trunk/doc/pcre.3

.TH PCRE 3
.SH NAME
PCRE - Perl-compatible regular expressions
.SH INTRODUCTION
.rs
.sp
The PCRE library is a set of functions that implement regular 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.)
.P
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.
.P
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 circumstances, the alternative function has some
advantages. For a discussion of the two matching algorithms, see the
.\" HREF
\fBpcrematching\fP
.\"
page.
.P
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
\fBpcrecpp\fP
.\"
page has details of this interface. Other people's contributions can be found
in the \fIContrib\fR directory at the primary FTP site, which is:
.sp
.\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
.\" </a>
ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
.P
Details of exactly which Perl regular expression features are and are not
supported by PCRE are given in separate documents. See the
.\" HREF
\fBpcrepattern\fR
.\"
and
.\" HREF
\fBpcrecompat\fR
.\"
pages.
.P
Some features of PCRE can be included, excluded, or changed when the library is
built. The
.\" HREF
\fBpcre_config()\fR
.\"
function makes it possible for a client to discover which features are
available. The features themselves are described in the
.\" HREF
\fBpcrebuild\fP
.\"
page. Documentation about building PCRE for various operating systems can be
found in the \fBREADME\fP file in the source distribution.
.P
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.
.
.
.SH "USER DOCUMENTATION"
.rs
.sp
The user documentation for PCRE comprises a number of different 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:
.sp
  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 \fBpcregrep\fP command
  pcrematching      discussion of the two matching algorithms
  pcrepartial       details of the partial matching facility
.\" JOIN
  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 sample program
  pcrestack         discussion of stack usage
  pcretest          description of the \fBpcretest\fP testing command
.sp
In addition, in the "man" and HTML formats, there is a short page for each
C library function, listing its arguments and results.
.
.
.SH LIMITATIONS
.rs
.sp
There are some size limitations in PCRE but it is hoped that they will never in
practice be relevant.
.P
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 \fBREADME\fP file in the source
distribution and the
.\" HREF
\fBpcrebuild\fP
.\"
documentation for details). In these cases the limit is substantially larger.
However, the speed of execution is slower.
.P
All values in repeating quantifiers must be less than 65536. The maximum
compiled length of subpattern with an explicit repeat count is 30000 bytes. The
maximum number of capturing subpatterns is 65535.
.P
There is no limit to the number of parenthesized subpatterns, but there can be
no more than 65535 capturing subpatterns.
.P
If a non-capturing subpattern with an unlimited repetition quantifier can match
an empty string, there is a limit of 1000 on the number of times it can be
repeated while not matching an empty string - if it does match an empty
string, the loop is immediately broken.
.P
The maximum length of name for a named subpattern is 32 characters, and the
maximum number of named subpatterns is 10000.
.P
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
\fBpcrestack\fP
.\"
documentation.
.sp
.\" HTML <a name="utf8support"></a>
.
.
.SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
.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 occasionally, so should not be very big.
.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, the Unicode script names such as Arabic or Han, and the derived
properties Any and L&. A full list is given in the
.\" HREF
\fBpcrepattern\fP
.\"
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.
.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. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
UTF-8 character if the value is greater than 127.
.P
3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
characters for values greater than \e177.
.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. This facility is not available in
the alternative matching function, \fBpcre_dfa_exec()\fP.
.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. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
(\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
.P
10. 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 between a
letter's cases. There are a small number of many-to-one mappings in Unicode;
these are not supported by PCRE.
.
.
.SH AUTHOR
.rs
.sp
.nf
Philip Hazel
University Computing Service
Cambridge CB2 3QH, England.
.fi
.P
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.
.
.
.SH REVISION
.rs
.sp
.nf
Last updated: 30 July 2007
Copyright (c) 1997-2007 University of Cambridge.
.fi
1	.TH PCRE 3
2	.SH NAME
3	PCRE - Perl-compatible regular expressions
4	.SH INTRODUCTION
5	.rs
6	.sp
7	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
9	differences. (Certain features that appeared in Python and PCRE before they
10	appeared in Perl are also available using the Python syntax.)
11	.P
12	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	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	PCRE is written in C and released as a C library. A number of people have
28	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	.sp
37	.\" 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	.P
41	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	.P
52	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	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	.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	"_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	.
73	.
74	.SH "USER DOCUMENTATION"
75	.rs
76	.sp
77	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	pcre this document
84	pcre-config show PCRE installation configuration information
85	pcreapi details of PCRE's native C API
86	pcrebuild options for building PCRE
87	pcrecallout details of the callout feature
88	pcrecompat discussion of Perl compatibility
89	pcrecpp details of the C++ wrapper
90	pcregrep description of the \fBpcregrep\fP command
91	pcrematching discussion of the two matching algorithms
92	pcrepartial details of the partial matching facility
93	.\" JOIN
94	pcrepattern syntax and semantics of supported
95	regular expressions
96	pcreperform discussion of performance issues
97	pcreposix the POSIX-compatible C API
98	pcreprecompile details of saving and re-using precompiled patterns
99	pcresample discussion of the sample program
100	pcrestack discussion of stack usage
101	pcretest description of the \fBpcretest\fP testing command
102	.sp
103	In addition, in the "man" and HTML formats, there is a short page for each
104	C library function, listing its arguments and results.
105	.
106	.
107	.SH LIMITATIONS
108	.rs
109	.sp
110	There are some size limitations in PCRE but it is hoped that they will never in
111	practice be relevant.
112	.P
113	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	internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
117	distribution and the
118	.\" HREF
119	\fBpcrebuild\fP
120	.\"
121	documentation for details). In these cases the limit is substantially larger.
122	However, the speed of execution is slower.
123	.P
124	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	.P
128	There is no limit to the number of parenthesized subpatterns, but there can be
129	no more than 65535 capturing subpatterns.
130	.P
131	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	repeated while not matching an empty string - if it does match an empty
134	string, the loop is immediately broken.
135	.P
136	The maximum length of name for a named subpattern is 32 characters, and the
137	maximum number of named subpatterns is 10000.
138	.P
139	The maximum length of a subject string is the largest positive number that an
140	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	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	.sp
150	.\" HTML <a name="utf8support"></a>
151	.
152	.
153	.SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
154	.rs
155	.sp
156	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	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	\fBpcre_compile()\fP
165	.\"
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	.P
170	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	to testing the PCRE_UTF8 flag occasionally, so should not be very big.
173	.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	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	.\" HREF
181	\fBpcrepattern\fP
182	.\"
183	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	.P
188	The following comments apply when PCRE is running in UTF-8 mode:
189	.P
190	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	.P
201	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	.P
204	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	bytes, for example: \ex{100}{3}.
209	.P
210	5. The dot metacharacter matches one UTF-8 character instead of a single byte.
211	.P
212	6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
213	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	.P
216	7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
217	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	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	8. Similarly, characters that match the POSIX named character classes are all
225	low-valued characters.
226	.P
227	9. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
228	(\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	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	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	.
240	.
241	.SH AUTHOR
242	.rs
243	.sp
244	.nf
245	Philip Hazel
246	University Computing Service
247	Cambridge CB2 3QH, England.
248	.fi
249	.P
250	Putting an actual email address here seems to have been a spam magnet, so I've
251	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	.
254	.
255	.SH REVISION
256	.rs
257	.sp
258	.nf
259	Last updated: 30 July 2007
260	Copyright (c) 1997-2007 University of Cambridge.
261	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"