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PCRECOMPAT(3)							 PCRECOMPAT(3)

NAME
       PCRE - Perl-compatible regular expressions

DIFFERENCES BETWEEN PCRE AND PERL

       This  document describes the differences in the ways that PCRE and Perl
       handle regular expressions. The differences  described  here  are  with
       respect to Perl versions 5.10 and above.

       1.  PCRE has only a subset of Perl's UTF-8 and Unicode support. Details
       of what it does have are given in the section on UTF-8 support  in  the
       main pcre page.

       2. PCRE does not allow repeat quantifiers on lookahead assertions. Perl
       permits them, but they do not mean what you might think.	 For  example,
       (?!a){3} does not assert that the next three characters are not "a". It
       just asserts that the next character is not "a" three times.

       3. Capturing subpatterns that occur inside  negative  lookahead	asser‐
       tions  are  counted,  but their entries in the offsets vector are never
       set. Perl sets its numerical variables from any such patterns that  are
       matched before the assertion fails to match something (thereby succeed‐
       ing), but only if the negative lookahead assertion  contains  just  one
       branch.

       4.  Though  binary zero characters are supported in the subject string,
       they are not allowed in a pattern string because it is passed as a nor‐
       mal C string, terminated by zero. The escape sequence \0 can be used in
       the pattern to represent a binary zero.

       5. The following Perl escape sequences are not supported: \l,  \u,  \L,
       \U, and \N. In fact these are implemented by Perl's general string-han‐
       dling and are not part of its pattern matching engine. If any of	 these
       are encountered by PCRE, an error is generated.

       6.  The Perl escape sequences \p, \P, and \X are supported only if PCRE
       is built with Unicode character property support. The  properties  that
       can  be tested with \p and \P are limited to the general category prop‐
       erties such as Lu and Nd, script names such as Greek or	Han,  and  the
       derived	properties  Any	 and  L&. PCRE does support the Cs (surrogate)
       property, which Perl does not; the  Perl	 documentation	says  "Because
       Perl hides the need for the user to understand the internal representa‐
       tion of Unicode characters, there is no need to implement the  somewhat
       messy concept of surrogates."

       7. PCRE does support the \Q...\E escape for quoting substrings. Charac‐
       ters in between are treated as literals.	 This  is  slightly  different
       from  Perl  in  that  $	and  @ are also handled as literals inside the
       quotes. In Perl, they cause variable interpolation (but of course  PCRE
       does not have variables). Note the following examples:

	   Pattern	      PCRE matches	Perl matches

	   \Qabc$xyz\E	      abc$xyz		abc followed by the
						  contents of $xyz
	   \Qabc\$xyz\E	      abc\$xyz		abc\$xyz
	   \Qabc\E\$\Qxyz\E   abc$xyz		abc$xyz

       The  \Q...\E  sequence  is recognized both inside and outside character
       classes.

       8. Fairly obviously, PCRE does not support the (?{code}) and (??{code})
       constructions.  However,	 there is support for recursive patterns. This
       is not available in Perl 5.8, but it is in Perl 5.10.  Also,  the  PCRE
       "callout"  feature allows an external function to be called during pat‐
       tern matching. See the pcrecallout documentation for details.

       9. Subpatterns that are called  recursively  or	as  "subroutines"  are
       always  treated	as  atomic  groups  in	PCRE. This is like Python, but
       unlike Perl. There is a discussion of an example that explains this  in
       more  detail  in	 the section on recursion differences from Perl in the
       pcrepattern page.

       10. There are some differences that are concerned with the settings  of
       captured	 strings  when	part  of  a  pattern is repeated. For example,
       matching "aba" against the  pattern  /^(a(b)?)+$/  in  Perl  leaves  $2
       unset, but in PCRE it is set to "b".

       11.  PCRE's handling of duplicate subpattern numbers and duplicate sub‐
       pattern names is not as general as Perl's. This is a consequence of the
       fact the PCRE works internally just with numbers, using an external ta‐
       ble to translate between numbers and names. In  particular,  a  pattern
       such  as	 (?|(?<a>A)|(?<b)B),  where the two capturing parentheses have
       the same number but different names, is not supported,  and  causes  an
       error  at compile time. If it were allowed, it would not be possible to
       distinguish which parentheses matched, because both names map  to  cap‐
       turing subpattern number 1. To avoid this confusing situation, an error
       is given at compile time.

       12. Perl recognizes comments in some  places  that  PCRE	 doesn't,  for
       example, between the ( and ? at the start of a subpattern.

       13. PCRE provides some extensions to the Perl regular expression facil‐
       ities.  Perl 5.10 includes new features that are not  in	 earlier  ver‐
       sions  of  Perl, some of which (such as named parentheses) have been in
       PCRE for some time. This list is with respect to Perl 5.10:

       (a) Although lookbehind assertions in  PCRE  must  match	 fixed	length
       strings,	 each alternative branch of a lookbehind assertion can match a
       different length of string. Perl requires them all  to  have  the  same
       length.

       (b)  If PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not set, the $
       meta-character matches only at the very end of the string.

       (c) If PCRE_EXTRA is set, a backslash followed by a letter with no spe‐
       cial meaning is faulted. Otherwise, like Perl, the backslash is quietly
       ignored.	 (Perl can be made to issue a warning.)

       (d) If PCRE_UNGREEDY is set, the greediness of the  repetition  quanti‐
       fiers is inverted, that is, by default they are not greedy, but if fol‐
       lowed by a question mark they are.

       (e) PCRE_ANCHORED can be used at matching time to force a pattern to be
       tried only at the first matching position in the subject string.

       (f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART,
       and PCRE_NO_AUTO_CAPTURE options for pcre_exec() have no	 Perl  equiva‐
       lents.

       (g)  The	 \R escape sequence can be restricted to match only CR, LF, or
       CRLF by the PCRE_BSR_ANYCRLF option.

       (h) The callout facility is PCRE-specific.

       (i) The partial matching facility is PCRE-specific.

       (j) Patterns compiled by PCRE can be saved and re-used at a later time,
       even on different hosts that have the other endianness.

       (k)  The	 alternative  matching function (pcre_dfa_exec()) matches in a
       different way and is not Perl-compatible.

       (l) PCRE recognizes some special sequences such as (*CR) at  the	 start
       of a pattern that set overall options that cannot be changed within the
       pattern.

AUTHOR

       Philip Hazel
       University Computing Service
       Cambridge CB2 3QH, England.

REVISION

       Last updated: 31 October 2010
       Copyright (c) 1997-2010 University of Cambridge.

								 PCRECOMPAT(3)
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