.TH PCRECOMPAT 3

.SH NAME

PCRE - Perl-compatible regular expressions

.SH "DIFFERENCES BETWEEN PCRE AND PERL"

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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.

.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:

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    Pattern            PCRE matches      Perl matches

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.\" 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

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The \eQ...\eE sequence is recognized both inside and outside character classes.

.P

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

.\" HREF

\fBpcrecallout\fP

.\"

documentation for details.

.P

9. Subpatterns that are called recursively or as "subroutines" are always

treated as atomic groups in PCRE. This is like Python, but unlike Perl.

.P

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".

.P

11. PCRE does support Perl 5.10's backtracking verbs (*ACCEPT), (*FAIL), (*F),

(*COMMIT), (*PRUNE), (*SKIP), and (*THEN), but only in the forms without an

argument. PCRE does not support (*MARK). If (*ACCEPT) is within capturing

parentheses, PCRE does not set that capture group; this is different to Perl.

.P

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:

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(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.

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(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.

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(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.)

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(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.

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(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.

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(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and PCRE_NO_AUTO_CAPTURE

options for \fBpcre_exec()\fP have no Perl equivalents.

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(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF

by the PCRE_BSR_ANYCRLF option.

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(h) The callout facility is PCRE-specific.

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

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(j) Patterns compiled by PCRE can be saved and re-used at a later time, even on

different hosts that have the other endianness.

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(k) The alternative matching function (\fBpcre_dfa_exec()\fP) matches in a

different way and is not Perl-compatible.

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(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.

.

.

.SH AUTHOR

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Philip Hazel

University Computing Service

Cambridge CB2 3QH, England.

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.

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.SH REVISION

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Last updated: 11 September 2007

Copyright (c) 1997-2007 University of Cambridge.

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