The current implementation of PCRE corresponds approximately with Perl 5.12, 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
HREF
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
HREF
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:
HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
</a>
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
HREF
pcrepattern
and
HREF
pcrecompat
pages. There is a syntax summary in the
HREF
pcresyntax
page.
Some features of PCRE can be included, excluded, or changed when the library is
built. The
HREF
pcre_config()
function makes it possible for a client to discover which features are
available. The features themselves are described in the
HREF
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. . .
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. . .
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
HREF
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
HREF
pcrestack
documentation.
.
.
HTML <a name="utf8support"></a>
In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
the code, and, in addition, you must call
HREF
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 \ep{..}, \eP{..}, and \eX 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
HREF
pcrepattern
documentation. Only the short names for properties are supported. For example,
\ep{L} matches a letter. Its Perl synonym, \ep{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.
.
.
HTML <a name="utf8strings"></a>
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. . .
2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8 characters for values greater than \e177.
3. Repeat quantifiers apply to complete UTF-8 characters, not to individual bytes, for example: \ex{100}{3}.
4. The dot metacharacter matches one UTF-8 character instead of a single byte.
5. The escape sequence \eC 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 \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
test characters of any code value, but, by default, 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 is built to
include Unicode property support, because to do otherwise would slow down PCRE
in many common cases. Note in particular that this applies to \eb and \eB,
because they are defined in terms of \ew and \eW. If you really want to test
for a wider sense of, say, "digit", you can use explicit Unicode property tests
such as \ep{Nd}. Alternatively, if you set the PCRE_UCP option, the way that
the character escapes work is changed so that Unicode properties are used to
determine which characters match. There are more details in the section on
HTML <a href="pcrepattern.html#genericchartypes">
</a>
generic character types
in the
HREF
pcrepattern
documentation.
7. Similarly, characters that match the POSIX named character classes are all low-valued characters, unless the PCRE_UCP option is set.
8. However, the horizontal and vertical whitespace matching escapes (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters, whether or not PCRE_UCP is set.
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. Furthermore, 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. . .
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. . .
Last updated: 13 November 2010 Copyright (c) 1997-2010 University of Cambridge.
