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Revision 197 - (hide annotations) (download)
Tue Jul 31 10:50:18 2007 UTC (7 years ago) by ph10
File size: 10867 byte(s)
Abolish the NULLWSLIMIT error at the expense of using more stack when an 
unlimited repeat could match an empty string. Also, doc tidies for a test 

1 nigel 41 .TH PCRE 3
2     .SH NAME
3 nigel 63 PCRE - Perl-compatible regular expressions
5 nigel 63 .rs
6     .sp
7 nigel 41 The PCRE library is a set of functions that implement regular expression
8 nigel 63 pattern matching using the same syntax and semantics as Perl, with just a few
9 nigel 93 differences. (Certain features that appeared in Python and PCRE before they
10     appeared in Perl are also available using the Python syntax.)
11 nigel 75 .P
12 nigel 93 The current implementation of PCRE (release 7.x) corresponds approximately with
13     Perl 5.10, including support for UTF-8 encoded strings and Unicode general
14     category properties. However, UTF-8 and Unicode support has to be explicitly
15     enabled; it is not the default. The Unicode tables correspond to Unicode
16     release 5.0.0.
17     .P
18     In addition to the Perl-compatible matching function, PCRE contains an
19 nigel 77 alternative matching function that matches the same compiled patterns in a
20     different way. In certain circumstances, the alternative function has some
21     advantages. For a discussion of the two matching algorithms, see the
22     .\" HREF
23     \fBpcrematching\fP
24     .\"
25     page.
26     .P
27 nigel 75 PCRE is written in C and released as a C library. A number of people have
28 nigel 77 written wrappers and interfaces of various kinds. In particular, Google Inc.
29     have provided a comprehensive C++ wrapper. This is now included as part of the
30     PCRE distribution. The
31     .\" HREF
32     \fBpcrecpp\fP
33     .\"
34     page has details of this interface. Other people's contributions can be found
35     in the \fIContrib\fR directory at the primary FTP site, which is:
36 nigel 75 .sp
37 nigel 63 .\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
38     .\" </a>
39     ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
40 nigel 75 .P
41 nigel 63 Details of exactly which Perl regular expression features are and are not
42     supported by PCRE are given in separate documents. See the
43     .\" HREF
44     \fBpcrepattern\fR
45     .\"
46     and
47     .\" HREF
48     \fBpcrecompat\fR
49     .\"
50     pages.
51 nigel 75 .P
52 nigel 63 Some features of PCRE can be included, excluded, or changed when the library is
53     built. The
54     .\" HREF
55     \fBpcre_config()\fR
56     .\"
57     function makes it possible for a client to discover which features are
58 nigel 75 available. The features themselves are described in the
59     .\" HREF
60     \fBpcrebuild\fP
61     .\"
62     page. Documentation about building PCRE for various operating systems can be
63     found in the \fBREADME\fP file in the source distribution.
64 nigel 77 .P
65     The library contains a number of undocumented internal functions and data
66     tables that are used by more than one of the exported external functions, but
67     which are not intended for use by external callers. Their names all begin with
68 nigel 83 "_pcre_", which hopefully will not provoke any name clashes. In some
69     environments, it is possible to control which external symbols are exported
70     when a shared library is built, and in these cases the undocumented symbols are
71     not exported.
72 nigel 75 .
73     .
75 nigel 63 .rs
76     .sp
77 nigel 75 The user documentation for PCRE comprises a number of different sections. In
78     the "man" format, each of these is a separate "man page". In the HTML format,
79     each is a separate page, linked from the index page. In the plain text format,
80     all the sections are concatenated, for ease of searching. The sections are as
81     follows:
82     .sp
83 nigel 63 pcre this document
84 ph10 153 pcre-config show PCRE installation configuration information
85 nigel 77 pcreapi details of PCRE's native C API
86 nigel 63 pcrebuild options for building PCRE
87     pcrecallout details of the callout feature
88     pcrecompat discussion of Perl compatibility
89 nigel 77 pcrecpp details of the C++ wrapper
90 nigel 75 pcregrep description of the \fBpcregrep\fP command
91 nigel 77 pcrematching discussion of the two matching algorithms
92 nigel 75 pcrepartial details of the partial matching facility
93     .\" JOIN
94 nigel 63 pcrepattern syntax and semantics of supported
95     regular expressions
96     pcreperform discussion of performance issues
97 nigel 77 pcreposix the POSIX-compatible C API
98 nigel 75 pcreprecompile details of saving and re-using precompiled patterns
99 nigel 63 pcresample discussion of the sample program
100 nigel 91 pcrestack discussion of stack usage
101 nigel 75 pcretest description of the \fBpcretest\fP testing command
102     .sp
103 nigel 63 In addition, in the "man" and HTML formats, there is a short page for each
104 nigel 77 C library function, listing its arguments and results.
105 nigel 75 .
106     .
107 nigel 41 .SH LIMITATIONS
108 nigel 63 .rs
109     .sp
110 nigel 41 There are some size limitations in PCRE but it is hoped that they will never in
111     practice be relevant.
112 nigel 75 .P
113 nigel 63 The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
114     compiled with the default internal linkage size of 2. If you want to process
115     regular expressions that are truly enormous, you can compile PCRE with an
116 nigel 75 internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
117 nigel 63 distribution and the
118     .\" HREF
119 nigel 75 \fBpcrebuild\fP
120 nigel 63 .\"
121 nigel 75 documentation for details). In these cases the limit is substantially larger.
122 nigel 93 However, the speed of execution is slower.
123 nigel 75 .P
124 nigel 91 All values in repeating quantifiers must be less than 65536. The maximum
125     compiled length of subpattern with an explicit repeat count is 30000 bytes. The
126     maximum number of capturing subpatterns is 65535.
127 nigel 75 .P
128 nigel 93 There is no limit to the number of parenthesized subpatterns, but there can be
129     no more than 65535 capturing subpatterns.
130 nigel 75 .P
131 ph10 191 If a non-capturing subpattern with an unlimited repetition quantifier can match
132     an empty string, there is a limit of 1000 on the number of times it can be
133 ph10 197 repeated while not matching an empty string - if it does match an empty
134 ph10 191 string, the loop is immediately broken.
135     .P
136 nigel 93 The maximum length of name for a named subpattern is 32 characters, and the
137     maximum number of named subpatterns is 10000.
138 nigel 91 .P
139 nigel 41 The maximum length of a subject string is the largest positive number that an
140 nigel 77 integer variable can hold. However, when using the traditional matching
141     function, PCRE uses recursion to handle subpatterns and indefinite repetition.
142     This means that the available stack space may limit the size of a subject
143 nigel 91 string that can be processed by certain patterns. For a discussion of stack
144     issues, see the
145     .\" HREF
146     \fBpcrestack\fP
147     .\"
148     documentation.
149 nigel 75 .sp
150 nigel 63 .\" HTML <a name="utf8support"></a>
151 nigel 75 .
152     .
154 nigel 63 .rs
155     .sp
156 nigel 75 From release 3.3, PCRE has had some support for character strings encoded in
157     the UTF-8 format. For release 4.0 this was greatly extended to cover most
158     common requirements, and in release 5.0 additional support for Unicode general
159     category properties was added.
160     .P
161 nigel 63 In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
162     the code, and, in addition, you must call
163     .\" HREF
164 nigel 75 \fBpcre_compile()\fP
165 nigel 63 .\"
166     with the PCRE_UTF8 option flag. When you do this, both the pattern and any
167     subject strings that are matched against it are treated as UTF-8 strings
168     instead of just strings of bytes.
169 nigel 75 .P
170 nigel 49 If you compile PCRE with UTF-8 support, but do not use it at run time, the
171     library will be a bit bigger, but the additional run time overhead is limited
172 nigel 93 to testing the PCRE_UTF8 flag occasionally, so should not be very big.
173 nigel 75 .P
174     If PCRE is built with Unicode character property support (which implies UTF-8
175     support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
176     The available properties that can be tested are limited to the general
177     category properties such as Lu for an upper case letter or Nd for a decimal
178 nigel 87 number, the Unicode script names such as Arabic or Han, and the derived
179     properties Any and L&. A full list is given in the
180 nigel 75 .\" HREF
181     \fBpcrepattern\fP
182     .\"
183 nigel 87 documentation. Only the short names for properties are supported. For example,
184     \ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
185     Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
186     compatibility with Perl 5.6. PCRE does not support this.
187 nigel 75 .P
188 nigel 63 The following comments apply when PCRE is running in UTF-8 mode:
189 nigel 75 .P
190 nigel 71 1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
191     are checked for validity on entry to the relevant functions. If an invalid
192     UTF-8 string is passed, an error return is given. In some situations, you may
193     already know that your strings are valid, and therefore want to skip these
194     checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
195     at compile time or at run time, PCRE assumes that the pattern or subject it
196     is given (respectively) contains only valid UTF-8 codes. In this case, it does
197     not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
198     PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
199     may crash.
200 nigel 75 .P
201 nigel 87 2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
202     UTF-8 character if the value is greater than 127.
203 nigel 75 .P
204 nigel 91 3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
205     characters for values greater than \e177.
206     .P
207     4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
208 nigel 75 bytes, for example: \ex{100}{3}.
209     .P
210 nigel 91 5. The dot metacharacter matches one UTF-8 character instead of a single byte.
211 nigel 75 .P
212 nigel 91 6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
213 nigel 77 but its use can lead to some strange effects. This facility is not available in
214     the alternative matching function, \fBpcre_dfa_exec()\fP.
215 nigel 75 .P
216 nigel 91 7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
217 nigel 63 test characters of any code value, but the characters that PCRE recognizes as
218     digits, spaces, or word characters remain the same set as before, all with
219 nigel 75 values less than 256. This remains true even when PCRE includes Unicode
220     property support, because to do otherwise would slow down PCRE in many common
221     cases. If you really want to test for a wider sense of, say, "digit", you
222     must use Unicode property tests such as \ep{Nd}.
223     .P
224 nigel 91 8. Similarly, characters that match the POSIX named character classes are all
225 nigel 75 low-valued characters.
226     .P
227 ph10 182 9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
228 ph10 181 (\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
229     .P
230     10. Case-insensitive matching applies only to characters whose values are less
231 nigel 75 than 128, unless PCRE is built with Unicode property support. Even when Unicode
232     property support is available, PCRE still uses its own character tables when
233     checking the case of low-valued characters, so as not to degrade performance.
234     The Unicode property information is used only for characters with higher
235 nigel 87 values. Even when Unicode property support is available, PCRE supports
236     case-insensitive matching only when there is a one-to-one mapping between a
237     letter's cases. There are a small number of many-to-one mappings in Unicode;
238     these are not supported by PCRE.
239 nigel 75 .
240 ph10 99 .
241 nigel 41 .SH AUTHOR
242 nigel 63 .rs
243     .sp
244 ph10 99 .nf
245 nigel 77 Philip Hazel
246 ph10 99 University Computing Service
247 nigel 93 Cambridge CB2 3QH, England.
248 ph10 99 .fi
249 nigel 77 .P
250     Putting an actual email address here seems to have been a spam magnet, so I've
251 ph10 153 taken it away. If you want to email me, use my two initials, followed by the
252     two digits 10, at the domain cam.ac.uk.
253 ph10 99 .
254     .
255     .SH REVISION
256     .rs
257 nigel 75 .sp
258 ph10 99 .nf
259 ph10 191 Last updated: 30 July 2007
260 ph10 99 Copyright (c) 1997-2007 University of Cambridge.
261     .fi


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