245 lines
10 KiB
Groff
245 lines
10 KiB
Groff
.TH PCRE 3
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.SH NAME
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PCRE - Perl-compatible regular expressions
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.SH INTRODUCTION
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.rs
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.sp
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The PCRE library is a set of functions that implement regular expression
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pattern matching using the same syntax and semantics as Perl, with just a few
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differences. The current implementation of PCRE (release 6.x) corresponds
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approximately with Perl 5.8, including support for UTF-8 encoded strings and
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Unicode general category properties. However, this support has to be explicitly
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enabled; it is not the default.
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.P
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In addition to the Perl-compatible matching function, PCRE also contains an
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alternative matching function that matches the same compiled patterns in a
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different way. In certain circumstances, the alternative function has some
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advantages. For a discussion of the two matching algorithms, see the
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.\" HREF
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\fBpcrematching\fP
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.\"
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page.
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.P
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PCRE is written in C and released as a C library. A number of people have
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written wrappers and interfaces of various kinds. In particular, Google Inc.
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have provided a comprehensive C++ wrapper. This is now included as part of the
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PCRE distribution. The
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.\" HREF
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\fBpcrecpp\fP
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.\"
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page has details of this interface. Other people's contributions can be found
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in the \fIContrib\fR directory at the primary FTP site, which is:
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.sp
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.\" HTML <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">
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.\" </a>
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ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre
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.P
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Details of exactly which Perl regular expression features are and are not
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supported by PCRE are given in separate documents. See the
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.\" HREF
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\fBpcrepattern\fR
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.\"
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and
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.\" HREF
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\fBpcrecompat\fR
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.\"
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pages.
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.P
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Some features of PCRE can be included, excluded, or changed when the library is
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built. The
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.\" HREF
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\fBpcre_config()\fR
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.\"
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function makes it possible for a client to discover which features are
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available. The features themselves are described in the
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.\" HREF
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\fBpcrebuild\fP
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.\"
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page. Documentation about building PCRE for various operating systems can be
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found in the \fBREADME\fP file in the source distribution.
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.P
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The library contains a number of undocumented internal functions and data
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tables that are used by more than one of the exported external functions, but
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which are not intended for use by external callers. Their names all begin with
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"_pcre_", which hopefully will not provoke any name clashes. In some
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environments, it is possible to control which external symbols are exported
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when a shared library is built, and in these cases the undocumented symbols are
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not exported.
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.
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.
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.SH "USER DOCUMENTATION"
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.rs
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.sp
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The user documentation for PCRE comprises a number of different sections. In
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the "man" format, each of these is a separate "man page". In the HTML format,
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each is a separate page, linked from the index page. In the plain text format,
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all the sections are concatenated, for ease of searching. The sections are as
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follows:
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.sp
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pcre this document
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pcreapi details of PCRE's native C API
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pcrebuild options for building PCRE
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pcrecallout details of the callout feature
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pcrecompat discussion of Perl compatibility
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pcrecpp details of the C++ wrapper
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pcregrep description of the \fBpcregrep\fP command
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pcrematching discussion of the two matching algorithms
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pcrepartial details of the partial matching facility
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.\" JOIN
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pcrepattern syntax and semantics of supported
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regular expressions
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pcreperform discussion of performance issues
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pcreposix the POSIX-compatible C API
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pcreprecompile details of saving and re-using precompiled patterns
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pcresample discussion of the sample program
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pcrestack discussion of stack usage
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pcretest description of the \fBpcretest\fP testing command
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.sp
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In addition, in the "man" and HTML formats, there is a short page for each
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C library function, listing its arguments and results.
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.
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.
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.SH LIMITATIONS
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.rs
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.sp
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There are some size limitations in PCRE but it is hoped that they will never in
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practice be relevant.
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.P
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The maximum length of a compiled pattern is 65539 (sic) bytes if PCRE is
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compiled with the default internal linkage size of 2. If you want to process
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regular expressions that are truly enormous, you can compile PCRE with an
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internal linkage size of 3 or 4 (see the \fBREADME\fP file in the source
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distribution and the
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.\" HREF
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\fBpcrebuild\fP
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.\"
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documentation for details). In these cases the limit is substantially larger.
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However, the speed of execution will be slower.
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.P
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All values in repeating quantifiers must be less than 65536. The maximum
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compiled length of subpattern with an explicit repeat count is 30000 bytes. The
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maximum number of capturing subpatterns is 65535.
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.P
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There is no limit to the number of non-capturing subpatterns, but the maximum
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depth of nesting of all kinds of parenthesized subpattern, including capturing
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subpatterns, assertions, and other types of subpattern, is 200.
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.P
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The maximum length of name for a named subpattern is 32, and the maximum number
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of named subpatterns is 10000.
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.P
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The maximum length of a subject string is the largest positive number that an
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integer variable can hold. However, when using the traditional matching
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function, PCRE uses recursion to handle subpatterns and indefinite repetition.
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This means that the available stack space may limit the size of a subject
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string that can be processed by certain patterns. For a discussion of stack
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issues, see the
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.\" HREF
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\fBpcrestack\fP
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.\"
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documentation.
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.sp
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.\" HTML <a name="utf8support"></a>
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.
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.
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.SH "UTF-8 AND UNICODE PROPERTY SUPPORT"
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.rs
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.sp
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From release 3.3, PCRE has had some support for character strings encoded in
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the UTF-8 format. For release 4.0 this was greatly extended to cover most
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common requirements, and in release 5.0 additional support for Unicode general
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category properties was added.
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.P
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In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
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the code, and, in addition, you must call
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.\" HREF
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\fBpcre_compile()\fP
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.\"
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with the PCRE_UTF8 option flag. When you do this, both the pattern and any
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subject strings that are matched against it are treated as UTF-8 strings
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instead of just strings of bytes.
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.P
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If you compile PCRE with UTF-8 support, but do not use it at run time, the
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library will be a bit bigger, but the additional run time overhead is limited
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to testing the PCRE_UTF8 flag in several places, so should not be very large.
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.P
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If PCRE is built with Unicode character property support (which implies UTF-8
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support), the escape sequences \ep{..}, \eP{..}, and \eX are supported.
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The available properties that can be tested are limited to the general
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category properties such as Lu for an upper case letter or Nd for a decimal
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number, the Unicode script names such as Arabic or Han, and the derived
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properties Any and L&. A full list is given in the
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.\" HREF
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\fBpcrepattern\fP
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.\"
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documentation. Only the short names for properties are supported. For example,
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\ep{L} matches a letter. Its Perl synonym, \ep{Letter}, is not supported.
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Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
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compatibility with Perl 5.6. PCRE does not support this.
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.P
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The following comments apply when PCRE is running in UTF-8 mode:
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.P
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1. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
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are checked for validity on entry to the relevant functions. If an invalid
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UTF-8 string is passed, an error return is given. In some situations, you may
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already know that your strings are valid, and therefore want to skip these
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checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
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at compile time or at run time, PCRE assumes that the pattern or subject it
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is given (respectively) contains only valid UTF-8 codes. In this case, it does
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not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
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PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
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may crash.
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.P
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2. An unbraced hexadecimal escape sequence (such as \exb3) matches a two-byte
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UTF-8 character if the value is greater than 127.
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.P
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3. Octal numbers up to \e777 are recognized, and match two-byte UTF-8
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characters for values greater than \e177.
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.P
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4. Repeat quantifiers apply to complete UTF-8 characters, not to individual
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bytes, for example: \ex{100}{3}.
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.P
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5. The dot metacharacter matches one UTF-8 character instead of a single byte.
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.P
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6. The escape sequence \eC can be used to match a single byte in UTF-8 mode,
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but its use can lead to some strange effects. This facility is not available in
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the alternative matching function, \fBpcre_dfa_exec()\fP.
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.P
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7. The character escapes \eb, \eB, \ed, \eD, \es, \eS, \ew, and \eW correctly
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test characters of any code value, but the characters that PCRE recognizes as
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digits, spaces, or word characters remain the same set as before, all with
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values less than 256. This remains true even when PCRE includes Unicode
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property support, because to do otherwise would slow down PCRE in many common
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cases. If you really want to test for a wider sense of, say, "digit", you
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must use Unicode property tests such as \ep{Nd}.
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.P
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8. Similarly, characters that match the POSIX named character classes are all
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low-valued characters.
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.P
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9. Case-insensitive matching applies only to characters whose values are less
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than 128, unless PCRE is built with Unicode property support. Even when Unicode
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property support is available, PCRE still uses its own character tables when
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checking the case of low-valued characters, so as not to degrade performance.
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The Unicode property information is used only for characters with higher
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values. Even when Unicode property support is available, PCRE supports
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case-insensitive matching only when there is a one-to-one mapping between a
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letter's cases. There are a small number of many-to-one mappings in Unicode;
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these are not supported by PCRE.
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.
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.SH AUTHOR
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.rs
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.sp
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Philip Hazel
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.br
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University Computing Service,
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.br
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Cambridge CB2 3QG, England.
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.P
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Putting an actual email address here seems to have been a spam magnet, so I've
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taken it away. If you want to email me, use my initial and surname, separated
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by a dot, at the domain ucs.cam.ac.uk.
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.sp
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.in 0
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Last updated: 05 June 2006
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.br
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Copyright (c) 1997-2006 University of Cambridge.
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