pcre
PCRE(3) PCRE(3)
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. (Certain features that appeared in Python
and PCRE before they appeared in Perl are also available using the
Python syntax.)
The current implementation of PCRE (release 7.x) corresponds approxi-
mately with Perl 5.10, including support for UTF-8 encoded strings and
Unicode general category properties. However, UTF-8 and Unicode sup-
port has to be explicitly enabled; it is not the default. The Unicode
tables correspond to Unicode release 5.0.0.
In addition to the Perl-compatible matching function, PCRE contains 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
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 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
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
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
pcresyntax quick syntax reference
pcreperform discussion of performance issues
pcreposix the POSIX-compatible C API
pcreprecompile details of saving and re-using precompiled pat-
terns
pcresample discussion of the sample program
pcrestack discussion of stack usage
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 com-
pile 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 documen-
tation.
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 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 lim-
ited 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 prop-
erties 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 rele-
vant functions. From release 7.3 of PCRE, the check is according the
rules of RFC 3629, which are themselves derived from the Unicode spec-
ification. 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, exclud-
ing 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 surro-
gate 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 con-
forms 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) han-
dles strings according to the more liberal rules of RFC 2279. However,
if the string does not even conform to RFC 2279, the result is unde-
fined. 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 indi-
vidual bytes, for example: \x{100}{3}.
4. The dot metacharacter matches one UTF-8 character instead of a sin-
gle 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 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}.
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 prop-
erty 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: 09 August 2007
Copyright (c) 1997-2007 University of Cambridge.
PCRE(3)
