PCRE - Perl-compatible regular expressions.
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
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 sys-
tems 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 POSIX 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
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 expressions 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 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 informa-
tion about the compiled regular expression.
The argument cflags is either zero, or contains one or more of the bits defined by the
The PCRE_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.
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 section).
The PCRE_NO_AUTO_CAPTURE option is set when the regular expression is passed for compila-
tion 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.
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]
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 num-
ber 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 PCRE:
Default Change with
. matches newline no PCRE_DOTALL
newline matches [^a] yes not changeable
$ matches \n at end yes PCRE_DOLLARENDONLY
$ matches \n in middle no PCRE_MULTILINE
^ matches \n in middle no PCRE_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
PCRE's behaviour is the same as Perl's, except that there is no equivalent for PCRE_DOL-
LAR_ENDONLY in Perl. In both PCRE and Perl, there is no way to stop newline from matching
The default POSIX newline handling can be obtained by setting PCRE_DOTALL and PCRE_DOL-
LAR_ENDONLY, but there is no way to make PCRE behave exactly as for the REG_NEWLINE
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:
The PCRE_NOTBOL option is set when calling the underlying PCRE matching function.
The PCRE_NOTEMPTY option is set when calling the underlying PCRE 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.
The PCRE_NOTEOL option is set when calling the underlying PCRE matching function.
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 loca-
tion), 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 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 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.
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 function is the
size of buffer needed to hold the whole message.
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.
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Last updated: 11 March 2009
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