/[pcre]/code/trunk/doc/html/pcre.html

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-revision 71 by nigel,
Sat Feb 24 21:40:24 2007 UTC
+revision 211 by ph10,
Thu Aug  9 09:52:43 2007 UTC
 Line 3
  <title>pcre specification</title>
  </head>
  <body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
- This HTML document has been generated automatically from the original man page.
+ <h1>pcre man page</h1>
- If there is any nonsense in it, please consult the man page, in case the
+ <p>
- conversion went wrong.<br>
+ Return to the <a href="index.html">PCRE index page</a>.
+ </p>
+ <p>
+ This page is part of the PCRE HTML documentation. It was generated automatically
+ from the original man page. If there is any nonsense in it, please consult the
+ man page, in case the conversion went wrong.
+ <br>
  <ul>
- <li><a name="TOC1" href="#SEC1">DESCRIPTION</a>
+ <li><a name="TOC1" href="#SEC1">INTRODUCTION</a>
  <li><a name="TOC2" href="#SEC2">USER DOCUMENTATION</a>
  <li><a name="TOC3" href="#SEC3">LIMITATIONS</a>
- <li><a name="TOC4" href="#SEC4">UTF-8 SUPPORT</a>
+ <li><a name="TOC4" href="#SEC4">UTF-8 AND UNICODE PROPERTY SUPPORT</a>
  <li><a name="TOC5" href="#SEC5">AUTHOR</a>
+ <li><a name="TOC6" href="#SEC6">REVISION</a>
  </ul>
- <br><a name="SEC1" href="#TOC1">DESCRIPTION</a><br>
+ <br><a name="SEC1" href="#TOC1">INTRODUCTION</a><br>
  <P>
  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. The current implementation of PCRE (release 4.x) corresponds
+ differences. (Certain features that appeared in Python and PCRE before they
- approximately with Perl 5.8, including support for UTF-8 encoded strings.
+ appeared in Perl are also available using the Python syntax.)
- However, this support has to be explicitly enabled; it is not the default.
  </P>
  <P>
- PCRE is written in C and released as a C library. However, a number of people
+ The current implementation of PCRE (release 7.x) corresponds approximately with
- have written wrappers and interfaces of various kinds. A C++ class is included
+ Perl 5.10, including support for UTF-8 encoded strings and Unicode general
- in these contributions, which can be found in the <i>Contrib</i> directory at
+ category properties. However, UTF-8 and Unicode support has to be explicitly
- the primary FTP site, which is:
+ enabled; it is not the default. The Unicode tables correspond to Unicode
- </P>
+ release 5.0.0.
+ </P>
+ <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
+ <a href="pcrematching.html"><b>pcrematching</b></a>
+ page.
+ </P>
+ <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
+ <a href="pcrecpp.html"><b>pcrecpp</b></a>
+ page has details of this interface. Other people's contributions can be found
+ in the <i>Contrib</i> directory at the primary FTP site, which is:
  <a href="ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre">ftp://ftp.csx.cam.ac.uk/pub/software/programming/pcre</a>
+ </P>
  <P>
  Details of exactly which Perl regular expression features are and are not
  supported by PCRE are given in separate documents. See the
  <a href="pcrepattern.html"><b>pcrepattern</b></a>
  and
  <a href="pcrecompat.html"><b>pcrecompat</b></a>
- pages.
+ pages. There is a syntax summary in the
+ <a href="pcresyntax.html"><b>pcresyntax</b></a>
+ page.
  </P>
  <P>
  Some features of PCRE can be included, excluded, or changed when the library is
  built. The
  <a href="pcre_config.html"><b>pcre_config()</b></a>
  function makes it possible for a client to discover which features are
- available. Documentation about building PCRE for various operating systems can
+ available. The features themselves are described in the
- be found in the <b>README</b> file in the source distribution.
+ <a href="pcrebuild.html"><b>pcrebuild</b></a>
+ page. Documentation about building PCRE for various operating systems can be
+ found in the <b>README</b> file in the source distribution.
  </P>
- <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
  <P>
- The user documentation for PCRE has been split up into a number of different
+ The library contains a number of undocumented internal functions and data
- sections. In the "man" format, each of these is a separate "man page". In the
+ tables that are used by more than one of the exported external functions, but
- HTML format, each is a separate page, linked from the index page. In the plain
+ which are not intended for use by external callers. Their names all begin with
- text format, all the sections are concatenated, for ease of searching. The
+ "_pcre_", which hopefully will not provoke any name clashes. In some
- sections are as follows:
+ 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.
  </P>
+ <br><a name="SEC2" href="#TOC1">USER DOCUMENTATION</a><br>
  <P>
+ 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:
  <pre>
    pcre              this document
-   pcreapi           details of PCRE's native API
+   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 <b>pcregrep</b> command
-   pcrepattern       syntax and semantics of supported
+   pcrematching      discussion of the two matching algorithms
-                       regular expressions
+   pcrepartial       details of the partial matching facility
+   pcrepattern       syntax and semantics of supported regular expressions
+   pcresyntax        quick syntax reference
    pcreperform       discussion of performance issues
-   pcreposix         the POSIX-compatible API
+   pcreposix         the POSIX-compatible C API
+   pcreprecompile    details of saving and re-using precompiled patterns
    pcresample        discussion of the sample program
-   pcretest          the <b>pcretest</b> testing command
+   pcrestack         discussion of stack usage
- </PRE>
+   pcretest          description of the <b>pcretest</b> testing command
- </P>
+ </pre>
- <P>
  In addition, in the "man" and HTML formats, there is a short page for each
- library function, listing its arguments and results.
+ C library function, listing its arguments and results.
  </P>
  <br><a name="SEC3" href="#TOC1">LIMITATIONS</a><br>
  <P>
-Line 84 
 regular expressions that are truly enorm
+Line 123 
 regular expressions that are truly enorm
  internal linkage size of 3 or 4 (see the <b>README</b> file in the source
  distribution and the
  <a href="pcrebuild.html"><b>pcrebuild</b></a>
- documentation for details). If these cases the limit is substantially larger.
+ documentation for details). In these cases the limit is substantially larger.
- However, the speed of execution will be slower.
+ However, the speed of execution is slower.
  </P>
  <P>
  All values in repeating quantifiers must be less than 65536.
- The maximum number of capturing subpatterns is 65535.
  </P>
  <P>
- There is no limit to the number of non-capturing subpatterns, but the maximum
+ There is no limit to the number of parenthesized subpatterns, but there can be
- depth of nesting of all kinds of parenthesized subpattern, including capturing
+ no more than 65535 capturing subpatterns.
- subpatterns, assertions, and other types of subpattern, is 200.
+ </P>
+ <P>
+ The maximum length of name for a named subpattern is 32 characters, and the
+ maximum number of named subpatterns is 10000.
  </P>
  <P>
  The maximum length of a subject string is the largest positive number that an
- integer variable can hold. However, PCRE uses recursion to handle subpatterns
+ integer variable can hold. However, when using the traditional matching
- and indefinite repetition. This means that the available stack space may limit
+ function, PCRE uses recursion to handle subpatterns and indefinite repetition.
- the size of a subject string that can be processed by certain patterns.
+ This means that the available stack space may limit the size of a subject
- </P>
+ string that can be processed by certain patterns. For a discussion of stack
- <a name="utf8support"></a><br><a name="SEC4" href="#TOC1">UTF-8 SUPPORT</a><br>
+ issues, see the
- <P>
+ <a href="pcrestack.html"><b>pcrestack</b></a>
- Starting at release 3.3, PCRE has had some support for character strings
+ documentation.
- encoded in the UTF-8 format. For release 4.0 this has been greatly extended to
+ <a name="utf8support"></a></P>
- cover most common requirements.
+ <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
+ <P>
+ 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>
  <P>
  In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
-Line 119 
 instead of just strings of bytes.
+Line 165 
 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 in several places, so should not be very large.
+ to testing the PCRE_UTF8 flag occasionally, so should not be very big.
  </P>
  <P>
- The following comments apply when PCRE is running in UTF-8 mode:
+ If PCRE is built with Unicode character property support (which implies UTF-8
- </P>
+ support), the escape sequences \p{..}, \P{..}, and \X are supported.
- <P>
+ The available properties that can be tested are limited to the general
-. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
+ category properties such as Lu for an upper case letter or Nd for a decimal
- are checked for validity on entry to the relevant functions. If an invalid
+ number, the Unicode script names such as Arabic or Han, and the derived
- UTF-8 string is passed, an error return is given. In some situations, you may
+ properties Any and L&. A full list is given in the
- already know that your strings are valid, and therefore want to skip these
+ <a href="pcrepattern.html"><b>pcrepattern</b></a>
- checks in order to improve performance. If you set the PCRE_NO_UTF8_CHECK flag
+ documentation. Only the short names for properties are supported. For example,
- at compile time or at run time, PCRE assumes that the pattern or subject it
+ \p{L} matches a letter. Its Perl synonym, \p{Letter}, is not supported.
- is given (respectively) contains only valid UTF-8 codes. In this case, it does
+ Furthermore, in Perl, many properties may optionally be prefixed by "Is", for
- not diagnose an invalid UTF-8 string. If you pass an invalid UTF-8 string to
+ compatibility with Perl 5.6. PCRE does not support this.
- PCRE when PCRE_NO_UTF8_CHECK is set, the results are undefined. Your program
+ <a name="utf8strings"></a></P>
- may crash.
+ <br><b>
- </P>
+ Validity of UTF-8 strings
- <P>
+ </b><br>
-. In a pattern, the escape sequence \x{...}, where the contents of the braces
+ <P>
- is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
+ When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
- code number is the given hexadecimal number, for example: \x{1234}. If a
+ are (by default) checked for validity on entry to the relevant functions. From
- non-hexadecimal digit appears between the braces, the item is not recognized.
+ release 7.3 of PCRE, the check is according the rules of RFC 3629, which are
- This escape sequence can be used either as a literal, or within a character
+ themselves derived from the Unicode specification. Earlier releases of PCRE
- class.
+ 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>
+ <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>
+ <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>
+ <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>
+ <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.
+ </P>
+ <br><b>
+ General comments about UTF-8 mode
+ </b><br>
+ <P>
+. An unbraced hexadecimal escape sequence (such as \xb3) matches a two-byte
+ UTF-8 character if the value is greater than 127.
  </P>
  <P>
-. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
+. Octal numbers up to \777 are recognized, and match two-byte UTF-8
- character if the value is greater than 127.
+ characters for values greater than \177.
  </P>
  <P>
 . Repeat quantifiers apply to complete UTF-8 characters, not to individual
  bytes, for example: \x{100}{3}.
  </P>
  <P>
 . The dot metacharacter matches one UTF-8 character instead of a single byte.
  </P>
  <P>
 . The escape sequence \C can be used to match a single byte in UTF-8 mode,
- but its use can lead to some strange effects.
+ but its use can lead to some strange effects. This facility is not available in
+ the alternative matching function, <b>pcre_dfa_exec()</b>.
  </P>
  <P>
 . The character escapes \b, \B, \d, \D, \s, \S, \w, and \W 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.
+ 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 \p{Nd}.
  </P>
  <P>
-. Case-insensitive matching applies only to characters whose values are less
+. Similarly, characters that match the POSIX named character classes are all
- than 256. PCRE does not support the notion of "case" for higher-valued
+ low-valued characters.
- characters.
  </P>
  <P>
-. PCRE does not support the use of Unicode tables and properties or the Perl
+. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
- escapes \p, \P, and \X.
+ (\h, \H, \v, and \V) do match all the appropriate Unicode characters.
+ </P>
+ <P>
+. 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.
  </P>
  <br><a name="SEC5" href="#TOC1">AUTHOR</a><br>
  <P>
- Philip Hazel &#60;ph10@cam.ac.uk&#62;
+ Philip Hazel
  <br>
- University Computing Service,
+ University Computing Service
  <br>
- Cambridge CB2 3QG, England.
+ Cambridge CB2 3QH, England.
  <br>
- Phone: +44 1223 334714
  </P>
  <P>
- Last updated: 20 August 2003
+ 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.
+ </P>
+ <br><a name="SEC6" href="#TOC1">REVISION</a><br>
+ <P>
+ Last updated: 09 August 2007
+ <br>
+ Copyright &copy; 1997-2007 University of Cambridge.
  <br>
- Copyright &copy; 1997-2003 University of Cambridge.
+ <p>
+ Return to the <a href="index.html">PCRE index page</a>.
+ </p>

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