the be- haviour may not be what you want. 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 replacementzfP, 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. 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 in the pattern. The following forms are recognized: $$ insert a dollar character $ insert the contents of group ${} insert the contents of group Either a group number or a group name can be given for . 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+=". Group insertion is done by calling pcre2_copy_byname() or pcre2_copy_bynumber() as appropriate. 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. There is one additional option, PCRE2_SUBSTITUTE_GLOBAL, which causes the function to iterate over the subject string, replacing every match- ing substring. If this is not set, only the first matching substring is replaced. The outlengthptr argument must point to a variable that contains the length, in code units, of the output buffer. It is updated to contain the length of the new string, excluding the trailing zero that is auto- matically added. The function 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 neg- ative error code is returned. Except for PCRE2_ERROR_NOMATCH (which is never returned), any errors from pcre2_match() or the substring copying functions are passed straight back. PCRE2_ERROR_BADREPLACEMENT is returned for an invalid replacement string (unrecognized sequence fol- lowing a dollar sign), and PCRE2_ERROR_NOMEMORY is returned if the out- put buffer is not big enough. 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. If none are set, PCRE2_ERROR_NOSUBSTRING is returned. The pcre2_substring_number_from_name() function returns the error PCRE2_ERROR_NOUNIQUESUBSTRING. 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 above. 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, and does not backtrack. This has different characteristics to the normal algorithm, and is not compatible with Perl. Some of the features of PCRE2 patterns are not supported. Never- theless, 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 support, see the pcre2matching documen- tation. 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_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 no more the three matched strings are 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 in the same way as for pcre2_match(). They are returned in reverse order of length; that is, the longest matching string is given first. If there were too many matches to fit into the ovector, the yield of the func- tion 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 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), pcre2stack(3), pcre2unicode(3). AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 27 November 2014 Copyright (c) 1997-2014 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 options whose names begin with --enable or --disable. These settings specify changes to the defaults for the configure command. Because of the way that configure works, --enable and --disable always come in pairs, so the complemen- tary option always exists as well, but as it specifies the default, it is not described. 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 have to set the PCRE2_UTF option when they call pcre2_compile() to compile a pattern. UTF support allows the libraries to process character code points up to 0x10ffff in the strings that they handle. It also provides support for accessing the Unicode properties of such 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 documentation. JUST-IN-TIME COMPILER SUPPORT Just-in-time compiler support is included in the build by specifying --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 called. 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. AVOIDING EXCESSIVE STACK USAGE When matching with the pcre2_match() function, PCRE2 implements back- tracking by making recursive calls to an internal function called match(). In environments where the size of the stack is limited, this can severely limit PCRE2's operation. (The Unix environment does not usually suffer from this problem, but it may sometimes be necessary to increase the maximum stack size. There is a discussion in the pcre2stack documentation.) An alternative approach to recursion that uses memory from the heap to remember data, instead of using recursive function calls, has been implemented to work round the problem of lim- ited stack size. If you want to build a version of PCRE2 that works this way, add --disable-stack-for-recursion to the configure command. By default, the system functions malloc() and free() are called to manage the heap memory that is required, but cus- tom memory management functions can be called instead. PCRE2 runs noticeably more slowly when built in this way. This option affects only the pcre2_match() function; it is not relevant for pcre2_dfa_match(). LIMITING PCRE2 RESOURCE USAGE Internally, PCRE2 has a function called match(), which it calls repeat- edly (sometimes recursively) when matching a pattern with the pcre2_match() function. By controlling the maximum number of times this function may be called during a single matching operation, a limit can be placed on the resources used by a single call to pcre2_match(). The limit can be changed at run time, as described in the pcre2api documen- tation. 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. In some environments it is desirable to limit the depth of recursive calls of match() more strictly than the total number of calls, in order to restrict the maximum amount of stack (or heap, if --disable-stack- for-recursion is specified) that is used. A second limit controls this; it defaults to the value that is set for --with-match-limit, which imposes no additional constraints. However, you can set a lower limit by adding, for example, --with-match-limit-recursion=10000 to the configure command. This value can also be overridden at run time. 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 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 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 long- est line that is guaranteed to be processable is the parameter size. You can change the default parameter value by adding, for example, --with-pcre2grep-bufsize=50K to the configure command. The caller of pcre2grep can override this value by using --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. 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. SEE ALSO pcre2api(3), pcre2-config(3). AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 23 November 2014 Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------ PCRE2CALLOUT(3) Library Functions Manual PCRE2CALLOUT(3) NAME PCRE2 - Perl-compatible regular expressions (revised API) SYNOPSIS #include int (*pcre2_callout)(pcre2_callout_block *, void *); 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) indicates the points 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. For example, this pattern has two callout points: (?C1)abc(?C2)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. For example, if PCRE2_AUTO_CALLOUT is used with the pattern A(\d{2}|--) it 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) This applies only to assertion conditions (because they are themselves independent groups). Automatic callouts can be used for tracking the progress of pattern matching. The pcre2test program has a pattern qualifier (/auto_call- out) that sets automatic callouts; when it is used, the output indi- cates how the pattern is being matched. This is useful information when you are trying to optimize the performance of a particular 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. 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+ and therefore the callouts that would be taken for the back- tracks do not occur. You can disable the auto-possessify feature by passing PCRE2_NO_AUTO_POSSESS to pcre2_compile(), or starting the pat- tern 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. 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. 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; The version field contains the version number of the block format. The current version is 0. The version number will change in future if addi- tional fields are added, but the intention is never to remove any of the existing fields. The callout_number field contains the number of the callout, as com- piled into the pattern (that is, the number after ?C for manual call- outs, and 255 for automatically generated callouts). The offset_vector field is a pointer to the vector of capturing offsets (the "ovector") that was passed to the matching function in the match data block. When pcre2_match() is used, the contents can be inspected in order to extract substrings that have been matched so far, in the same way as for extracting substrings after a match has completed. For the DFA matching function, this field is not useful. 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. When the pcre2_match() is used, the capture_top field contains one more than the number of the highest numbered captured substring so far. If no substrings have been captured, the value of capture_top is one. This is always the case when the DFA functions are used, because they do not support captured substrings. The capture_last field contains the number of the most recently cap- tured substring. However, when a recursion exits, the value reverts to what it was outside the recursion, as do the values of all captured substrings. If no substrings have been captured, the value of cap- ture_last is 0. This is always the case for the DFA matching functions. The pattern_position field contains the offset to the next item to be matched in the pattern string. The next_item_length field contains the length of the next item to be matched in the pattern string. When the callout immediately precedes an alternation bar, a closing parenthesis, or the end of the pattern, the length is zero. When the callout precedes an opening parenthesis, the length is that of the entire subpattern. 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. 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 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. AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 25 November 2014 Copyright (c) 1997-2014 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 5.10 and above. 1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does have are given in the pcre2unicode page. 2. PCRE2 allows repeat quantifiers only on parenthesized assertions, 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 repeat quantifiers on other assertions such as \b, but these do not seem to have any use. 3. Capturing subpatterns that occur inside negative lookahead asser- tions are counted, but their entries in the offsets vector are never set. Perl sometimes (but not always) sets its numerical variables from inside negative assertions. 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 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 Unicode characters, there is no need to implement the somewhat messy concept of surro- gates." 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 for recursive pat- terns. This is not available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE2 "callout" feature allows an external function to be called during pattern matching. See the pcre2callout documentation for details. 8. Subpatterns that are called as subroutines (whether or not recur- sively) are always treated as atomic groups in PCRE2. This is like Python, but unlike Perl. Captured values that are set outside a sub- routine call can be reference from inside in PCRE2, but not in Perl. There is a discussion that explains these differences in more detail in the section on recursion differences from Perl in the pcre2pattern page. 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 examples 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)|(? is matched against the string 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. The appearance of \K in the pattern has no effect for a partial match. Consider this pat- tern: /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: 14 October 2014 Copyright (c) 1997-2014 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 in the alternative matching function pcre2_dfa_match(), nor is it supported in UTF mode by the JIT optimization. If JIT optimization is requested for a UTF pattern that contains \C, it will not succeed, and so the match- ing 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. The entire string is checked before any other processing takes place. 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: PCRE_UTF16_ERR1 Missing low surrogate at end of string PCRE_UTF16_ERR2 Invalid low surrogate follows high surrogate PCRE_UTF16_ERR3 Isolated low surrogate Errors in UTF-32 strings The following negative error codes are given for invalid UTF-32 strings: PCRE_UTF32_ERR1 Surrogate character (range from 0xd800 to 0xdfff) PCRE_UTF32_ERR2 Code point is greater than 0x10ffff AUTHOR Philip Hazel University Computing Service Cambridge, England. REVISION Last updated: 23 November 2014 Copyright (c) 1997-2014 University of Cambridge. ------------------------------------------------------------------------------