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revision 211 by ph10, Thu Aug 9 09:52:43 2007 UTC revision 678 by ph10, Sun Aug 28 15:23:03 2011 UTC
# 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. (Certain features that appeared in Python and PCRE before they  differences. Some features that appeared in Python and PCRE before they
10  appeared in Perl are also available using the Python syntax.)  appeared in Perl are also available using the Python syntax, there is some
11  .P  support for one or two .NET and Oniguruma syntax items, and there is an option
12  The current implementation of PCRE (release 7.x) corresponds approximately with  for requesting some minor changes that give better JavaScript compatibility.
13  Perl 5.10, including support for UTF-8 encoded strings and Unicode general  .P
14  category properties. However, UTF-8 and Unicode support has to be explicitly  The current implementation of PCRE corresponds approximately with Perl 5.12,
15  enabled; it is not the default. The Unicode tables correspond to Unicode  including support for UTF-8 encoded strings and Unicode general category
16  release 5.0.0.  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 6.0.0.
18  .P  .P
19  In addition to the Perl-compatible matching function, PCRE contains an  In addition to the Perl-compatible matching function, PCRE contains an
20  alternative matching function that matches the same compiled patterns in a  alternative function that matches the same compiled patterns in a different
21  different way. In certain circumstances, the alternative function has some  way. In certain circumstances, the alternative function has some advantages.
22  advantages. For a discussion of the two matching algorithms, see the  For a discussion of the two matching algorithms, see the
23  .\" HREF  .\" HREF
24  \fBpcrematching\fP  \fBpcrematching\fP
25  .\"  .\"
# Line 32  PCRE distribution. The Line 33  PCRE distribution. The
33  \fBpcrecpp\fP  \fBpcrecpp\fP
34  .\"  .\"
35  page has details of this interface. Other people's contributions can be found  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:  in the \fIContrib\fP directory at the primary FTP site, which is:
37  .sp  .sp
38  .\" 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">
39  .\" </a>  .\" </a>
# Line 41  ftp://ftp.csx.cam.ac.uk/pub/software/pro Line 42  ftp://ftp.csx.cam.ac.uk/pub/software/pro
42  Details of exactly which Perl regular expression features are and are not  Details of exactly which Perl regular expression features are and are not
43  supported by PCRE are given in separate documents. See the  supported by PCRE are given in separate documents. See the
44  .\" HREF  .\" HREF
45  \fBpcrepattern\fR  \fBpcrepattern\fP
46  .\"  .\"
47  and  and
48  .\" HREF  .\" HREF
49  \fBpcrecompat\fR  \fBpcrecompat\fP
50  .\"  .\"
51  pages. There is a syntax summary in the  pages. There is a syntax summary in the
52  .\" HREF  .\" HREF
53  \fBpcresyntax\fR  \fBpcresyntax\fP
54  .\"  .\"
55  page.  page.
56  .P  .P
57  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
58  built. The  built. The
59  .\" HREF  .\" HREF
60  \fBpcre_config()\fR  \fBpcre_config()\fP
61  .\"  .\"
62  function makes it possible for a client to discover which features are  function makes it possible for a client to discover which features are
63  available. The features themselves are described in the  available. The features themselves are described in the
# Line 64  available. The features themselves are d Line 65  available. The features themselves are d
65  \fBpcrebuild\fP  \fBpcrebuild\fP
66  .\"  .\"
67  page. Documentation about building PCRE for various operating systems can be  page. Documentation about building PCRE for various operating systems can be
68  found in the \fBREADME\fP file in the source distribution.  found in the \fBREADME\fP and \fBNON-UNIX-USE\fP files in the source
69    distribution.
70  .P  .P
71  The library contains a number of undocumented internal functions and data  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  tables that are used by more than one of the exported external functions, but
# Line 81  not exported. Line 83  not exported.
83  The user documentation for PCRE comprises a number of different sections. In  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,  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,  each is a separate page, linked from the index page. In the plain text format,
86  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
87  follows:  of searching. The sections are as follows:
88  .sp  .sp
89    pcre              this document    pcre              this document
90    pcre-config       show PCRE installation configuration information    pcre-config       show PCRE installation configuration information
# Line 91  follows: Line 93  follows:
93    pcrecallout       details of the callout feature    pcrecallout       details of the callout feature
94    pcrecompat        discussion of Perl compatibility    pcrecompat        discussion of Perl compatibility
95    pcrecpp           details of the C++ wrapper    pcrecpp           details of the C++ wrapper
96      pcredemo          a demonstration C program that uses PCRE
97    pcregrep          description of the \fBpcregrep\fP command    pcregrep          description of the \fBpcregrep\fP command
98      pcrejit           discussion of the just-in-time optimization support
99      pcrelimits        details of size and other limits
100    pcrematching      discussion of the two matching algorithms    pcrematching      discussion of the two matching algorithms
101    pcrepartial       details of the partial matching facility    pcrepartial       details of the partial matching facility
102  .\" JOIN  .\" JOIN
103    pcrepattern       syntax and semantics of supported    pcrepattern       syntax and semantics of supported
104                        regular expressions                        regular expressions
   pcresyntax        quick syntax reference  
105    pcreperform       discussion of performance issues    pcreperform       discussion of performance issues
106    pcreposix         the POSIX-compatible C API    pcreposix         the POSIX-compatible C API
107    pcreprecompile    details of saving and re-using precompiled patterns    pcreprecompile    details of saving and re-using precompiled patterns
108    pcresample        discussion of the sample program    pcresample        discussion of the pcredemo program
109    pcrestack         discussion of stack usage    pcrestack         discussion of stack usage
110      pcresyntax        quick syntax reference
111    pcretest          description of the \fBpcretest\fP testing command    pcretest          description of the \fBpcretest\fP testing command
112      pcreunicode       discussion of Unicode and UTF-8 support
113  .sp  .sp
114  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
115  C library function, listing its arguments and results.  C library function, listing its arguments and results.
116  .  .
117  .  .
 .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. 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.  
 .  
 .\" 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}.  
 .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.  
 .  
 .  
118  .SH AUTHOR  .SH AUTHOR
119  .rs  .rs
120  .sp  .sp
# Line 287  two digits 10, at the domain cam.ac.uk. Line 133  two digits 10, at the domain cam.ac.uk.
133  .rs  .rs
134  .sp  .sp
135  .nf  .nf
136  Last updated: 09 August 2007  Last updated: 24 August 2011
137  Copyright (c) 1997-2007 University of Cambridge.  Copyright (c) 1997-2011 University of Cambridge.
138  .fi  .fi

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