pcreposix - POSIX API for Perl-compatible regular expressions.
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 pcre documentation for a description of the native API, which contains additional
The functions described here are just wrapper functions that ultimately call the 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 com-
mand for linking an application which 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 options REG_EXTENDED and REG_NOSUB are defined with the value
zero. They have no effect, but since programs that are written to the POSIX interface
often use them, this makes it easier to slot in PCRE as a replacement library. Other POSIX
options are not even defined.
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.
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 which is used as a base for storing informa-
tion about the compiled expression.
The argument cflags is either zero, or contains one or more of the bits defined by the
The PCRE_CASELESS option is set when the expression is passed for compilation to the
The PCRE_MULTILINE option is set when the expression is passed for compilation to the
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 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 publicized: re_nsub contains the
number of capturing subpatterns in the regular expression. Various error codes are defined
in the header file.
MATCHING A PATTERN
The function regexec() is called to match a pre-compiled pattern preg against a given
string, which is terminated by a zero byte, subject to the options in eflags. These can
The PCRE_NOTBOL option is set when calling the underlying PCRE matching function.
The PCRE_NOTEOL option is set when calling the underlying PCRE matching function.
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.
Philip Hazel <email@example.com>
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