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PCRE(3) PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
INTRODUCTION
The PCRE library is a set of functions that implement regular expression pattern
matching using the same syntax and semantics as Perl, with just a few differences.
The current implementation of PCRE (release 5.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.
PCRE is written in C and released as a C library. A number of people have written
wrappers and interfaces of various kinds. A C++ class is included in these contri-
butions, 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 pcrepattern 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 themselves are described in the pcrebuild
page. 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 comprises a number of different 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 sec-
tions are concatenated, 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
pcrepartial details of the partial matching facility
pcrepattern syntax and semantics of supported
regular expressions
pcreperform discussion of performance issues
pcreposix the POSIX-compatible 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 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 expres-
sions 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. How-
ever, the speed of execution will be slower.
All values in repeating quantifiers must be less than 65536. The maximum 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 subpat-
terns, assertions, and other types of subpattern, is 200.
The maximum length of a subject string is the largest positive number that an inte-
ger variable can hold. However, PCRE uses recursion to handle subpatterns and
indefinite 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 support 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 sup-
port), the escape sequences \p{..}, \P{..}, and \X are supported. 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 documentation. 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 example: \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 charac-
ter if the value is greater than 127.
4. Repeat quantifiers apply to complete UTF-8 characters, not to individual bytes,
for example: \x{100}{3}.
5. The dot metacharacter matches one UTF-8 character instead of a single 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 charac-
ters of any code value, but the characters that PCRE recognizes 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 prop-
erty information is used only for characters with higher values.
AUTHOR
Philip Hazel <ph10 AT cam.uk>
University Computing Service,
Cambridge CB2 3QG, England.
Phone: +44 1223 334714
Last updated: 09 September 2004
Copyright (c) 1997-2004 University of Cambridge.
PCRE(3)
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