NAME

       PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

       #include <pcre2posix.h>

       int regcomp(regex_t *preg, const char *pattern,
            int cflags);

       int regexec(const regex_t *preg, const char *string,
            size_t nmatch, regmatch_t pmatch[], int eflags);

       size_t regerror(int errcode, const regex_t *preg,
            char *errbuf, size_t errbuf_size);

       void regfree(regex_t *preg);

DESCRIPTION

       This  set of functions provides a POSIX-style API for the PCRE2 regular expression 8-bit library. See the
       pcre2api documentation  for  a  description  of  PCRE2's  native  API,  which  contains  much  additional
       functionality. There are no POSIX-style wrappers for PCRE2's 16-bit and 32-bit libraries.

       The  functions described here are just wrapper functions that ultimately call the PCRE2 native API. Their
       prototypes are defined in the pcre2posix.h header file, and on Unix systems the library itself is  called
       libpcre2-posix.a,  so  can  be accessed by adding -lpcre2-posix to the command for linking an application
       that uses them. Because the POSIX functions call the native ones, it is also necessary to add -lpcre2-8.

       Those POSIX option bits that can reasonably be mapped to PCRE2 native options have been  implemented.  In
       addition,  the option REG_EXTENDED is defined with the value zero. This has no effect, but since programs
       that are written to the POSIX interface often use it, this  makes  it  easier  to  slot  in  PCRE2  as  a
       replacement library. Other POSIX options are not even defined.

       There  are also some options that are not defined by POSIX. These have been added at the request of users
       who want to make use of certain PCRE2-specific features via the POSIX calling interface 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  header for these functions is supplied as pcre2posix.h to avoid any potential clash with other POSIX
       libraries. It can, of course, be renamed or aliased as regex.h, which is the "correct" name. It  provides
       two  structure  types,  regex_t  for  compiled  internal  forms,  and  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.

COMPILING A PATTERN

       The function regcomp() is called to compile a pattern into an internal form. By default, the pattern is a
       C string terminated by a binary zero (but see REG_PEND below). The  preg  argument  is  a  pointer  to  a
       regex_t  structure  that is used as a base for storing information about the compiled regular expression.
       (It is also used for input when REG_PEND is set.)

       The argument cflags is either zero, or contains one or more of the bits defined by the following macros:

         REG_DOTALL

       The PCRE2_DOTALL option is set when the regular expression  is  passed  for  compilation  to  the  native
       function. Note that REG_DOTALL is not part of the POSIX standard.

         REG_ICASE

       The  PCRE2_CASELESS  option  is  set  when the regular expression is passed for compilation to the native
       function.

         REG_NEWLINE

       The PCRE2_MULTILINE option is set when the regular expression is passed for  compilation  to  the  native
       function.  Note  that  this does not mimic the defined POSIX behaviour for REG_NEWLINE (see the following
       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 regexec() for matching, the nmatch and pmatch
       arguments are ignored, and no captured strings are returned. Versions of the PCRE library prior to  10.22
       used  to set the PCRE2_NO_AUTO_CAPTURE compile option, but this no longer happens because it disables the
       use of back references.

         REG_PEND

       If this option is set, the reg_endp field in the preg structure (which has the type const char *) must be
       set  to point to the character beyond the end of the pattern before calling regcomp(). The pattern itself
       may now contain binary zeroes, 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 matchine \d, \w, etc., instead of just recognizing ASCII
       values. Note that REG_UCP is not part of the POSIX standard.

         REG_UNGREEDY

       The PCRE2_UNGREEDY option is set when the regular expression is passed  for  compilation  to  the  native
       function. Note that REG_UNGREEDY is not part of the POSIX standard.

         REG_UTF

       The PCRE2_UTF option is set when the regular expression is passed for compilation to the native function.
       This causes the pattern itself and all data strings used for matching it to be treated as UTF-8  strings.
       Note that REG_UTF is not part of the POSIX standard.

       In the absence of these flags, no options are passed to the native function.  This means the the regex is
       compiled with PCRE2 default semantics. In particular, the  way  it  handles  newline  characters  in  the
       subject string is the Perl way, not the POSIX way. Note that setting PCRE2_MULTILINE has only some of the
       effects specified for REG_NEWLINE.  It  does  not  affect  the  way  newlines  are  matched  by  the  dot
       metacharacter (they are not) or by a negative class such as [^a] (they are).

       The  yield  of  regcomp()  is zero on success, and non-zero otherwise. The preg structure is filled in on
       success, and one 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 regcomp() is non-zero, you must not  attempt  to  use  the  contents  of  the  preg
       structure.  If, for example, you pass it to regexec(), the result is undefined and your program is likely
       to crash.

MATCHING NEWLINE CHARACTERS

       This area is not simple, because POSIX and Perl take different views of things.  It is  not  possible  to
       get  PCRE2 to obey POSIX semantics, but then PCRE2 was never intended to be a POSIX engine. The following
       table lists the different possibilities for matching newline characters in Perl and PCRE2:

                                 Default   Change with

         . matches newline          no     PCRE2_DOTALL
         newline matches [^a]       yes    not changeable
         $ matches \n at end        yes    PCRE2_DOLLAR_ENDONLY
         $ matches \n in middle     no     PCRE2_MULTILINE
         ^ matches \n in middle     no     PCRE2_MULTILINE

       This is the equivalent table for a POSIX-compatible pattern matcher:

                                 Default   Change with

         . matches newline          yes    REG_NEWLINE
         newline matches [^a]       yes    REG_NEWLINE
         $ matches \n at end        no     REG_NEWLINE
         $ matches \n in middle     no     REG_NEWLINE
         ^ matches \n in middle     no     REG_NEWLINE

       This behaviour is not what happens when PCRE2 is called via its POSIX API. By default, PCRE2's  behaviour
       is the same as Perl's, except that there is no equivalent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2
       and Perl, there is no way to stop newline from matching [^a].

       Default POSIX newline handling can be obtained by  setting  PCRE2_DOTALL  and  PCRE2_DOLLAR_ENDONLY  when
       calling pcre2_compile() directly, but there is no way to make PCRE2 behave exactly as for the REG_NEWLINE
       action.  When  using  the  POSIX  API,  passing  REG_NEWLINE  to  PCRE2's   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 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 is 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 zeroes
       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 regexec() are ignored (except possibly as input for REG_STARTEND).

       The value of nmatch may be zero, and the value pmatch may be NULL (unless REG_STARTEND is set);  in  both
       these cases no data about any matched strings is returned.

       Otherwise, the portion of the string that was matched, and also any captured substrings, are returned via
       the pmatch argument, which points to an array of nmatch structures of  type  regmatch_t,  containing  the
       members  rm_so  and rm_eo. These contain the byte offset to the first character of each substring and the
       offset to the first character after the end of each substring,  respectively.  The  0th  element  of  the
       vector  relates  to  the  entire  portion  of  string that was matched; subsequent elements relate to the
       capturing subpatterns of the regular expression. Unused entries in the array have both structure  members
       set to -1.

       A  successful  match  yields  a zero return; various error codes are defined in the header file, of which
       REG_NOMATCH is the "expected" failure code.

ERROR MESSAGES

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

MEMORY USAGE

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

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge, England.

REVISION

       Last updated: 15 June 2017
       Copyright (c) 1997-2017 University of Cambridge.