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SYNOPSIS OF POSIX API
int regcomp(regex_t *preg, const char *pattern,
int regexec(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);
This set of functions provides a POSIX-style API to the PCRE regular
expression package. See the pcreapi documentation for a description of
PCRE's native API, which contains much additional functionality.
The functions described here are just wrapper functions that ultimately
call the PCRE native API. Their prototypes are defined in the
pcreposix.h header file, and on Unix systems the library itself is
called pcreposix.a, so can be accessed by adding -lpcreposix to the
command for linking an application that uses them. Because the POSIX
functions call the native ones, it is also necessary to add -lpcre.
I have implemented only those option bits that can be reasonably mapped
to PCRE native options. 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 PCRE as a replacement library. Other POSIX options are not even
When PCRE 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 PCRE 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-byte encoding domains it is probably
even less compatible.
The header for these functions is supplied as pcreposix.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.
compilation to the native function. Note that REG_DOTALL is not part of
the POSIX standard.
The PCRE_CASELESS option is set when the regular expression is passed
for compilation to the native function.
The PCRE_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-
The PCRE_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.
The PCRE_UTF8 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_UTF8 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 PCRE 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
PCRE_MULTILINE has only some of the effects specified for REG_NEWLINE.
It does not affect the way newlines are matched by . (they aren't) 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.
MATCHING NEWLINE CHARACTERS
This area is not simple, because POSIX and Perl take different views of
things. It is not possible to get PCRE to obey POSIX semantics, but
then PCRE was never intended to be a POSIX engine. The following table
lists the different possibilities for matching newline characters in
Default Change with
$ matches \n in middle no REG_NEWLINE
^ matches \n in middle no REG_NEWLINE
PCRE's behaviour is the same as Perl's, except that there is no equiva-
lent for PCRE_DOLLAR_ENDONLY in Perl. In both PCRE and Perl, there is
no way to stop newline from matching [^a].
The default POSIX newline handling can be obtained by setting
PCRE_DOTALL and PCRE_DOLLAR_ENDONLY, but there is no way to make PCRE
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
The PCRE_NOTBOL option is set when calling the underlying PCRE matching
The PCRE_NOTEOL option is set when calling the underlying PCRE matching
The string is considered to start at string + pmatch.rm_so and to
have a terminating NUL located at string + pmatch.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.
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 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.
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-
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Last updated: 05 April 2008
Copyright (c) 1997-2008 University of Cambridge.