/usr/man2/cat.3/pcre.3.Z(/usr/man2/cat.3/pcre.3.Z)
NAME
PCRE - Perl-compatible regular expressions
INTRODUCTION
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
6.x) corresponds approximately with Perl 5.8, including support for
UTF-8 encoded strings and Unicode general category properties. However,
this support has to be explicitly enabled; it is not the default.
In addition to the Perl-compatible matching function, PCRE also con-
tains an alternative matching function that matches the same compiled
patterns in a different way. In certain circumstances, the alternative
function has some advantages. For a discussion of the two matching
algorithms, see the pcrematching page.
PCRE is written in C and released as a C library. A number of people
have written wrappers and interfaces of various kinds. In particular,
Google Inc. have provided a comprehensive C++ wrapper. This is now
included as part of the PCRE distribution. The pcrecpp page has details
of this interface. Other people's contributions 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. The features them-
selves are described in the pcrebuild page. Documentation about build-
ing PCRE for various operating systems can be found in the README file
in the source distribution.
The library contains a number of undocumented internal functions and
data tables that are used by more than one of the exported external
functions, but which are not intended for use by external callers.
Their names all begin with "_pcre_", which hopefully will not provoke
any name clashes. In some environments, it is possible to control which
external symbols are exported when a shared library is built, and in
these cases the undocumented symbols are not exported.
USER DOCUMENTATION
The user documentation for PCRE comprises a number of different sec-
tions. 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 concatenated, for ease
of searching. The sections are as follows:
pcre this document
pcreapi details of PCRE's native C API
pcrebuild options for building PCRE
pcrecallout details of the callout feature
pcrecompat discussion of Perl compatibility
pcrecpp details of the C++ wrapper
pcregrep description of the pcregrep command
pcrematching discussion of the two matching algorithms
pcrepartial details of the partial matching facility
pcrepattern syntax and semantics of supported
regular expressions
pcreperform discussion of performance issues
pcreposix the POSIX-compatible C API
pcreprecompile details of saving and re-using precompiled patterns
pcresample discussion of the sample program
pcretest description of the pcretest testing command
In addition, in the "man" and HTML formats, there is a short page for
each C 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).
In 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, when using the traditional
matching function, PCRE uses recursion to handle subpatterns and indef-
inite repetition. This means that the available stack space may limit
the size of a subject string that can be processed by certain patterns.
UTF-8 AND UNICODE PROPERTY SUPPORT
From release 3.3, PCRE has had some support for character strings
encoded in the UTF-8 format. For release 4.0 this was greatly extended
to cover most common requirements, and in release 5.0 additional sup-
port for Unicode general category properties was added.
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.
If PCRE is built with Unicode character property support (which implies
UTF-8 support), the escape sequences \p{..}, \P{..}, and \X are sup-
ported. The available properties that can be tested are limited to the
general category properties such as Lu for an upper case letter or Nd
for a decimal number. A full list is given in the pcrepattern documen-
tation. The PCRE library is increased in size by about 90K when Unicode
property support is included.
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. This facility is
not available in the alternative matching function, pcre_dfa_exec().
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. This remains true even when PCRE
includes Unicode property support, because to do otherwise would slow
down PCRE in many common cases. If you really want to test for a wider
sense of, say, "digit", you must use Unicode property tests such as
\p{Nd}.
8. Similarly, characters that match the POSIX named character classes
are all low-valued characters.
9. Case-insensitive matching applies only to characters whose values
are less than 128, unless PCRE is built with Unicode property support.
Even when Unicode property support is available, PCRE still uses its
own character tables when checking the case of low-valued characters,
so as not to degrade performance. The Unicode property information is
used only for characters with higher values.
AUTHOR
Philip Hazel
University Computing Service,
Cambridge CB2 3QG, England.
Putting an actual email address here seems to have been a spam magnet,
so I've taken it away. If you want to email me, use my initial and sur-
name, separated by a dot, at the domain ucs.cam.ac.uk.
Last updated: 07 March 2005
Copyright (c) 1997-2005 University of Cambridge.
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