PCRE2POSIX(3) Introduction to Library Functions PCRE2POSIX(3)

NAME

PCRE2 - Perl-compatible regular expressions (revised API)

SYNOPSIS

#include <pcre2posix.h>

int pcre2_regcomp(regex_t *preg, const char *pattern,
int cflags);

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);

DESCRIPTION

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 functionality.

IMPORTANT NOTE: The functions described here are NOT thread-safe, and
should not be used in multi-threaded applications. They are also
limited to processing subjects that are not bigger than 2GB. Use the
native API instead.

These functions 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_. However,
the pcre2posix.h header also contains macro definitions that convert
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 library.

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.

On Windows systems, if you are linking to a DLL version of the
library, it is recommended that PCRE2POSIX_SHARED is defined before
including the pcre2posix.h header, as it will allow for a more
efficient way to invoke the functions by adding the
__declspec(dllimport) decorator.

Although they were not defined as prototypes 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 functions. 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
identifying error codes.

USING THE POSIX FUNCTIONS

Note that these functions are just POSIX-style wrappers for PCRE2's
native API. They do not give POSIX regular expression behaviour, and
they are not thread-safe or even POSIX compatible.

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
expressions 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 regex_t structure used by pcre2_regcomp() is
defined in pcre2posix.h and is not the same as the structure used by
other libraries that provide POSIX-style matching.

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 section).

REG_NOSPEC

The PCRE2_LITERAL option is set when the regular expression is passed
for compilation to the native function. This disables all meta
characters 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.

REG_NOSUB

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 PCRE2
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
backreferences.

REG_PEND

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 characters. 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 other systems.

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 matching \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 that 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 metacharacter (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
otherwise. 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 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 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 pass PCRE2_DOLLAR_ENDONLY.

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 can be:

REG_NOTBOL

The PCRE2_NOTBOL option is set when calling the underlying PCRE2
matching 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
behaviour in some situations.

REG_NOTEOL

The PCRE2_NOTEOL option is set when calling the underlying PCRE2
matching function.

REG_STARTEND

When this option is set, the subject string starts at string +
pmatch[0].rm_so and ends at string + pmatch[0].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[0].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[0].rm_so, but this differs from other
implementations.)

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 returned.

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_STARTEND).

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.

regmatch_t as well as the regoff_t typedef it uses are defined in
pcre2posix.h and are not warranted to have the same size or layout as
other similarly named types from other libraries that provide POSIX-
style matching.

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 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 buffer 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.

MEMORY USAGE

Compiling a regular expression causes memory to be allocated and
associated with the preg structure. The function pcre2_regfree()
frees all such memory, after which preg may no longer be used as a
compiled expression.

AUTHOR

Philip Hazel
Retired from University Computing Service
Cambridge, England.

REVISION

Last updated: 27 November 2024
Copyright (c) 1997-2024 University of Cambridge.

PCRE2 10.45 27 November 2024 PCRE2POSIX(3)