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. Some features that appeared in
Python and PCRE before they appeared in Perl are also available using
the Python syntax, there is some support for one or two .NET and
Oniguruma syntax items, and there is an option for requesting some
minor changes that give better JavaScript compatibility.
The current implementation of PCRE corresponds approximately with Perl
5.10, including support for UTF-8 encoded strings and Unicode general
category properties. However, UTF-8 and Unicode support has to be
explicitly enabled; it is not the default. The Unicode tables
correspond to Unicode release 5.2.0.
In addition to the Perl-compatible matching function, PCRE contains an
alternative 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
pcrepattern and pcrecompat pages. There is a syntax summary in the
pcresyntax page.
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
and NON-UNIX-USE files 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
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, except the pcredemo
section, are concatenated, for ease of searching. The sections are as
follows:
pcre this document
pcre-config show PCRE installation configuration information
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
pcredemo a demonstration C program that uses PCRE
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 pcredemo program
pcrestack discussion of stack usage
pcresyntax quick syntax reference
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 is slower.
All values in repeating quantifiers must be less than 65536.
There is no limit to the number of parenthesized subpatterns, but there
can be no more than 65535 capturing subpatterns.
The maximum length of name for a named subpattern is 32 characters, and
the maximum number of named subpatterns is 10000.
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
indefinite repetition. This means that the available stack space may
limit the size of a subject string that can be processed by certain
patterns. For a discussion of stack issues, see the pcrestack
documentation.
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, or the pattern must start with the
sequence (*UTF8). When either of these is the case, both the pattern
and any subject strings that are matched against it are treated as
UTF-8 strings instead of strings of 1-byte characters.
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 occasionally, so should not be
very big.
If PCRE is built with Unicode character property support (which implies
UTF-8 support), 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, the Unicode script names such as Arabic or
Han, and the derived properties Any and L&. A full list is given in the
pcrepattern documentation. Only the short names for properties are
supported. For example, \p{L} matches a letter. Its Perl synonym,
\p{Letter}, is not supported. Furthermore, in Perl, many properties
may optionally be prefixed by "Is", for compatibility with Perl 5.6.
PCRE does not support this.
Validity of UTF-8 strings
When you set the PCRE_UTF8 flag, the strings passed as patterns and
subjects are (by default) checked for validity on entry to the relevant
functions. From release 7.3 of PCRE, the check is according the rules
of RFC 3629, which are themselves derived from the Unicode
specification. Earlier releases of PCRE followed the rules of RFC 2279,
which allows the full range of 31-bit values (0 to 0x7FFFFFFF). The
current check allows only values in the range U+0 to U+10FFFF,
excluding U+D800 to U+DFFF.
The excluded code points are the "Low Surrogate Area" of Unicode, of
which the Unicode Standard says this: "The Low Surrogate Area does not
contain any character assignments, consequently no character code
charts or namelists are provided for this area. Surrogates are reserved
for use with UTF-16 and then must be used in pairs." The code points
that are encoded by UTF-16 pairs are available as independent code
points in the UTF-8 encoding. (In other words, the whole surrogate
thing is a fudge for UTF-16 which unfortunately messes up UTF-8.)
If an invalid UTF-8 string is passed to PCRE, an error return
(PCRE_ERROR_BADUTF8) 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 when PCRE_NO_UTF8_CHECK is set,
what happens depends on why the string is invalid. If the string
conforms to the "old" definition of UTF-8 (RFC 2279), it is processed
as a string of characters in the range 0 to 0x7FFFFFFF. In other words,
apart from the initial validity test, PCRE (when in UTF-8 mode) handles
strings according to the more liberal rules of RFC 2279. However, if
the string does not even conform to RFC 2279, the result is undefined.
Your program may crash.
If you want to process strings of values in the full range 0 to
0x7FFFFFFF, encoded in a UTF-8-like manner as per the old RFC, you can
set PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in
this situation, you will have to apply your own validity check.
General comments about UTF-8 mode
1. An unbraced hexadecimal escape sequence (such as \xb3) matches a
two-byte UTF-8 character if the value is greater than 127.
2. Octal numbers up to \777 are recognized, and match two-byte UTF-8
characters for values greater than \177.
3. Repeat quantifiers apply to complete UTF-8 characters, not to
individual bytes, for example: \x{100}{3}.
4. The dot metacharacter matches one UTF-8 character instead of a
single byte.
5. 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().
6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
test characters 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}. Note that this also applies to \b, because it is defined in
terms of \w and \W.
7. Similarly, characters that match the POSIX named character classes
are all low-valued characters.
8. However, the Perl 5.10 horizontal and vertical whitespace matching
escapes (\h, \H, \v, and \V) do match all the appropriate Unicode
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. Even when Unicode property
support is available, PCRE supports case-insensitive matching only when
there is a one-to-one mapping between a letter’s cases. There are a
small number of many-to-one mappings in Unicode; these are not
supported by PCRE.
AUTHOR
Philip Hazel
University Computing Service
Cambridge CB2 3QH, 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 two initials,
followed by the two digits 10, at the domain cam.ac.uk.
REVISION
Last updated: 01 March 2010
Copyright (c) 1997-2010 University of Cambridge.