pcrecompat(3) — Linux manual page


PCRECOMPAT(3)             Library Functions Manual             PCRECOMPAT(3)

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       PCRE - Perl-compatible regular expressions


       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 Unicode support. Details of what
       it does have are given in the pcreunicode page.

       2. PCRE allows repeat quantifiers only on parenthesized assertions,
       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 (in principle:
       PCRE optimizes this to run the assertion just once). Perl allows
       repeat quantifiers on other assertions such as \b, but these do not
       seem to have any use.

       3. Capturing subpatterns that occur inside negative lookahead
       assertions are counted, but their entries in the offsets vector are
       never set. Perl sometimes (but not always) sets its numerical
       variables from inside negative assertions.

       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
       normal 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 when followed by a character name or Unicode value. (\N on
       its own, matching a non-newline character, is supported.) In fact
       these are implemented by Perl's general string-handling and are not
       part of its pattern matching engine. If any of these are encountered
       by PCRE, an error is generated by default. However, if the
       PCRE_JAVASCRIPT_COMPAT option is set, \U and \u are interpreted as
       JavaScript interprets them.

       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
       properties 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 representation 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.
       Characters 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

       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 pattern matching. See the pcrecallout documentation for

       9. Subpatterns that are called as subroutines (whether or not
       recursively) are always treated as atomic groups in PCRE. This is
       like Python, but unlike Perl.  Captured values that are set outside a
       subroutine call can be reference from inside in PCRE, but not in
       Perl. There is a discussion that explains these differences in more
       detail in the section on recursion differences from Perl in the
       pcrepattern page.

       10. If any of the backtracking control verbs are used in a subpattern
       that is called as a subroutine (whether or not recursively), their
       effect is confined to that subpattern; it does not extend to the
       surrounding pattern. This is not always the case in Perl. In
       particular, if (*THEN) is present in a group that is called as a
       subroutine, its action is limited to that group, even if the group
       does not contain any | characters. Note that such subpatterns are
       processed as anchored at the point where they are tested.

       11. If a pattern contains more than one backtracking control verb,
       the first one that is backtracked onto acts. For example, in the
       pattern A(*COMMIT)B(*PRUNE)C a failure in B triggers (*COMMIT), but a
       failure in C triggers (*PRUNE). Perl's behaviour is more complex; in
       many cases it is the same as PCRE, but there are examples where it

       12. Most backtracking verbs in assertions have their normal actions.
       They are not confined to the assertion.

       13. 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".

       14. PCRE's handling of duplicate subpattern numbers and duplicate
       subpattern 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 table 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 capturing subpattern number 1. To avoid
       this confusing situation, an error is given at compile time.

       15. Perl recognizes comments in some places that PCRE does not, for
       example, between the ( and ? at the start of a subpattern. If the /x
       modifier is set, Perl allows white space between ( and ? (though
       current Perls warn that this is deprecated) but PCRE never does, even
       if the PCRE_EXTENDED option is set.

       16. Perl, when in warning mode, gives warnings for character classes
       such as [A-\d] or [a-[:digit:]]. It then treats the hyphens as
       literals. PCRE has no warning features, so it gives an error in these
       cases because they are almost certainly user mistakes.

       17. In PCRE, the upper/lower case character properties Lu and Ll are
       not affected when case-independent matching is specified. For
       example, \p{Lu} always matches an upper case letter. I think Perl has
       changed in this respect; in the release at the time of writing
       (5.16), \p{Lu} and \p{Ll} match all letters, regardless of case, when
       case independence is specified.

       18. PCRE provides some extensions to the Perl regular expression
       facilities.  Perl 5.10 includes new features that are not in earlier
       versions 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

       (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
       special 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
       quantifiers is inverted, that is, by default they are not greedy, but
       if followed 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.

       pcre_exec() have no Perl equivalents.

       (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.
       However, this does not apply to optimized data created by the just-
       in-time compiler.

       (k) The alternative matching functions (pcre_dfa_exec(),
       pcre16_dfa_exec() and pcre32_dfa_exec(),) match in a different way
       and are 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         top

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

REVISION         top

       Last updated: 10 November 2013
       Copyright (c) 1997-2013 University of Cambridge.

COLOPHON         top

       This page is part of the PCRE (Perl Compatible Regular Expressions)
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PCRE 8.34                     10 November 2013                 PCRECOMPAT(3)