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. Some features that appeared in Python and PCRE before they
appeared in Perl are also available using the Python syntax, there is some
support for one or two .NET and Oniguruma syntax items, and there is an option
for requesting some minor changes that give better JavaScript compatibility.
.P
The current implementation of PCRE 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 5.1.
.P
In addition to the Perl-compatible matching function, PCRE contains an
alternative 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. There is a syntax summary in the
.\" HREF
\fBpcresyntax\fR
.\"
page.
.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 and \fBNON-UNIX-USE\fP files 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, except the \fBpcredemo\fP section, 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
  pcredemo          a demonstration C program that uses PCRE
  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 pcredemo program
  pcrestack         discussion of stack usage
  pcresyntax        quick syntax reference
  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.
.P
There is no limit to the number of parenthesized subpatterns, but there can be
no more than 65535 capturing subpatterns.
.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.
.
.
.\" 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, or the pattern must start with the sequence
(*UTF8). When either of these is the case, both the pattern and any subject
strings that are matched against it are treated as UTF-8 strings instead of
strings of 1-byte characters.
.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.
.
.
.\" HTML <a name="utf8strings"></a>
.
.SS "Validity of UTF-8 strings"
.rs
.sp
When you set the PCRE_UTF8 flag, the strings passed as patterns 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 0x7FFFFFFF). The current check allows only values in the range U+0 to
U+10FFFF, excluding U+D800 to U+DFFF.
.P
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
provided 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 unfortunately messes up
UTF-8.)
.P
If an invalid UTF-8 string is passed to PCRE, an error return
(PCRE_ERROR_BADUTF8) 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.
.P
If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK 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 0x7FFFFFFF. In other words, apart from the initial 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.
.P
If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
encoded in a UTF-8-like manner as per the old RFC, you can set
PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
situation, you will have to apply your own validity check.
.
.SS "General comments about UTF-8 mode"
.rs
.sp
1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
UTF-8 character if the value is greater than 127.
.P
2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
characters for values greater than \e177.
.P
3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
bytes, for example: \ex{100}{3}.
.P
4. The dot metacharacter matches one UTF-8 character instead of a single byte.
.P
5. 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
6. 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}. Note that this also applies to
\eb, because it is defined in terms of \ew and \eW.
.P
7. Similarly, characters that match the POSIX named character classes are all
low-valued characters.
.P
8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
(\eh, \eH, \ev, and \eV) do match all the appropriate Unicode 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. 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: 28 September 2009
Copyright (c) 1997-2009 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. Some features that appeared in Python and PCRE before they
10	appeared in Perl are also available using the Python syntax, there is some
11	support for one or two .NET and Oniguruma syntax items, and there is an option
12	for requesting some minor changes that give better JavaScript compatibility.
13	.P
14	The current implementation of PCRE corresponds approximately with Perl 5.10,
15	including support for UTF-8 encoded strings and Unicode general category
16	properties. However, UTF-8 and Unicode support has to be explicitly enabled; it
17	is not the default. The Unicode tables correspond to Unicode release 5.1.
18	.P
19	In addition to the Perl-compatible matching function, PCRE contains an
20	alternative function that matches the same compiled patterns in a different
21	way. In certain circumstances, the alternative function has some advantages.
22	For a discussion of the two matching algorithms, see the
23	.\" HREF
24	\fBpcrematching\fP
25	.\"
26	page.
27	.P
28	PCRE is written in C and released as a C library. A number of people have
29	written wrappers and interfaces of various kinds. In particular, Google Inc.
30	have provided a comprehensive C++ wrapper. This is now included as part of the
31	PCRE distribution. The
32	.\" HREF
33	\fBpcrecpp\fP
34	.\"
35	page has details of this interface. Other people's contributions can be found
36	in the \fIContrib\fR directory at the primary FTP site, which is:
37	.sp
38	.\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
39	.\" </a>
40	ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
41	.P
42	Details of exactly which Perl regular expression features are and are not
43	supported by PCRE are given in separate documents. See the
44	.\" HREF
45	\fBpcrepattern\fR
46	.\"
47	and
48	.\" HREF
49	\fBpcrecompat\fR
50	.\"
51	pages. There is a syntax summary in the
52	.\" HREF
53	\fBpcresyntax\fR
54	.\"
55	page.
56	.P
57	Some features of PCRE can be included, excluded, or changed when the library is
58	built. The
59	.\" HREF
60	\fBpcre_config()\fR
61	.\"
62	function makes it possible for a client to discover which features are
63	available. The features themselves are described in the
64	.\" HREF
65	\fBpcrebuild\fP
66	.\"
67	page. Documentation about building PCRE for various operating systems can be
68	found in the \fBREADME\fP and \fBNON-UNIX-USE\fP files in the source
69	distribution.
70	.P
71	The library contains a number of undocumented internal functions and data
72	tables that are used by more than one of the exported external functions, but
73	which are not intended for use by external callers. Their names all begin with
74	"_pcre_", which hopefully will not provoke any name clashes. In some
75	environments, it is possible to control which external symbols are exported
76	when a shared library is built, and in these cases the undocumented symbols are
77	not exported.
78	.
79	.
80	.SH "USER DOCUMENTATION"
81	.rs
82	.sp
83	The user documentation for PCRE comprises a number of different sections. In
84	the "man" format, each of these is a separate "man page". In the HTML format,
85	each is a separate page, linked from the index page. In the plain text format,
86	all the sections, except the \fBpcredemo\fP section, are concatenated, for ease
87	of searching. The sections are as follows:
88	.sp
89	pcre this document
90	pcre-config show PCRE installation configuration information
91	pcreapi details of PCRE's native C API
92	pcrebuild options for building PCRE
93	pcrecallout details of the callout feature
94	pcrecompat discussion of Perl compatibility
95	pcrecpp details of the C++ wrapper
96	pcredemo a demonstration C program that uses PCRE
97	pcregrep description of the \fBpcregrep\fP command
98	pcrematching discussion of the two matching algorithms
99	pcrepartial details of the partial matching facility
100	.\" JOIN
101	pcrepattern syntax and semantics of supported
102	regular expressions
103	pcreperform discussion of performance issues
104	pcreposix the POSIX-compatible C API
105	pcreprecompile details of saving and re-using precompiled patterns
106	pcresample discussion of the pcredemo program
107	pcrestack discussion of stack usage
108	pcresyntax quick syntax reference
109	pcretest description of the \fBpcretest\fP testing command
110	.sp
111	In addition, in the "man" and HTML formats, there is a short page for each
112	C library function, listing its arguments and results.
113	.
114	.
115	.SH LIMITATIONS
116	.rs
117	.sp
118	There are some size limitations in PCRE but it is hoped that they will never in
119	practice be relevant.
120	.P
121	The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
122	compiled with the default internal linkage size of 2. If you want to process
123	regular expressions that are truly enormous, you can compile PCRE with an
124	internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
125	distribution and the
126	.\" HREF
127	\fBpcrebuild\fP
128	.\"
129	documentation for details). In these cases the limit is substantially larger.
130	However, the speed of execution is slower.
131	.P
132	All values in repeating quantifiers must be less than 65536.
133	.P
134	There is no limit to the number of parenthesized subpatterns, but there can be
135	no more than 65535 capturing subpatterns.
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
140	The maximum length of a subject string is the largest positive number that an
141	integer variable can hold. However, when using the traditional matching
142	function, PCRE uses recursion to handle subpatterns and indefinite repetition.
143	This means that the available stack space may limit the size of a subject
144	string that can be processed by certain patterns. For a discussion of stack
145	issues, see the
146	.\" HREF
147	\fBpcrestack\fP
148	.\"
149	documentation.
150	.
151	.
152	.\" HTML <a name="utf8support"></a>
153	.
154	.SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
155	.rs
156	.sp
157	From release 3.3, PCRE has had some support for character strings encoded in
158	the UTF-8 format. For release 4.0 this was greatly extended to cover most
159	common requirements, and in release 5.0 additional support for Unicode general
160	category properties was added.
161	.P
162	In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
163	the code, and, in addition, you must call
164	.\" HREF
165	\fBpcre_compile()\fP
166	.\"
167	with the PCRE_UTF8 option flag, or the pattern must start with the sequence
168	(*UTF8). When either of these is the case, both the pattern and any subject
169	strings that are matched against it are treated as UTF-8 strings instead of
170	strings of 1-byte characters.
171	.P
172	If you compile PCRE with UTF-8 support, but do not use it at run time, the
173	library will be a bit bigger, but the additional run time overhead is limited
174	to testing the PCRE_UTF8 flag occasionally, so should not be very big.
175	.P
176	If PCRE is built with Unicode character property support (which implies UTF-8
177	support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
178	The available properties that can be tested are limited to the general
179	category properties such as Lu for an upper case letter or Nd for a decimal
180	number, the Unicode script names such as Arabic or Han, and the derived
181	properties Any and L&. A full list is given in the
182	.\" HREF
183	\fBpcrepattern\fP
184	.\"
185	documentation. Only the short names for properties are supported. For example,
186	\ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
187	Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
188	compatibility with Perl 5.6. PCRE does not support this.
189	.
190	.
191	.\" HTML <a name="utf8strings"></a>
192	.
193	.SS "Validity of UTF-8 strings"
194	.rs
195	.sp
196	When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
197	are (by default) checked for validity on entry to the relevant functions. From
198	release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
199	themselves derived from the Unicode specification. Earlier releases of PCRE
200	followed the rules of RFC 2279, which allows the full range of 31-bit values (0
201	to 0x7FFFFFFF). The current check allows only values in the range U+0 to
202	U+10FFFF, excluding U+D800 to U+DFFF.
203	.P
204	The excluded code points are the "Low Surrogate Area" of Unicode, of which the
205	Unicode Standard says this: "The Low Surrogate Area does not contain any
206	character assignments, consequently no character code charts or namelists are
207	provided for this area. Surrogates are reserved for use with UTF-16 and then
208	must be used in pairs." The code points that are encoded by UTF-16 pairs are
209	available as independent code points in the UTF-8 encoding. (In other words,
210	the whole surrogate thing is a fudge for UTF-16 which unfortunately messes up
211	UTF-8.)
212	.P
213	If an invalid UTF-8 string is passed to PCRE, an error return
214	(PCRE_ERROR_BADUTF8) is given. In some situations, you may already know that
215	your strings are valid, and therefore want to skip these checks in order to
216	improve performance. If you set the PCRE_NO_UTF8_CHECK flag at compile time or
217	at run time, PCRE assumes that the pattern or subject it is given
218	(respectively) contains only valid UTF-8 codes. In this case, it does not
219	diagnose an invalid UTF-8 string.
220	.P
221	If you pass an invalid UTF-8 string when PCRE_NO_UTF8_CHECK is set, what
222	happens depends on why the string is invalid. If the string conforms to the
223	"old" definition of UTF-8 (RFC 2279), it is processed as a string of characters
224	in the range 0 to 0x7FFFFFFF. In other words, apart from the initial validity
225	test, PCRE (when in UTF-8 mode) handles strings according to the more liberal
226	rules of RFC 2279. However, if the string does not even conform to RFC 2279,
227	the result is undefined. Your program may crash.
228	.P
229	If you want to process strings of values in the full range 0 to 0x7FFFFFFF,
230	encoded in a UTF-8-like manner as per the old RFC, you can set
231	PCRE_NO_UTF8_CHECK to bypass the more restrictive test. However, in this
232	situation, you will have to apply your own validity check.
233	.
234	.SS "General comments about UTF-8 mode"
235	.rs
236	.sp
237	1. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
238	UTF-8 character if the value is greater than 127.
239	.P
240	2. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
241	characters for values greater than \e177.
242	.P
243	3. Repeat quantifiers apply to complete UTF-8 characters, not to individual
244	bytes, for example: \ex{100}{3}.
245	.P
246	4. The dot metacharacter matches one UTF-8 character instead of a single byte.
247	.P
248	5. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
249	but its use can lead to some strange effects. This facility is not available in
250	the alternative matching function, \fBpcre_dfa_exec()\fP.
251	.P
252	6. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
253	test characters of any code value, but the characters that PCRE recognizes as
254	digits, spaces, or word characters remain the same set as before, all with
255	values less than 256. This remains true even when PCRE includes Unicode
256	property support, because to do otherwise would slow down PCRE in many common
257	cases. If you really want to test for a wider sense of, say, "digit", you
258	must use Unicode property tests such as \ep{Nd}. Note that this also applies to
259	\eb, because it is defined in terms of \ew and \eW.
260	.P
261	7. Similarly, characters that match the POSIX named character classes are all
262	low-valued characters.
263	.P
264	8. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
265	(\eh, \eH, \ev, and \eV) do match all the appropriate Unicode characters.
266	.P
267	9. Case-insensitive matching applies only to characters whose values are less
268	than 128, unless PCRE is built with Unicode property support. Even when Unicode
269	property support is available, PCRE still uses its own character tables when
270	checking the case of low-valued characters, so as not to degrade performance.
271	The Unicode property information is used only for characters with higher
272	values. Even when Unicode property support is available, PCRE supports
273	case-insensitive matching only when there is a one-to-one mapping between a
274	letter's cases. There are a small number of many-to-one mappings in Unicode;
275	these are not supported by PCRE.
276	.
277	.
278	.SH AUTHOR
279	.rs
280	.sp
281	.nf
282	Philip Hazel
283	University Computing Service
284	Cambridge CB2 3QH, England.
285	.fi
286	.P
287	Putting an actual email address here seems to have been a spam magnet, so I've
288	taken it away. If you want to email me, use my two initials, followed by the
289	two digits 10, at the domain cam.ac.uk.
290	.
291	.
292	.SH REVISION
293	.rs
294	.sp
295	.nf
296	Last updated: 28 September 2009
297	Copyright (c) 1997-2009 University of Cambridge.
298	.fi
Name	Value
svn:eol-style	native
svn:keywords	"Author Date Id Revision Url"