# pcre2posix

## NAME



           PCRE2 - Perl-compatible regular expressions (revised API)
    


## SYNOPSIS



    
           **#include <pcre2posix.h>**
    
           **int pcre2_regcomp(regex_t ***_preg_**, const char ***_pattern_**,**
                **int** _cflags_**);**
    
           **int pcre2_regexec(const regex_t ***_preg_**, const char ***_string_**,**
                **size_t** _nmatch_**, regmatch_t** _pmatch_**[], int** _eflags_**);**
    
           **size_t pcre2_regerror(int** _errcode_**, const regex_t ***_preg_**,**
                **char ***_errbuf_**, size_t** _errbuf_size_**);**
    
           **void pcre2_regfree(regex_t ***_preg_**);**
    


## DESCRIPTION



    
           This set of functions provides a POSIX-style API for the PCRE2
           regular expression 8-bit library. There are no POSIX-style
           wrappers for PCRE2's 16-bit and 32-bit libraries. See the **pcre2api**
           documentation for a description of PCRE2's native API, which
           contains much additional functionality.
    
           **IMPORTANT NOTE** : The functions described here are NOT thread-safe,
           and should not be used in multi-threaded applications. They are
           also limited to processing subjects that are not bigger than 2GB.
           Use the native API instead.
    
           These functions are wrapper functions that ultimately call the
           PCRE2 native API. Their prototypes are defined in the **pcre2posix.h**
           header file, and they all have unique names starting with **pcre2_**.
           However, the **pcre2posix.h** header also contains macro definitions
           that convert the standard POSIX names such **regcomp()** into
           **pcre2_regcomp()** etc. This means that a program can use the usual
           POSIX names without running the risk of accidentally linking with
           POSIX functions from a different library.
    
           On Unix-like systems the PCRE2 POSIX library is called
           **libpcre2-posix** , so can be accessed by adding **-lpcre2-posix** to the
           command for linking an application. Because the POSIX functions
           call the native ones, it is also necessary to add **-lpcre2-8**.
    
           On Windows systems, if you are linking to a DLL version of the
           library, it is recommended that **PCRE2POSIX_SHARED** is defined
           before including the **pcre2posix.h** header, as it will allow for a
           more efficient way to invoke the functions by adding the
           **__declspec(dllimport)** decorator.
    
           Although they were not defined as prototypes in **pcre2posix.h** ,
           releases 10.33 to 10.36 of the library contained functions with
           the POSIX names **regcomp()** etc. These simply passed their arguments
           to the PCRE2 functions. These functions were provided for
           backwards compatibility with earlier versions of PCRE2, which had
           only POSIX names. However, this has proved troublesome in
           situations where a program links with several libraries, some of
           which use PCRE2's POSIX interface while others use the real POSIX
           functions.  For this reason, the POSIX names have been removed
           since release 10.37.
    
           Calling the header file **pcre2posix.h** avoids any conflict with
           other POSIX libraries. It can, of course, be renamed or aliased as
           **regex.h** , which is the "correct" name, if there is no clash. It
           provides two structure types, _regex_t_ for compiled internal forms,
           and _regmatch_t_ for returning captured substrings. It also defines
           some constants whose names start with "REG_"; these are used for
           setting options and identifying error codes.
    


## USING THE POSIX FUNCTIONS



    
           Note that these functions are just POSIX-style wrappers for
           PCRE2's native API.  They do not give POSIX regular expression
           behaviour, and they are not thread-safe or even POSIX compatible.
    
           Those POSIX option bits that can reasonably be mapped to PCRE2
           native options have been implemented. In addition, the option
           REG_EXTENDED is defined with the value zero. This has no effect,
           but since programs that are written to the POSIX interface often
           use it, this makes it easier to slot in PCRE2 as a replacement
           library. Other POSIX options are not even defined.
    
           There are also some options that are not defined by POSIX. These
           have been added at the request of users who want to make use of
           certain PCRE2-specific features via the POSIX calling interface or
           to add BSD or GNU functionality.
    
           When PCRE2 is called via these functions, it is only the API that
           is POSIX-like in style. The syntax and semantics of the regular
           expressions themselves are still those of Perl, subject to the
           setting of various PCRE2 options, as described below. "POSIX-like
           in style" means that the API approximates to the POSIX definition;
           it is not fully POSIX-compatible, and in multi-unit encoding
           domains it is probably even less compatible.
    
           The descriptions below use the actual names of the functions, but,
           as described above, the standard POSIX names (without the **pcre2_**
           prefix) may also be used.
    


## COMPILING A PATTERN



    
           The function **pcre2_regcomp()** is called to compile a pattern into
           an internal form. By default, the pattern is a C string terminated
           by a binary zero (but see REG_PEND below). The _preg_ argument is a
           pointer to a **regex_t** structure that is used as a base for storing
           information about the compiled regular expression. It is also used
           for input when REG_PEND is set. The **regex_t** structure used by
           **pcre2_regcomp()** is defined in **pcre2posix.h** and is not the same as
           the structure used by other libraries that provide POSIX-style
           matching.
    
           The argument _cflags_ is either zero, or contains one or more of the
           bits defined by the following macros:
    
             REG_DOTALL
    
           The PCRE2_DOTALL option is set when the regular expression is
           passed for compilation to the native function. Note that
           REG_DOTALL is not part of the POSIX standard.
    
             REG_ICASE
    
           The PCRE2_CASELESS option is set when the regular expression is
           passed for compilation to the native function.
    
             REG_NEWLINE
    
           The PCRE2_MULTILINE option is set when the regular expression is
           passed for compilation to the native function. Note that this does
           _not_ mimic the defined POSIX behaviour for REG_NEWLINE (see the
           following section).
    
             REG_NOSPEC
    
           The PCRE2_LITERAL option is set when the regular expression is
           passed for compilation to the native function. This disables all
           meta characters in the pattern, causing it to be treated as a
           literal string. The only other options that are allowed with
           REG_NOSPEC are REG_ICASE, REG_NOSUB, REG_PEND, and REG_UTF. Note
           that REG_NOSPEC is not part of the POSIX standard.
    
             REG_NOSUB
    
           When a pattern that is compiled with this flag is passed to
           **pcre2_regexec()** for matching, the _nmatch_ and _pmatch_ arguments are
           ignored, and no captured strings are returned. Versions of the
           PCRE2 library prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE
           compile option, but this no longer happens because it disables the
           use of backreferences.
    
             REG_PEND
    
           If this option is set, the **reg_endp** field in the _preg_ structure
           (which has the type const char *) must be set to point to the
           character beyond the end of the pattern before calling
           **pcre2_regcomp()**. The pattern itself may now contain binary zeros,
           which are treated as data characters. Without REG_PEND, a binary
           zero terminates the pattern and the **re_endp** field is ignored. This
           is a GNU extension to the POSIX standard and should be used with
           caution in software intended to be portable to other systems.
    
             REG_UCP
    
           The PCRE2_UCP option is set when the regular expression is passed
           for compilation to the native function. This causes PCRE2 to use
           Unicode properties when matching \d, \w, etc., instead of just
           recognizing ASCII values. Note that REG_UCP is not part of the
           POSIX standard.
    
             REG_UNGREEDY
    
           The PCRE2_UNGREEDY option is set when the regular expression is
           passed for compilation to the native function. Note that
           REG_UNGREEDY is not part of the POSIX standard.
    
             REG_UTF
    
           The PCRE2_UTF option is set when the regular expression is passed
           for compilation to the native function. This causes the pattern
           itself and all data strings used for matching it to be treated as
           UTF-8 strings. Note that REG_UTF is not part of the POSIX
           standard.
    
           In the absence of these flags, no options are passed to the native
           function.  This means that the regex is compiled with PCRE2
           default semantics. In particular, the way it handles newline
           characters in the subject string is the Perl way, not the POSIX
           way. Note that setting PCRE2_MULTILINE has only _some_ of the
           effects specified for REG_NEWLINE. It does not affect the way
           newlines are matched by the dot metacharacter (they are not) or by
           a negative class such as [^a] (they are).
    
           The yield of **pcre2_regcomp()** is zero on success, and non-zero
           otherwise.  The _preg_ structure is filled in on success, and one
           other member of the structure (as well as _re_endp_) is public:
           _re_nsub_ contains the number of capturing subpatterns in the
           regular expression. Various error codes are defined in the header
           file.
    
           NOTE: If the yield of **pcre2_regcomp()** is non-zero, you must not
           attempt to use the contents of the _preg_ structure. If, for
           example, you pass it to **pcre2_regexec()** , the result is undefined
           and your program is likely to crash.
    


## MATCHING NEWLINE CHARACTERS



    
           This area is not simple, because POSIX and Perl take different
           views of things.  It is not possible to get PCRE2 to obey POSIX
           semantics, but then PCRE2 was never intended to be a POSIX engine.
           The following table lists the different possibilities for matching
           newline characters in Perl and PCRE2:
    
                                     Default   Change with
    
             . matches newline          no     PCRE2_DOTALL
             newline matches [^a]       yes    not changeable
             $ matches \n at end        yes    PCRE2_DOLLAR_ENDONLY
             $ matches \n in middle     no     PCRE2_MULTILINE
             ^ matches \n in middle     no     PCRE2_MULTILINE
    
           This is the equivalent table for a POSIX-compatible pattern
           matcher:
    
                                     Default   Change with
    
             . matches newline          yes    REG_NEWLINE
             newline matches [^a]       yes    REG_NEWLINE
             $ matches \n at end        no     REG_NEWLINE
             $ matches \n in middle     no     REG_NEWLINE
             ^ matches \n in middle     no     REG_NEWLINE
    
           This behaviour is not what happens when PCRE2 is called via its
           POSIX API. By default, PCRE2's behaviour is the same as Perl's,
           except that there is no equivalent for PCRE2_DOLLAR_ENDONLY in
           Perl. In both PCRE2 and Perl, there is no way to stop newline from
           matching [^a].
    
           Default POSIX newline handling can be obtained by setting
           PCRE2_DOTALL and PCRE2_DOLLAR_ENDONLY when calling **pcre2_compile()**
           directly, but there is no way to make PCRE2 behave exactly as for
           the REG_NEWLINE action. When using the POSIX API, passing
           REG_NEWLINE to PCRE2's **pcre2_regcomp()** function causes
           PCRE2_MULTILINE to be passed to **pcre2_compile()** , and REG_DOTALL
           passes PCRE2_DOTALL. There is no way to pass PCRE2_DOLLAR_ENDONLY.
    


## MATCHING A PATTERN



    
           The function **pcre2_regexec()** is called to match a compiled pattern
           _preg_ against a given _string_ , which is by default terminated by a
           zero byte (but see REG_STARTEND below), subject to the options in
           _eflags_.  These can be:
    
             REG_NOTBOL
    
           The PCRE2_NOTBOL option is set when calling the underlying PCRE2
           matching function.
    
             REG_NOTEMPTY
    
           The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2
           matching function. Note that REG_NOTEMPTY is not part of the POSIX
           standard. However, setting this option can give more POSIX-like
           behaviour in some situations.
    
             REG_NOTEOL
    
           The PCRE2_NOTEOL option is set when calling the underlying PCRE2
           matching function.
    
             REG_STARTEND
    
           When this option is set, the subject string starts at _string_ +
           _pmatch[0].rm_so_ and ends at _string_ + _pmatch[0].rm_eo_ , which should
           point to the first character beyond the string. There may be
           binary zeros within the subject string, and indeed, using
           REG_STARTEND is the only way to pass a subject string that
           contains a binary zero.
    
           Whatever the value of _pmatch[0].rm_so_ , the offsets of the matched
           string and any captured substrings are still given relative to the
           start of _string_ itself. (Before PCRE2 release 10.30 these were
           given relative to _string_ + _pmatch[0].rm_so_ , but this differs from
           other implementations.)
    
           This is a BSD extension, compatible with but not specified by IEEE
           Standard 1003.2 (POSIX.2), and should be used with caution in
           software intended to be portable to other systems. Note that a
           non-zero _rm_so_ does not imply REG_NOTBOL; REG_STARTEND affects
           only the location and length of the string, not how it is matched.
           Setting REG_STARTEND and passing _pmatch_ as NULL are mutually
           exclusive; the error REG_INVARG is returned.
    
           If the pattern was compiled with the REG_NOSUB flag, no data about
           any matched strings is returned. The _nmatch_ and _pmatch_ arguments
           of **pcre2_regexec()** are ignored (except possibly as input for
           REG_STARTEND).
    
           The value of _nmatch_ may be zero, and the value _pmatch_ may be NULL
           (unless REG_STARTEND is set); in both these cases no data about
           any matched strings is returned.
    
           Otherwise, the portion of the string that was matched, and also
           any captured substrings, are returned via the _pmatch_ argument,
           which points to an array of _nmatch_ structures of type _regmatch_t_ ,
           containing the members _rm_so_ and _rm_eo_. These contain the byte
           offset to the first character of each substring and the offset to
           the first character after the end of each substring, respectively.
           The 0th element of the vector relates to the entire portion of
           _string_ that was matched; subsequent elements relate to the
           capturing subpatterns of the regular expression. Unused entries in
           the array have both structure members set to -1.
    
           _regmatch_t_ as well as the _regoff_t_ typedef it uses are defined in
           **pcre2posix.h** and are not warranted to have the same size or layout
           as other similarly named types from other libraries that provide
           POSIX-style matching.
    
           A successful match yields a zero return; various error codes are
           defined in the header file, of which REG_NOMATCH is the "expected"
           failure code.
    


## ERROR MESSAGES



    
           The **pcre2_regerror()** function maps a non-zero errorcode from
           either **pcre2_regcomp()** or **pcre2_regexec()** to a printable message.
           If _preg_ is not NULL, the error should have arisen from the use of
           that structure. A message terminated by a binary zero is placed in
           _errbuf_. If the buffer is too short, only the first _errbuf_size_ - 1
           characters of the error message are used. The yield of the
           function is the size of buffer needed to hold the whole message,
           including the terminating zero. This value is greater than
           _errbuf_size_ if the message was truncated.
    


## MEMORY USAGE



    
           Compiling a regular expression causes memory to be allocated and
           associated with the _preg_ structure. The function **pcre2_regfree()**
           frees all such memory, after which _preg_ may no longer be used as a
           compiled expression.
    


## REVISION



    
           Last updated: 27 November 2024
           Copyright (c) 1997-2024 University of Cambridge.
    


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