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3  <title>pcre specification</title>  <title>pcre specification</title>
4  </head>  </head>
5  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
6  This HTML document has been generated automatically from the original man page.  <h1>pcre man page</h1>
7  If there is any nonsense in it, please consult the man page, in case the  <p>
8  conversion went wrong.<br>  Return to the <a href="index.html">PCRE index page</a>.
9    </p>
10    <p>
11    This page is part of the PCRE HTML documentation. It was generated automatically
12    from the original man page. If there is any nonsense in it, please consult the
13    man page, in case the conversion went wrong.
14    <br>
15  <ul>  <ul>
16  <li><a name="TOC1" href="#SEC1">DESCRIPTION</a>  <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
17  <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>  <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
18  <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>  <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
19  <li><a name="TOC4" href="#SEC4">UTF-8 SUPPORT</a>  <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
20  <li><a name="TOC5" href="#SEC5">AUTHOR</a>  <li><a name="TOC5" href="#SEC5">AUTHOR</a>
21    <li><a name="TOC6" href="#SEC6">REVISION</a>
22  </ul>  </ul>
23  <br><a name="SEC1" href="#TOC1">DESCRIPTION</a><br>  <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
24  <P>  <P>
25  The PCRE library is a set of functions that implement regular expression  The PCRE library is a set of functions that implement regular expression
26  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
27  differences. The current implementation of PCRE (release 4.x) corresponds  differences. (Certain features that appeared in Python and PCRE before they
28  approximately with Perl 5.8, including support for UTF-8 encoded strings.  appeared in Perl are also available using the Python syntax.)
 However, this support has to be explicitly enabled; it is not the default.  
29  </P>  </P>
30  <P>  <P>
31  PCRE is written in C and released as a C library. However, a number of people  The current implementation of PCRE (release 7.x) corresponds approximately with
32  have written wrappers and interfaces of various kinds. A C++ class is included  Perl 5.10, including support for UTF-8 encoded strings and Unicode general
33  in these contributions, which can be found in the <i>Contrib</i> directory at  category properties. However, UTF-8 and Unicode support has to be explicitly
34  the primary FTP site, which is:  enabled; it is not the default. The Unicode tables correspond to Unicode
35  </P>  release 5.0.0.
36    </P>
37    <P>
38    In addition to the Perl-compatible matching function, PCRE contains an
39    alternative matching function that matches the same compiled patterns in a
40    different way. In certain circumstances, the alternative function has some
41    advantages. For a discussion of the two matching algorithms, see the
42    <a href="pcrematching.html"><b>pcrematching</b></a>
43    page.
44    </P>
45    <P>
46    PCRE is written in C and released as a C library. A number of people have
47    written wrappers and interfaces of various kinds. In particular, Google Inc.
48    have provided a comprehensive C++ wrapper. This is now included as part of the
49    PCRE distribution. The
50    <a href="pcrecpp.html"><b>pcrecpp</b></a>
51    page has details of this interface. Other people's contributions can be found
52    in the <i>Contrib</i> directory at the primary FTP site, which is:
53  <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>  <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
54    </P>
55  <P>  <P>
56  Details of exactly which Perl regular expression features are and are not  Details of exactly which Perl regular expression features are and are not
57  supported by PCRE are given in separate documents. See the  supported by PCRE are given in separate documents. See the
58  <a href="pcrepattern.html"><b>pcrepattern</b></a>  <a href="pcrepattern.html"><b>pcrepattern</b></a>
59  and  and
60  <a href="pcrecompat.html"><b>pcrecompat</b></a>  <a href="pcrecompat.html"><b>pcrecompat</b></a>
61  pages.  pages. There is a syntax summary in the
62    <a href="pcresyntax.html"><b>pcresyntax</b></a>
63    page.
64  </P>  </P>
65  <P>  <P>
66  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
67  built. The  built. The
68  <a href="pcre_config.html"><b>pcre_config()</b></a>  <a href="pcre_config.html"><b>pcre_config()</b></a>
69  function makes it possible for a client to discover which features are  function makes it possible for a client to discover which features are
70  available. Documentation about building PCRE for various operating systems can  available. The features themselves are described in the
71  be found in the <b>README</b> file in the source distribution.  <a href="pcrebuild.html"><b>pcrebuild</b></a>
72    page. Documentation about building PCRE for various operating systems can be
73    found in the <b>README</b> file in the source distribution.
74  </P>  </P>
 <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>  
75  <P>  <P>
76  The user documentation for PCRE has been split up into a number of different  The library contains a number of undocumented internal functions and data
77  sections. In the "man" format, each of these is a separate "man page". In the  tables that are used by more than one of the exported external functions, but
78  HTML format, each is a separate page, linked from the index page. In the plain  which are not intended for use by external callers. Their names all begin with
79  text format, all the sections are concatenated, for ease of searching. The  "_pcre_", which hopefully will not provoke any name clashes. In some
80  sections are as follows:  environments, it is possible to control which external symbols are exported
81    when a shared library is built, and in these cases the undocumented symbols are
82    not exported.
83  </P>  </P>
84    <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
85  <P>  <P>
86    The user documentation for PCRE comprises a number of different sections. In
87    the "man" format, each of these is a separate "man page". In the HTML format,
88    each is a separate page, linked from the index page. In the plain text format,
89    all the sections are concatenated, for ease of searching. The sections are as
90    follows:
91  <pre>  <pre>
92    pcre              this document    pcre              this document
93    pcreapi           details of PCRE's native API    pcre-config       show PCRE installation configuration information
94      pcreapi           details of PCRE's native C API
95    pcrebuild         options for building PCRE    pcrebuild         options for building PCRE
96    pcrecallout       details of the callout feature    pcrecallout       details of the callout feature
97    pcrecompat        discussion of Perl compatibility    pcrecompat        discussion of Perl compatibility
98      pcrecpp           details of the C++ wrapper
99    pcregrep          description of the <b>pcregrep</b> command    pcregrep          description of the <b>pcregrep</b> command
100    pcrepattern       syntax and semantics of supported    pcrematching      discussion of the two matching algorithms
101                        regular expressions    pcrepartial       details of the partial matching facility
102      pcrepattern       syntax and semantics of supported regular expressions
103      pcresyntax        quick syntax reference
104    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
105    pcreposix         the POSIX-compatible API    pcreposix         the POSIX-compatible C API
106      pcreprecompile    details of saving and re-using precompiled patterns
107    pcresample        discussion of the sample program    pcresample        discussion of the sample program
108    pcretest          the <b>pcretest</b> testing command    pcrestack         discussion of stack usage
109  </PRE>    pcretest          description of the <b>pcretest</b> testing command
110  </P>  </pre>
 <P>  
111  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
112  library function, listing its arguments and results.  C library function, listing its arguments and results.
113  </P>  </P>
114  <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>  <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
115  <P>  <P>
# Line 84  regular expressions that are truly enorm Line 123  regular expressions that are truly enorm
123  internal linkage size of 3 or 4 (see the <b>README</b> file in the source  internal linkage size of 3 or 4 (see the <b>README</b> file in the source
124  distribution and the  distribution and the
125  <a href="pcrebuild.html"><b>pcrebuild</b></a>  <a href="pcrebuild.html"><b>pcrebuild</b></a>
126  documentation for details). If these cases the limit is substantially larger.  documentation for details). In these cases the limit is substantially larger.
127  However, the speed of execution will be slower.  However, the speed of execution is slower.
128  </P>  </P>
129  <P>  <P>
130  All values in repeating quantifiers must be less than 65536.  All values in repeating quantifiers must be less than 65536.
 The maximum number of capturing subpatterns is 65535.  
131  </P>  </P>
132  <P>  <P>
133  There is no limit to the number of non-capturing subpatterns, but the maximum  There is no limit to the number of parenthesized subpatterns, but there can be
134  depth of nesting of all kinds of parenthesized subpattern, including capturing  no more than 65535 capturing subpatterns.
135  subpatterns, assertions, and other types of subpattern, is 200.  </P>
136    <P>
137    The maximum length of name for a named subpattern is 32 characters, and the
138    maximum number of named subpatterns is 10000.
139  </P>  </P>
140  <P>  <P>
141  The maximum length of a subject string is the largest positive number that an  The maximum length of a subject string is the largest positive number that an
142  integer variable can hold. However, PCRE uses recursion to handle subpatterns  integer variable can hold. However, when using the traditional matching
143  and indefinite repetition. This means that the available stack space may limit  function, PCRE uses recursion to handle subpatterns and indefinite repetition.
144  the size of a subject string that can be processed by certain patterns.  This means that the available stack space may limit the size of a subject
145  </P>  string that can be processed by certain patterns. For a discussion of stack
146  <a name="utf8support"></a><br><a name="SEC4" href="#TOC1">UTF-8 SUPPORT</a><br>  issues, see the
147  <P>  <a href="pcrestack.html"><b>pcrestack</b></a>
148  Starting at release 3.3, PCRE has had some support for character strings  documentation.
149  encoded in the UTF-8 format. For release 4.0 this has been greatly extended to  <a name="utf8support"></a></P>
150  cover most common requirements.  <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
151    <P>
152    From release 3.3, PCRE has had some support for character strings encoded in
153    the UTF-8 format. For release 4.0 this was greatly extended to cover most
154    common requirements, and in release 5.0 additional support for Unicode general
155    category properties was added.
156  </P>  </P>
157  <P>  <P>
158  In order process UTF-8 strings, you must build PCRE to include UTF-8 support in  In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
# Line 119  instead of just strings of bytes. Line 165  instead of just strings of bytes.
165  <P>  <P>
166  If you compile PCRE with UTF-8 support, but do not use it at run time, the  If you compile PCRE with UTF-8 support, but do not use it at run time, the
167  library will be a bit bigger, but the additional run time overhead is limited  library will be a bit bigger, but the additional run time overhead is limited
168  to testing the PCRE_UTF8 flag in several places, so should not be very large.  to testing the PCRE_UTF8 flag occasionally, so should not be very big.
169    </P>
170    <P>
171    If PCRE is built with Unicode character property support (which implies UTF-8
172    support), the escape sequences \p{..}, \P{..}, and \X are supported.
173    The available properties that can be tested are limited to the general
174    category properties such as Lu for an upper case letter or Nd for a decimal
175    number, the Unicode script names such as Arabic or Han, and the derived
176    properties Any and L&. A full list is given in the
177    <a href="pcrepattern.html"><b>pcrepattern</b></a>
178    documentation. Only the short names for properties are supported. For example,
179    \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
180    Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
181    compatibility with Perl 5.6. PCRE does not support this.
182  </P>  </P>
183  <P>  <P>
184  The following comments apply when PCRE is running in UTF-8 mode:  The following comments apply when PCRE is running in UTF-8 mode:
185  </P>  </P>
186  <P>  <P>
187  1. PCRE assumes that the strings it is given contain valid UTF-8 codes. It does  1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
188  not diagnose invalid UTF-8 strings. If you pass invalid UTF-8 strings to PCRE,  are checked for validity on entry to the relevant functions. If an invalid
189  the results are undefined.  UTF-8 string is passed, an error return is given. In some situations, you may
190    already know that your strings are valid, and therefore want to skip these
191    checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
192    at compile time or at run time, PCRE assumes that the pattern or subject it
193    is given (respectively) contains only valid UTF-8 codes. In this case, it does
194    not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
195    PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
196    may crash.
197  </P>  </P>
198  <P>  <P>
199  2. In a pattern, the escape sequence \x{...}, where the contents of the braces  2. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
200  is a string of hexadecimal digits, is interpreted as a UTF-8 character whose  UTF-8 character if the value is greater than 127.
 code number is the given hexadecimal number, for example: \x{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.  
201  </P>  </P>
202  <P>  <P>
203  3. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8  3. Octal numbers up to \777 are recognized, and match two-byte UTF-8
204  character if the value is greater than 127.  characters for values greater than \177.
205  </P>  </P>
206  <P>  <P>
207  4. Repeat quantifiers apply to complete UTF-8 characters, not to individual  4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
# Line 150  bytes, for example: \x{100}{3}. Line 212  bytes, for example: \x{100}{3}.
212  </P>  </P>
213  <P>  <P>
214  6. The escape sequence \C can be used to match a single byte in UTF-8 mode,  6. The escape sequence \C can be used to match a single byte in UTF-8 mode,
215  but its use can lead to some strange effects.  but its use can lead to some strange effects. This facility is not available in
216    the alternative matching function, <b>pcre_dfa_exec()</b>.
217  </P>  </P>
218  <P>  <P>
219  7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly  7. The character escapes \b, \B, \d, \D, \s, \S, \w, and \W correctly
220  test characters of any code value, but the characters that PCRE recognizes as  test characters of any code value, but the characters that PCRE recognizes as
221  digits, spaces, or word characters remain the same set as before, all with  digits, spaces, or word characters remain the same set as before, all with
222  values less than 256.  values less than 256. This remains true even when PCRE includes Unicode
223    property support, because to do otherwise would slow down PCRE in many common
224    cases. If you really want to test for a wider sense of, say, "digit", you
225    must use Unicode property tests such as \p{Nd}.
226  </P>  </P>
227  <P>  <P>
228  8. Case-insensitive matching applies only to characters whose values are less  8. Similarly, characters that match the POSIX named character classes are all
229  than 256. PCRE does not support the notion of "case" for higher-valued  low-valued characters.
 characters.  
230  </P>  </P>
231  <P>  <P>
232  9. PCRE does not support the use of Unicode tables and properties or the Perl  9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
233  escapes \p, \P, and \X.  (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
234    </P>
235    <P>
236    10. Case-insensitive matching applies only to characters whose values are less
237    than 128, unless PCRE is built with Unicode property support. Even when Unicode
238    property support is available, PCRE still uses its own character tables when
239    checking the case of low-valued characters, so as not to degrade performance.
240    The Unicode property information is used only for characters with higher
241    values. Even when Unicode property support is available, PCRE supports
242    case-insensitive matching only when there is a one-to-one mapping between a
243    letter's cases. There are a small number of many-to-one mappings in Unicode;
244    these are not supported by PCRE.
245  </P>  </P>
246  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
247  <P>  <P>
248  Philip Hazel &#60;ph10@cam.ac.uk&#62;  Philip Hazel
249  <br>  <br>
250  University Computing Service,  University Computing Service
251  <br>  <br>
252  Cambridge CB2 3QG, England.  Cambridge CB2 3QH, England.
253  <br>  <br>
 Phone: +44 1223 334714  
254  </P>  </P>
255  <P>  <P>
256  Last updated: 04 February 2003  Putting an actual email address here seems to have been a spam magnet, so I've
257    taken it away. If you want to email me, use my two initials, followed by the
258    two digits 10, at the domain cam.ac.uk.
259    </P>
260    <br><a name="SEC6" href="#TOC1">REVISION</a><br>
261    <P>
262    Last updated: 06 August 2007
263    <br>
264    Copyright &copy; 1997-2007 University of Cambridge.
265  <br>  <br>
266  Copyright &copy; 1997-2003 University of Cambridge.  <p>
267    Return to the <a href="index.html">PCRE index page</a>.
268    </p>

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