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

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-revision 75 by nigel,
Sat Feb 24 21:40:37 2007 UTC
+revision 461 by ph10,
Mon Oct  5 10:59:35 2009 UTC
 Line 18 
 man page, in case the conversion went wr
  <li><a name="TOC3" href="#SEC3">LIMITATIONS</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">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 5.x) corresponds
+ differences. Some features that appeared in Python and PCRE before they
- approximately with Perl 5.8, including support for UTF-8 encoded strings and
+ appeared in Perl are also available using the Python syntax, there is some
- Unicode general category properties. However, this support has to be explicitly
+ support for one or two .NET and Oniguruma syntax items, and there is an option
- enabled; it is not the default.
+ for requesting some minor changes that give better JavaScript compatibility.
+ </P>
+ <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>
+ <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
+ <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. A C++ class is included in
+ written wrappers and interfaces of various kinds. In particular, Google Inc.
- these contributions, which can be found in the <i>Contrib</i> directory at the
+ have provided a comprehensive C++ wrapper. This is now included as part of the
- primary FTP site, which is:
+ 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>
-Line 41 
 supported by PCRE are given in separate
+Line 59 
 supported by PCRE are given in separate
  <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
-Line 51 
 function makes it possible for a client
+Line 71 
 function makes it possible for a client
  available. The features themselves are described in the
  <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.
+ found in the <b>README</b> and <b>NON-UNIX-USE</b> files in the source
+ distribution.
+ </P>
+ <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.
  </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
+ all the sections, except the <b>pcredemo</b> section, are concatenated, for ease
- follows:
+ 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
+   pcredemo          a demonstration C program that uses PCRE
    pcregrep          description of the <b>pcregrep</b> command
+   pcrematching      discussion of the two matching algorithms
    pcrepartial       details of the partial matching facility
    pcrepattern       syntax and semantics of supported regular expressions
    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
+   pcresample        discussion of the pcredemo program
+   pcrestack         discussion of stack usage
+   pcresyntax        quick syntax reference
    pcretest          description of the <b>pcretest</b> testing command
  </pre>
  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 91 
 internal linkage size of 3 or 4 (see the
+Line 127 
 internal linkage size of 3 or 4 (see the
  distribution and the
  <a href="pcrebuild.html"><b>pcrebuild</b></a>
  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
+ string that can be processed by certain patterns. For a discussion of stack
+ issues, see the
+ <a href="pcrestack.html"><b>pcrestack</b></a>
+ documentation.
  <a name="utf8support"></a></P>
  <br><a name="SEC4" href="#TOC1">UTF-8 AND UNICODE PROPERTY SUPPORT</a><br>
  <P>
-Line 119 
 category properties was added.
+Line 161 
 category properties was added.
  In order process UTF-8 strings, you must build PCRE to include UTF-8 support in
  the code, and, in addition, you must call
  <a href="pcre_compile.html"><b>pcre_compile()</b></a>
- with the PCRE_UTF8 option flag. When you do this, both the pattern and any
+ with the PCRE_UTF8 option flag, or the pattern must start with the sequence
- subject strings that are matched against it are treated as UTF-8 strings
+ (*UTF8). When either of these is the case, both the pattern and any subject
- instead of just strings of bytes.
+ strings that are matched against it are treated as UTF-8 strings instead of
+ strings of 1-byte characters.
  </P>
  <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>
  If PCRE is built with Unicode character property support (which implies UTF-8
  support), the escape sequences \p{..}, \P{..}, and \X 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. A full list is given in the
+ number, the Unicode script names such as Arabic or Han, and the derived
+ properties Any and L&. A full list is given in the
  <a href="pcrepattern.html"><b>pcrepattern</b></a>
- documentation. The PCRE library is increased in size by about 90K when Unicode
+ documentation. Only the short names for properties are supported. For example,
- property support is included.
+ \p{L} matches a letter. Its Perl synonym, \p{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.
+ <a name="utf8strings"></a></P>
+ <br><b>
+ Validity of UTF-8 strings
+ </b><br>
+ <P>
+ 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>
+ <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 following comments apply when PCRE is running in UTF-8 mode:
+. Octal numbers up to \777 are recognized, and match two-byte UTF-8
+ characters for values greater than \177.
  </P>
  <P>
-. When you set the PCRE_UTF8 flag, the strings passed as patterns and subjects
+. Repeat quantifiers apply to complete UTF-8 characters, not to individual
- 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>
- <P>
-. In a pattern, the escape sequence \x{...}, where the contents of the braces
- is a string of hexadecimal digits, is interpreted as a UTF-8 character whose
- code number is the given hexadecimal number, for example: \x{1234}. If a
- non-hexadecimal digit appears between the braces, the item is not recognized.
- This escape sequence can be used either as a literal, or within a character
- class.
- </P>
- <P>
-. The original hexadecimal escape sequence, \xhh, matches a two-byte UTF-8
- character if the value is greater than 127.
- </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. 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}.
+ must use Unicode property tests such as \p{Nd}. Note that this also applies to
+ \b, because it is defined in terms of \w and \W.
  </P>
  <P>
 . Similarly, characters that match the POSIX named character classes are all
  low-valued characters.
  </P>
  <P>
+. However, the Perl 5.10 horizontal and vertical whitespace matching escapes
+ (\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.
+ 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
  <br>
- University Computing Service,
+ Cambridge CB2 3QH, England.
  <br>
- Cambridge CB2 3QG, England.
+ </P>
+ <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.
+ </P>
+ <br><a name="SEC6" href="#TOC1">REVISION</a><br>
+ <P>
+ Last updated: 28 September 2009
  <br>
- Phone: +44 1223 334714
+ Copyright &copy; 1997-2009 University of Cambridge.
- Last updated: 09 September 2004
  <br>
- Copyright &copy; 1997-2004 University of Cambridge.
  <p>
  Return to the <a href="index.html">PCRE index page</a>.
  </p>

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