PCRE2POSIX(3)PCRE2POSIX(3)NAME
PCRE2 - Perl-compatible regular expressions (revised API)
SYNOPSIS
#include <pcre2posix.h>
int regcomp(regex_t *preg, const char *pattern,
int cflags);
int regexec(const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch[], int eflags);
size_t regerror(int errcode, const regex_t *preg,
char *errbuf, size_t errbuf_size);
void regfree(regex_t *preg);
DESCRIPTION
This set of functions provides a POSIX-style API for the PCRE2 regular
expression 8-bit library. See the pcre2api documentation for a descrip‐
tion of PCRE2's native API, which contains much additional functional‐
ity. There 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 expres‐
sions themselves are still those of Perl, subject to the setting of
various PCRE2 options, as described below. "POSIX-like in style" means
that the API approximates to the POSIX definition; it is not fully
POSIX-compatible, and in multi-unit encoding domains it is probably
even less compatible.
The header for these functions is supplied as pcre2posix.h to avoid any
potential clash with other POSIX libraries. It can, of course, be
renamed or aliased as regex.h, which is the "correct" name. It provides
two structure types, regex_t for compiled internal forms, and reg‐
match_t for returning captured substrings. It also defines some con‐
stants whose names start with "REG_"; these are used for setting
options and identifying error codes.
COMPILING A PATTERN
The function regcomp() is called to compile a pattern into an internal
form. The pattern is a C string terminated by a binary zero, and is
passed in the argument pattern. The preg argument is a pointer to a
regex_t structure that is used as a base for storing information about
the compiled regular expression.
The argument cflags is either zero, or contains one or more of the bits
defined by the following macros:
REG_DOTALL
The PCRE2_DOTALL option is set when the regular expression is passed
for compilation to the native function. Note that REG_DOTALL is not
part of the POSIX standard.
REG_ICASE
The PCRE2_CASELESS option is set when the regular expression is passed
for compilation to the native function.
REG_NEWLINE
The PCRE2_MULTILINE option is set when the regular expression is passed
for compilation to the native function. Note that this does not mimic
the defined POSIX behaviour for REG_NEWLINE (see the following sec‐
tion).
REG_NOSUB
The PCRE2_NO_AUTO_CAPTURE option is set when the regular expression is
passed for compilation to the native function. In addition, when a pat‐
tern that is compiled with this flag is passed to regexec() for match‐
ing, 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 metacharac‐
ter (they are not) or by a negative class such as [^a] (they are).
The yield of regcomp() is zero on success, and non-zero otherwise. The
preg structure is filled in on success, and one member of the structure
is public: re_nsub contains the number of capturing subpatterns in the
regular expression. Various error codes are defined in the header file.
NOTE: If the yield of regcomp() is non-zero, you must not attempt to
use the contents of the preg structure. If, for example, you pass it to
regexec(), the result is undefined and your program is likely to crash.
MATCHING NEWLINE CHARACTERS
This area is not simple, because POSIX and Perl take different views of
things. It is not possible to get PCRE2 to obey POSIX semantics, but
then PCRE2 was never intended to be a POSIX engine. The following table
lists the different possibilities for matching newline characters in
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 POSIX:
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
PCRE2's behaviour is the same as Perl's, except that there is no equiv‐
alent for PCRE2_DOLLAR_ENDONLY in Perl. In both PCRE2 and Perl, there
is no way to stop newline from matching [^a].
The default POSIX newline handling can be obtained by setting
PCRE2_DOTALL and PCRE2_DOLLAR_ENDONLY, but there is no way to make
PCRE2 behave exactly as for the REG_NEWLINE action.
MATCHING A PATTERN
The function regexec() is called to match a compiled pattern preg
against a given string, which is by default terminated by a zero byte
(but see REG_STARTEND below), subject to the options in eflags. These
can be:
REG_NOTBOL
The PCRE2_NOTBOL option is set when calling the underlying PCRE2 match‐
ing function.
REG_NOTEMPTY
The PCRE2_NOTEMPTY option is set when calling the underlying PCRE2
matching function. Note that REG_NOTEMPTY is not part of the POSIX
standard. However, setting this option can give more POSIX-like behav‐
iour in some situations.
REG_NOTEOL
The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match‐
ing function.
REG_STARTEND
The string is considered to start at string + pmatch[0].rm_so and to
have a terminating NUL located at string + pmatch[0].rm_eo (there need
not actually be a NUL at that location), regardless of the value of
nmatch. This is a BSD extension, compatible with but not specified by
IEEE Standard 1003.2 (POSIX.2), and should be used with caution in
software intended to be portable to other systems. Note that a non-zero
rm_so does not imply REG_NOTBOL; REG_STARTEND affects only the location
of the string, not how it is matched.
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 cap‐
tured substrings, are returned via the pmatch argument, which points to
an array of nmatch structures of type regmatch_t, containing the mem‐
bers rm_so and rm_eo. These contain the byte offset to the first char‐
acter 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. The length of the message,
including the zero, is limited to errbuf_size. The yield of the func‐
tion is the size of buffer needed to hold the whole message.
MEMORY USAGE
Compiling a regular expression causes memory to be allocated and asso‐
ciated with the preg structure. The function regfree() frees all such
memory, after which preg may no longer be used as a compiled expres‐
sion.
AUTHOR
Philip Hazel
University Computing Service
Cambridge, England.
REVISION
Last updated: 20 October 2014
Copyright (c) 1997-2014 University of Cambridge.
PCRE2 10.00 20 October 2014 PCRE2POSIX(3)
