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 is no POSIX-style wrapper 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 other options that are not defined by POSIX. These have been added  at
       the request of users who want to make use of certain PCRE2-specific features via the POSIX
       calling interface.

       When PCRE2 is called via these functions, it is only the API that is POSIX-like in  style.
       The  syntax  and  semantics of the regular 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.  The  pattern
       is a C string terminated by a binary zero, and is passed in the argument pattern. The preg
       argument is a pointer to  a  regex_t  structure  that  is  used  as  a  base  for  storing
       information about the compiled regular expression.

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

         REG_DOTALL

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

         REG_ICASE

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

         REG_NEWLINE

       The PCRE2_MULTILINE option is set when the regular expression is passed for compilation to
       the  native  function.  Note  that  this  does  not  mimic the defined POSIX behaviour for
       REG_NEWLINE (see the following section).

         REG_NOSUB

       The PCRE2_NO_AUTO_CAPTURE option  is  set  when  the  regular  expression  is  passed  for
       compilation to the native function. In addition, 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.

         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 member of the structure is public: re_nsub contains the
       number of capturing subpatterns in the regular expression. Various error codes are defined
       in the header file.

       NOTE:  If  the yield of regcomp() is non-zero, you must not attempt to use the contents of
       the preg structure. If, for example, you pass it to regexec(), the result is undefined and
       your program is likely to crash.

MATCHING NEWLINE CHARACTERS

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

                                 Default   Change with

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

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

                                 Default   Change with

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

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

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

       The string is considered to start at string + pmatch[0].rm_so and to  have  a  terminating
       NUL  located  at  string  +  pmatch[0].rm_eo  (there  need  not  actually be a NUL at that
       location), regardless of the value of nmatch. This is a BSD extension, compatible with but
       not  specified  by  IEEE  Standard  1003.2  (POSIX.2),  and should be used with caution in
       software intended to be portable to other systems. Note that a  non-zero  rm_so  does  not
       imply  REG_NOTBOL;  REG_STARTEND  affects  only  the location of the string, not how it is
       matched. Setting REG_STARTEND and passing pmatch as NULL are mutually exclusive; the error
       REG_INVARG is returned.

       If  the pattern was compiled with the REG_NOSUB flag, no data about any matched strings is
       returned. The nmatch and pmatch arguments of regexec() are ignored.

       If the value of nmatch is zero, or if the value pmatch is NULL, 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: 29 November 2015
       Copyright (c) 1997-2015 University of Cambridge.