PCRE(3)PCRE(3)NAME
PCRE - Perl-compatible regular expressions
DESCRIPTION
The PCRE library is a set of functions that implement regular expres‐
sion pattern matching using the same syntax and semantics as Perl, with
just a few differences. The current implementation of PCRE (release
4.x) corresponds approximately with Perl 5.8, including support for
UTF-8 encoded strings. However, this support has to be explicitly
enabled; it is not the default.
PCRE is written in C and released as a C library. However, a number of
people have written wrappers and interfaces of various kinds. A C++
class is included in these contributions, which can be found in the
Contrib directory at the primary FTP site, which is:
ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
Details of exactly which Perl regular expression features are and are
not supported by PCRE are given in separate documents. See the pcrepat‐
tern and pcrecompat pages.
Some features of PCRE can be included, excluded, or changed when the
library is built. The pcre_config() function makes it possible for a
client to discover which features are available. Documentation about
building PCRE for various operating systems can be found in the README
file in the source distribution.
USER DOCUMENTATION
The user documentation for PCRE has been split up into a number of dif‐
ferent sections. In the "man" format, each of these is a separate "man
page". In the HTML format, each is a separate page, linked from the
index page. In the plain text format, all the sections are concate‐
nated, for ease of searching. The sections are as follows:
pcre this document
pcreapi details of PCRE's native API
pcrebuild options for building PCRE
pcrecallout details of the callout feature
pcrecompat discussion of Perl compatibility
pcregrep description of the pcregrep command
pcrepattern syntax and semantics of supported
regular expressions
pcreperform discussion of performance issues
pcreposix the POSIX-compatible API
pcresample discussion of the sample program
pcretest the pcretest testing command
In addition, in the "man" and HTML formats, there is a short page for
each library function, listing its arguments and results.
LIMITATIONS
There are some size limitations in PCRE but it is hoped that they will
never in practice be relevant.
The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE
is compiled with the default internal linkage size of 2. If you want to
process regular expressions that are truly enormous, you can compile
PCRE with an internal linkage size of 3 or 4 (see the README file in
the source distribution and the pcrebuild documentation for details).
If these cases the limit is substantially larger. However, the speed
of execution will be slower.
All values in repeating quantifiers must be less than 65536. The maxi‐
mum number of capturing subpatterns is 65535.
There is no limit to the number of non-capturing subpatterns, but the
maximum depth of nesting of all kinds of parenthesized subpattern,
including capturing subpatterns, assertions, and other types of subpat‐
tern, is 200.
The maximum length of a subject string is the largest positive number
that an integer variable can hold. However, PCRE uses recursion to han‐
dle subpatterns and indefinite repetition. This means that the avail‐
able stack space may limit the size of a subject string that can be
processed by certain patterns.
UTF-8 SUPPORT
Starting at release 3.3, PCRE has had some support for character
strings encoded in the UTF-8 format. For release 4.0 this has been
greatly extended to cover most common requirements.
In order process UTF-8 strings, you must build PCRE to include UTF-8
support in the code, and, in addition, you must call pcre_compile()
with the PCRE_UTF8 option flag. When you do this, both the pattern and
any subject strings that are matched against it are treated as UTF-8
strings instead of just strings of bytes.
If you compile PCRE with UTF-8 support, but do not use it at run time,
the library will be a bit bigger, but the additional run time overhead
is limited to testing the PCRE_UTF8 flag in several places, so should
not be very large.
The following comments apply when PCRE is running in UTF-8 mode:
1. When you set the PCRE_UTF8 flag, the strings passed as patterns and
subjects are checked for validity on entry to the relevant functions.
If an invalid UTF-8 string is passed, an error return is given. In some
situations, you may already know that your strings are valid, and
therefore want to skip these checks in order to improve performance. If
you set the PCRE_NO_UTF8_CHECK flag at compile time or at run time,
PCRE assumes that the pattern or subject it is given (respectively)
contains only valid UTF-8 codes. In this case, it does not diagnose an
invalid UTF-8 string. If you pass an invalid UTF-8 string to PCRE when
PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program may
crash.
2. In a pattern, the escape sequence \x{...}, where the contents of the
braces is a string of hexadecimal digits, is interpreted as a UTF-8
character whose code number is the given hexadecimal number, for exam‐
ple: \x{1234}. If a non-hexadecimal digit appears between the braces,
the item is not recognized. This escape sequence can be used either as
a literal, or within a character class.
3. The original hexadecimal escape sequence, \xhh, matches a two-byte
UTF-8 character if the value is greater than 127.
4. Repeat quantifiers apply to complete UTF-8 characters, not to indi‐
vidual bytes, for example: \x{100}{3}.
5. The dot metacharacter matches one UTF-8 character instead of a sin‐
gle byte.
6. The escape sequence \C can be used to match a single byte in UTF-8
mode, but its use can lead to some strange effects.
7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
test characters of any code value, but the characters that PCRE recog‐
nizes as digits, spaces, or word characters remain the same set as
before, all with values less than 256.
8. Case-insensitive matching applies only to characters whose values
are less than 256. PCRE does not support the notion of "case" for
higher-valued characters.
9. PCRE does not support the use of Unicode tables and properties or
the Perl escapes \p, \P, and \X.
AUTHOR
Philip Hazel <ph10@cam.ac.uk>
University Computing Service,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Last updated: 20 August 2003Copyright (c) 1997-2003 University of Cambridge.
PCRE(3)
