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1  .TH PCRE 3  .TH PCRE 3 "11 November 2012" "PCRE 8.32"
2  .SH NAME  .SH NAME
3  PCRE - Perl-compatible regular expressions  PCRE - Perl-compatible regular expressions
4  .SH INTRODUCTION  .SH INTRODUCTION
# Line 6  PCRE - Perl-compatible regular expressio Line 6  PCRE - Perl-compatible regular expressio
6  .sp  .sp
7  The PCRE library is a set of functions that implement regular expression  The PCRE library is a set of functions that implement regular expression
8  pattern matching using the same syntax and semantics as Perl, with just a few  pattern matching using the same syntax and semantics as Perl, with just a few
9  differences. The current implementation of PCRE (release 5.x) corresponds  differences. Some features that appeared in Python and PCRE before they
10  approximately with Perl 5.8, including support for UTF-8 encoded strings and  appeared in Perl are also available using the Python syntax, there is some
11  Unicode general category properties. However, this support has to be explicitly  support for one or two .NET and Oniguruma syntax items, and there is an option
12  enabled; it is not the default.  for requesting some minor changes that give better JavaScript compatibility.
13    .P
14    Starting with release 8.30, it is possible to compile two separate PCRE
15    libraries: the original, which supports 8-bit character strings (including
16    UTF-8 strings), and a second library that supports 16-bit character strings
17    (including UTF-16 strings). The build process allows either one or both to be
18    built. The majority of the work to make this possible was done by Zoltan
19    Herczeg.
20    .P
21    Starting with release 8.32 it is possible to compile a third separate PCRE
22    library, which supports 32-bit character strings (including
23    UTF-32 strings). The build process allows any set of the 8-, 16- and 32-bit
24    libraries. The work to make this possible was done by Christian Persch.
25    .P
26    The three libraries contain identical sets of functions, except that the names
27    in the 16-bit library start with \fBpcre16_\fP instead of \fBpcre_\fP, and the
28    names in the 32-bit library start with \fBpcre32_\fP instead of \fBpcre_\fP. To
29    avoid over-complication and reduce the documentation maintenance load, most of
30    the documentation describes the 8-bit library, with the differences for the
31    16-bit and 32-bit libraries described separately in the
32    .\" HREF
33    \fBpcre16\fP
34    and
35    .\" HREF
36    \fBpcre32\fP
37    .\"
38    pages. References to functions or structures of the form \fIpcre[16|32]_xxx\fP
39    should be read as meaning "\fIpcre_xxx\fP when using the 8-bit library,
40    \fIpcre16_xxx\fP when using the 16-bit library, or \fIpcre32_xxx\fP when using
41    the 32-bit library".
42    .P
43    The current implementation of PCRE corresponds approximately with Perl 5.12,
44    including support for UTF-8/16/32 encoded strings and Unicode general category
45    properties. However, UTF-8/16/32 and Unicode support has to be explicitly
46    enabled; it is not the default. The Unicode tables correspond to Unicode
47    release 6.2.0.
48    .P
49    In addition to the Perl-compatible matching function, PCRE contains an
50    alternative function that matches the same compiled patterns in a different
51    way. In certain circumstances, the alternative function has some advantages.
52    For a discussion of the two matching algorithms, see the
53    .\" HREF
54    \fBpcrematching\fP
55    .\"
56    page.
57  .P  .P
58  PCRE is written in C and released as a C library. A number of people have  PCRE is written in C and released as a C library. A number of people have
59  written wrappers and interfaces of various kinds. A C++ class is included in  written wrappers and interfaces of various kinds. In particular, Google Inc.
60  these contributions, which can be found in the \fIContrib\fR directory at the  have provided a comprehensive C++ wrapper for the 8-bit library. This is now
61  primary FTP site, which is:  included as part of the PCRE distribution. The
62    .\" HREF
63    \fBpcrecpp\fP
64    .\"
65    page has details of this interface. Other people's contributions can be found
66    in the \fIContrib\fP directory at the primary FTP site, which is:
67  .sp  .sp
68  .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">  .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
69  .\" </a>  .\" </a>
# Line 23  ftp://ftp.csx.cam.ac.uk/pub/software/pro Line 72  ftp://ftp.csx.cam.ac.uk/pub/software/pro
72  Details of exactly which Perl regular expression features are and are not  Details of exactly which Perl regular expression features are and are not
73  supported by PCRE are given in separate documents. See the  supported by PCRE are given in separate documents. See the
74  .\" HREF  .\" HREF
75  \fBpcrepattern\fR  \fBpcrepattern\fP
76  .\"  .\"
77  and  and
78  .\" HREF  .\" HREF
79  \fBpcrecompat\fR  \fBpcrecompat\fP
80  .\"  .\"
81  pages.  pages. There is a syntax summary in the
82    .\" HREF
83    \fBpcresyntax\fP
84    .\"
85    page.
86  .P  .P
87  Some features of PCRE can be included, excluded, or changed when the library is  Some features of PCRE can be included, excluded, or changed when the library is
88  built. The  built. The
89  .\" HREF  .\" HREF
90  \fBpcre_config()\fR  \fBpcre_config()\fP
91  .\"  .\"
92  function makes it possible for a client to discover which features are  function makes it possible for a client to discover which features are
93  available. The features themselves are described in the  available. The features themselves are described in the
# Line 42  available. The features themselves are d Line 95  available. The features themselves are d
95  \fBpcrebuild\fP  \fBpcrebuild\fP
96  .\"  .\"
97  page. Documentation about building PCRE for various operating systems can be  page. Documentation about building PCRE for various operating systems can be
98  found in the \fBREADME\fP file in the source distribution.  found in the \fBREADME\fP and \fBNON-AUTOTOOLS_BUILD\fP files in the source
99    distribution.
100    .P
101    The libraries contains a number of undocumented internal functions and data
102    tables that are used by more than one of the exported external functions, but
103    which are not intended for use by external callers. Their names all begin with
104    "_pcre_" or "_pcre16_" or "_pcre32_", which hopefully will not provoke any name
105    clashes. In some environments, it is possible to control which external symbols
106    are exported when a shared library is built, and in these cases the
107    undocumented symbols are not exported.
108    .
109    .
110    .SH "SECURITY CONSIDERATIONS"
111    .rs
112    .sp
113    If you are using PCRE in a non-UTF application that permits users to supply
114    arbitrary patterns for compilation, you should be aware of a feature that
115    allows users to turn on UTF support from within a pattern, provided that PCRE
116    was built with UTF support. For example, an 8-bit pattern that begins with
117    "(*UTF8)" or "(*UTF)" turns on UTF-8 mode, which interprets patterns and
118    subjects as strings of UTF-8 characters instead of individual 8-bit characters.
119    This causes both the pattern and any data against which it is matched to be
120    checked for UTF-8 validity. If the data string is very long, such a check might
121    use sufficiently many resources as to cause your application to lose
122    performance.
123    .P
124    The best way of guarding against this possibility is to use the
125    \fBpcre_fullinfo()\fP function to check the compiled pattern's options for UTF.
126    .P
127    If your application is one that supports UTF, be aware that validity checking
128    can take time. If the same data string is to be matched many times, you can use
129    the PCRE_NO_UTF[8|16|32]_CHECK option for the second and subsequent matches to
130    save redundant checks.
131    .P
132    Another way that performance can be hit is by running a pattern that has a very
133    large search tree against a string that will never match. Nested unlimited
134    repeats in a pattern are a common example. PCRE provides some protection
135    against this: see the PCRE_EXTRA_MATCH_LIMIT feature in the
136    .\" HREF
137    \fBpcreapi\fP
138    .\"
139    page.
140  .  .
141  .  .
142  .SH "USER DOCUMENTATION"  .SH "USER DOCUMENTATION"
# Line 51  found in the \fBREADME\fP file in the so Line 145  found in the \fBREADME\fP file in the so
145  The user documentation for PCRE comprises a number of different sections. In  The user documentation for PCRE comprises a number of different sections. In
146  the "man" format, each of these is a separate "man page". In the HTML format,  the "man" format, each of these is a separate "man page". In the HTML format,
147  each is a separate page, linked from the index page. In the plain text format,  each is a separate page, linked from the index page. In the plain text format,
148  all the sections are concatenated, for ease of searching. The sections are as  all the sections, except the \fBpcredemo\fP section, are concatenated, for ease
149  follows:  of searching. The sections are as follows:
150  .sp  .sp
151    pcre              this document    pcre              this document
152    pcreapi           details of PCRE's native API    pcre16            details of the 16-bit library
153      pcre32            details of the 32-bit library
154      pcre-config       show PCRE installation configuration information
155      pcreapi           details of PCRE's native C API
156    pcrebuild         options for building PCRE    pcrebuild         options for building PCRE
157    pcrecallout       details of the callout feature    pcrecallout       details of the callout feature
158    pcrecompat        discussion of Perl compatibility    pcrecompat        discussion of Perl compatibility
159    pcregrep          description of the \fBpcregrep\fP command    pcrecpp           details of the C++ wrapper for the 8-bit library
160      pcredemo          a demonstration C program that uses PCRE
161      pcregrep          description of the \fBpcregrep\fP command (8-bit only)
162      pcrejit           discussion of the just-in-time optimization support
163      pcrelimits        details of size and other limits
164      pcrematching      discussion of the two matching algorithms
165    pcrepartial       details of the partial matching facility    pcrepartial       details of the partial matching facility
166  .\" JOIN  .\" JOIN
167    pcrepattern       syntax and semantics of supported    pcrepattern       syntax and semantics of supported
168                        regular expressions                        regular expressions
169    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
170    pcreposix         the POSIX-compatible API    pcreposix         the POSIX-compatible C API for the 8-bit library
171    pcreprecompile    details of saving and re-using precompiled patterns    pcreprecompile    details of saving and re-using precompiled patterns
172    pcresample        discussion of the sample program    pcresample        discussion of the pcredemo program
173      pcrestack         discussion of stack usage
174      pcresyntax        quick syntax reference
175    pcretest          description of the \fBpcretest\fP testing command    pcretest          description of the \fBpcretest\fP testing command
176      pcreunicode       discussion of Unicode and UTF-8/16/32 support
177  .sp  .sp
178  In addition, in the "man" and HTML formats, there is a short page for each  In addition, in the "man" and HTML formats, there is a short page for each
179  library function, listing its arguments and results.  C library function, listing its arguments and results.
180  .  .
181  .  .
182  .SH LIMITATIONS  .SH AUTHOR
183  .rs  .rs
184  .sp  .sp
185  There are some size limitations in PCRE but it is hoped that they will never in  .nf
186  practice be relevant.  Philip Hazel
187  .P  University Computing Service
188  The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is  Cambridge CB2 3QH, England.
189  compiled with the default internal linkage size of 2. If you want to process  .fi
190  regular expressions that are truly enormous, you can compile PCRE with an  .P
191  internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source  Putting an actual email address here seems to have been a spam magnet, so I've
192  distribution and the  taken it away. If you want to email me, use my two initials, followed by the
193  .\" HREF  two digits 10, at the domain cam.ac.uk.
 \fBpcrebuild\fP  
 .\"  
 documentation for details). In these cases the limit is substantially larger.  
 However, the speed of execution will be slower.  
 .P  
 All values in repeating quantifiers must be less than 65536.  
 The maximum number of capturing subpatterns is 65535.  
 .P  
 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  
 subpatterns, assertions, and other types of subpattern, is 200.  
 .P  
 The maximum length of a subject string is the largest positive number that an  
 integer 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.  
 .sp  
 .\" HTML <a name="utf8support"></a>  
194  .  .
195  .  .
196  .SH "UTF-8 AND UNICODE PROPERTY SUPPORT"  .SH REVISION
 .rs  
 .sp  
 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.  
 .P  
 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  
 \fBpcre_compile()\fP  
 .\"  
 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.  
 .P  
 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.  
 .P  
 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. A full list is given in the  
 .\" HREF  
 \fBpcrepattern\fP  
 .\"  
 documentation. The PCRE library is increased in size by about 90K when Unicode  
 property support is included.  
 .P  
 The following comments apply when PCRE is running in UTF-8 mode:  
 .P  
 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.  
 .P  
 2. In a pattern, the escape sequence \ex{...}, 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: \ex{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.  
 .P  
 3. The original hexadecimal escape sequence, \exhh, matches a two-byte UTF-8  
 character if the value is greater than 127.  
 .P  
 4. Repeat quantifiers apply to complete UTF-8 characters, not to individual  
 bytes, for example: \ex{100}{3}.  
 .P  
 5. The dot metacharacter matches one UTF-8 character instead of a single byte.  
 .P  
 6. 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.  
 .P  
 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW 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 \ep{Nd}.  
 .P  
 8. Similarly, characters that match the POSIX named character classes are all  
 low-valued characters.  
 .P  
 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.  
 .  
 .SH AUTHOR  
197  .rs  .rs
198  .sp  .sp
199  Philip Hazel <ph10@cam.ac.uk>  .nf
200  .br  Last updated: 11 November 2012
201  University Computing Service,  Copyright (c) 1997-2012 University of Cambridge.
202  .br  .fi
 Cambridge CB2 3QG, England.  
 .br  
 Phone: +44 1223 334714  
 .sp  
 .in 0  
 Last updated: 09 September 2004  
 .br  
 Copyright (c) 1997-2004 University of Cambridge.  

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