PCRE2POSIX(3) Library Functions Manual PCRE2POSIX(3)
PCRE2 - Perl-compatible regular expressions (revised API)
int pcre2_regcomp(regex_t *preg, const char *pattern,
int pcre2_regexec(const regex_t *preg, const char *string,
size_t nmatch, regmatch_t pmatch, int eflags);
size_t pcre2_regerror(int errcode, const regex_t *preg,
char *errbuf, size_t errbuf_size);
void pcre2_regfree(regex_t *preg);
This set of functions provides a POSIX-style API for the PCRE2 regular
expression 8-bit library. There are no POSIX-style wrappers for PCRE2's
16-bit and 32-bit libraries. See the pcre2api documentation for a
description of PCRE2's native API, which contains much additional func-
The functions described here are wrapper functions that ultimately call
the PCRE2 native API. Their prototypes are defined in the pcre2posix.h
header file, and they all have unique names starting with pcre2_. How-
ever, the pcre2posix.h header also contains macro definitions that con-
vert the standard POSIX names such regcomp() into pcre2_regcomp() etc.
This means that a program can use the usual POSIX names without running
the risk of accidentally linking with POSIX functions from a different
On Unix-like systems the PCRE2 POSIX library is called libpcre2-posix,
so can be accessed by adding -lpcre2-posix to the command for linking
an application. Because the POSIX functions call the native ones, it is
also necessary to add -lpcre2-8.
Although they were not defined as protypes in pcre2posix.h, releases
10.33 to 10.36 of the library contained functions with the POSIX names
regcomp() etc. These simply passed their arguments to the PCRE2 func-
tions. These functions were provided for backwards compatibility with
earlier versions of PCRE2, which had only POSIX names. However, this
has proved troublesome in situations where a program links with several
libraries, some of which use PCRE2's POSIX interface while others use
the real POSIX functions. For this reason, the POSIX names have been
removed since release 10.37.
Calling the header file pcre2posix.h avoids any conflict with other
POSIX libraries. It can, of course, be renamed or aliased as regex.h,
which is the "correct" name, if there is no clash. 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 identi-
fying error codes.
USING THE POSIX FUNCTIONS
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 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 or to add BSD
or GNU functionality.
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 descriptions below use the actual names of the functions, but, as
described above, the standard POSIX names (without the pcre2_ prefix)
may also be used.
COMPILING A PATTERN
The function pcre2_regcomp() is called to compile a pattern into an
internal form. By default, the pattern is a C string terminated by a
binary zero (but see REG_PEND below). 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. (It is also used for input when
REG_PEND is set.)
The argument cflags is either zero, or contains one or more of the bits
defined by the following macros:
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.
The PCRE2_CASELESS option is set when the regular expression is passed
for compilation to the native function.
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-
The PCRE2_LITERAL option is set when the regular expression is passed
for compilation to the native function. This disables all meta charac-
ters in the pattern, causing it to be treated as a literal string. The
only other options that are allowed with REG_NOSPEC are REG_ICASE,
REG_NOSUB, REG_PEND, and REG_UTF. Note that REG_NOSPEC is not part of
the POSIX standard.
When a pattern that is compiled with this flag is passed to
pcre2_regexec() for matching, the nmatch and pmatch arguments are
ignored, and no captured strings are returned. Versions of the PCRE
library prior to 10.22 used to set the PCRE2_NO_AUTO_CAPTURE compile
option, but this no longer happens because it disables the use of back-
If this option is set, the reg_endp field in the preg structure (which
has the type const char *) must be set to point to the character beyond
the end of the pattern before calling pcre2_regcomp(). The pattern
itself may now contain binary zeros, which are treated as data charac-
ters. Without REG_PEND, a binary zero terminates the pattern and the
re_endp field is ignored. This is a GNU extension to the POSIX standard
and should be used with caution in software intended to be portable to
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.
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.
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 pcre2_regcomp() is zero on success, and non-zero other-
wise. The preg structure is filled in on success, and one other member
of the structure (as well as re_endp) 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 pcre2_regcomp() is non-zero, you must not attempt
to use the contents of the preg structure. If, for example, you pass it
to pcre2_regexec(), the result is undefined and your program is likely
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
pcre2_regcomp() function causes PCRE2_MULTILINE to be passed to
pcre2_compile(), and REG_DOTALL passes PCRE2_DOTALL. There is no way to
MATCHING A PATTERN
The function pcre2_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 PCRE2_NOTBOL option is set when calling the underlying PCRE2 match-
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.
The PCRE2_NOTEOL option is set when calling the underlying PCRE2 match-
When this option is set, the subject string starts at string +
pmatch.rm_so and ends at string + pmatch.rm_eo, which should
point to the first character beyond the string. There may be binary
zeros within the subject string, and indeed, using REG_STARTEND is the
only way to pass a subject string that contains a binary zero.
Whatever the value of pmatch.rm_so, the offsets of the matched
string and any captured substrings are still given relative to the
start of string itself. (Before PCRE2 release 10.30 these were given
relative to string + pmatch.rm_so, but this differs from other
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 and
length of the string, not how it is matched. Setting REG_STARTEND and
passing pmatch as NULL are mutually exclusive; the error REG_INVARG is
If the pattern was compiled with the REG_NOSUB flag, no data about any
matched strings is returned. The nmatch and pmatch arguments of
pcre2_regexec() are ignored (except possibly as input for REG_STAR-
The value of nmatch may be zero, and the value pmatch may be NULL
(unless REG_STARTEND is set); in both these cases 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"
The pcre2_regerror() function maps a non-zero errorcode from either
pcre2_regcomp() or pcre2_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 buf-
fer 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.
Compiling a regular expression causes memory to be allocated and asso-
ciated with the preg structure. The function pcre2_regfree() frees all
such memory, after which preg may no longer be used as a compiled
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Last updated: 26 April 2021
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