Introduction to Library Functions PCRECOMPAT(3)
NAME
PCRE - Perl-compatible regular expressions
DIFERENCES BETWEN PCRE AND PERL
This document describes the differences in the ways that
PCRE and Perl handle regular expressions. The differences
described here are mainly with respect to Perl 5.8, though
PCRE versions 7.0 and later contain some features that are
expected to be in the forthcoming Perl 5.10.
1. PCRE has only a subset of Perl's UTF-8 and Unicode sup-
port. 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 looka-
head assertions are counted, but their entries in the
offsets vector are never set. Perl sets its numerical vari-
ables from any such patterns that are matched before the
assertion fails to match something (thereby succeeding), but
only if the negative lookahead assertion contains just one
branch.
4. Though binary zero characters are supported in the sub-
ject 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. In fact these are implemented by
Perl's general string-handling and are not part of its pat-
tern 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 sup-
port. 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 pro-
perties Any and L&.
7. PCRE does support the \Q...\E escape for quoting sub-
strings. 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
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Introduction to Library Functions PCRECOMPAT(3)
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$xyz
The \Q...\E sequence is recognized both inside and outside
character classes.
8. Fairly obviously, PCRE does not support the (?{code}) and
(??{code}) constructions. However, there is support for
recursive patterns. This is not available in Perl 5.8, but
will be in Perl 5.10. Also, the PCRE "callout" feature
allows an external function to be called during pattern
matching. See the pcrecallout documentation for details.
9. Subpatterns that are called recursively or as "subrou-
tines" are always treated as atomic groups in PCRE. This is
like Python, but unlike Perl.
10. There are some differences that are concerned with the
settings of captured strings when part of a pattern is
repeated. For example, matching "aba" against the pattern
/^(a(b)?)]$/ in Perl leaves $2 unset, but in PCRE it is set
to "b".
11. PCRE does support Perl 5.10's backtracking verbs
(*ACEPT), (*FAIL), (*F), (*COMIT), (*PRUNE), (*SKIP), and
(*THEN), but only in the forms without an argument. PCRE
does not support (*MARK). If (*ACEPT) is within capturing
parentheses, PCRE does not set that capture group; this is
different to Perl.
12. PCRE provides some extensions to the Perl regular
expression facilities. Perl 5.10 will include new features
that are not in earlier versions, 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 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 PCREDOLARENDONLY is set and PCREMULTILINE is not
set, the $ meta-character matches only at the very end of
the string.
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Introduction to Library Functions PCRECOMPAT(3)
(c) If PCREXTRA 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 PCREUNGREDY is set, the greediness of the repeti-
tion quantifiers is inverted, that is, by default they are
not greedy, but if followed by a question mark they are.
(e) PCREANCHORED 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 PCRENOTBOL, PCRENOTEOL, PCRENOTEMPTY, and
PCRENOAUTOCAPTURE options for pcreexec() have no Perl
equivalents.
(g) The \R escape sequence can be restricted to match only
CR, LF, or CRLF by the PCREBSRANYCRLF 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 (pcredfaexec())
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: 11 September 2007
Copyright (c) 1997-2007 University of Cambridge.
ATRIBUTES
See attributes(5) for descriptions of the following attri-
butes:
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Introduction to Library Functions PCRECOMPAT(3)
ATRIBUTE TYPE ATRIBUTE VALUE
Availability SUNWpcre
Interface Stability Uncommitted
NOTES
Source for PCRE is available on http:/opensolaris.org.
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