forked from Mirrors/freeswitch
9da5d7e90f
git-svn-id: http://svn.freeswitch.org/svn/freeswitch/trunk@3732 d0543943-73ff-0310-b7d9-9358b9ac24b2
127 lines
5.1 KiB
Groff
127 lines
5.1 KiB
Groff
.TH PCRECOMPAT 3
|
|
.SH NAME
|
|
PCRE - Perl-compatible regular expressions
|
|
.SH "DIFFERENCES BETWEEN PCRE AND PERL"
|
|
.rs
|
|
.sp
|
|
This document describes the differences in the ways that PCRE and Perl handle
|
|
regular expressions. The differences described here are with respect to Perl
|
|
5.8.
|
|
.P
|
|
1. PCRE has only a subset of Perl's UTF-8 and Unicode support. Details of what
|
|
it does have are given in the
|
|
.\" HTML <a href="pcre.html#utf8support">
|
|
.\" </a>
|
|
section on UTF-8 support
|
|
.\"
|
|
in the main
|
|
.\" HREF
|
|
\fBpcre\fP
|
|
.\"
|
|
page.
|
|
.P
|
|
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.
|
|
.P
|
|
3. Capturing subpatterns that occur inside negative lookahead assertions 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 succeeding), but only if the
|
|
negative lookahead assertion contains just one branch.
|
|
.P
|
|
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 \e0 can be used in the pattern to
|
|
represent a binary zero.
|
|
.P
|
|
5. The following Perl escape sequences are not supported: \el, \eu, \eL,
|
|
\eU, and \eN. 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.
|
|
.P
|
|
6. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE is
|
|
built with Unicode character property support. The properties that can be
|
|
tested with \ep and \eP 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&.
|
|
.P
|
|
7. PCRE does support the \eQ...\eE 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:
|
|
.sp
|
|
Pattern PCRE matches Perl matches
|
|
.sp
|
|
.\" JOIN
|
|
\eQabc$xyz\eE abc$xyz abc followed by the
|
|
contents of $xyz
|
|
\eQabc\e$xyz\eE abc\e$xyz abc\e$xyz
|
|
\eQabc\eE\e$\eQxyz\eE abc$xyz abc$xyz
|
|
.sp
|
|
The \eQ...\eE sequence is recognized both inside and outside character classes.
|
|
.P
|
|
8. Fairly obviously, PCRE does not support the (?{code}) and (?p{code})
|
|
constructions. However, there is support for recursive patterns using the
|
|
non-Perl items (?R), (?number), and (?P>name). Also, the PCRE "callout" feature
|
|
allows an external function to be called during pattern matching. See the
|
|
.\" HREF
|
|
\fBpcrecallout\fP
|
|
.\"
|
|
documentation for details.
|
|
.P
|
|
9. There are some differences that are concerned with the settings of captured
|
|
strings when part of a pattern is repeated. For example, matching "aba" against
|
|
the pattern /^(a(b)?)+$/ in Perl leaves $2 unset, but in PCRE it is set to "b".
|
|
.P
|
|
10. PCRE provides some extensions to the Perl regular expression facilities:
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(c) If PCRE_EXTRA is set, a backslash followed by a letter with no special
|
|
meaning is faulted. Otherwise, like Perl, the backslash is ignored. (Perl can
|
|
be made to issue a warning.)
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(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.
|
|
.sp
|
|
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE
|
|
options for \fBpcre_exec()\fP have no Perl equivalents.
|
|
.sp
|
|
(g) The (?R), (?number), and (?P>name) constructs allows for recursive pattern
|
|
matching (Perl can do this using the (?p{code}) construct, which PCRE cannot
|
|
support.)
|
|
.sp
|
|
(h) PCRE supports named capturing substrings, using the Python syntax.
|
|
.sp
|
|
(i) PCRE supports the possessive quantifier "++" syntax, taken from Sun's Java
|
|
package.
|
|
.sp
|
|
(j) The (R) condition, for testing recursion, is a PCRE extension.
|
|
.sp
|
|
(k) The callout facility is PCRE-specific.
|
|
.sp
|
|
(l) The partial matching facility is PCRE-specific.
|
|
.sp
|
|
(m) Patterns compiled by PCRE can be saved and re-used at a later time, even on
|
|
different hosts that have the other endianness.
|
|
.sp
|
|
(n) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a
|
|
different way and is not Perl-compatible.
|
|
.P
|
|
.in 0
|
|
Last updated: 06 June 2006
|
|
.br
|
|
Copyright (c) 1997-2006 University of Cambridge.
|