Introduction to Library Functions PCRE(3)
NAME
PCRE - Perl-compatible regular expressions
INTRODUCTION
The PCRE library is a set of functions that implement regu-
lar expression 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.
There is also some support for certain .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 (release 7.x) 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 5.0.0.
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 cir-
cumstances, 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 pcreconfig() 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 vari-
ous operating systems can be found in the README file in the
source distribution.
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Introduction to Library Functions PCRE(3)
The library contains a number of undocumented internal func-
tions 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 exter-
nal 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 dif-
ferent 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 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 algo-
rithms
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 precom-
piled patterns
pcresample discussion of the sample program
pcrestack discussion of stack usage
pcretest description of the pcretest testing com-
mand
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.
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Introduction to Library Functions PCRE(3)
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 subpat-
terns, but there can be no more than 65535 capturing subpat-
terns.
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 posi-
tive 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 sub-
ject string that can be processed by certain patterns. For a
discussion of stack issues, see the pcrestack documentation.
UTF-8 AND UNICODE PROPERTY SUPORT
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 pcrecompile() with the PCREUTF8 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 addi-
tional run time overhead is limited to testing the PCREUTF8
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
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Introduction to Library Functions PCRE(3)
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 pro-
perties are supported. For example, \p{L} matches a letter.
Its Perl synonym, \p{Letter}, is not supported. Further-
more, 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 PCREUTF8 flag, the strings passed as pat-
terns 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 0x7F).
The current check allows only values in the range U]0 to
U]10F, excluding U]D800 to U]DF.
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 pro-
vided 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 unfor-
tunately messes up UTF-8.)
If an invalid UTF-8 string is passed to PCRE, an error
return (PCRERORBADUTF8) is given. In some situations,
you may already know that your strings are valid, and there-
fore want to skip these checks in order to improve perfor-
mance. If you set the PCRENOUTF8CHECK flag at compile
time or at run time, PCRE assumes that the pattern or sub-
ject 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 PCRENOUTF8CHECK
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 0x7F. In other words, apart from the ini-
tial 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.
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If you want to process strings of values in the full range 0
to 0x7F, encoded in a UTF-8-like manner as per the old
RFC, you can set PCRENOUTF8CHECK to bypass the more res-
trictive 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, pcredfaexec().
6. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W
correctly test characters of any code value, but the charac-
ters that PCRE recognizes as digits, spaces, or word charac-
ters 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 charac-
ter 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
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Introduction to Library Functions PCRE(3)
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: 12 April 2008
Copyright (c) 1997-2008 University of Cambridge.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWpcre
Interface Stability Uncommitted
NOTES
Source for PCRE is available on http:/opensolaris.org.
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