d4de0e6f1e
I was über lazy at first, so took libs from SM. But actually it's quite easy to compile, so let's update to latest version \o/.
236 lines
9.6 KiB
HTML
236 lines
9.6 KiB
HTML
<html>
|
|
<head>
|
|
<title>pcrecompat specification</title>
|
|
</head>
|
|
<body bgcolor="#FFFFFF" text="#00005A" link="#0066FF" alink="#3399FF" vlink="#2222BB">
|
|
<h1>pcrecompat man page</h1>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE index page</a>.
|
|
</p>
|
|
<p>
|
|
This page is part of the PCRE HTML documentation. It was generated automatically
|
|
from the original man page. If there is any nonsense in it, please consult the
|
|
man page, in case the conversion went wrong.
|
|
<br>
|
|
<br><b>
|
|
DIFFERENCES BETWEEN PCRE AND PERL
|
|
</b><br>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
1. PCRE has only a subset of Perl's Unicode support. Details of what it does
|
|
have are given in the
|
|
<a href="pcreunicode.html"><b>pcreunicode</b></a>
|
|
page.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<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 \0 can be used in the pattern to
|
|
represent a binary zero.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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."
|
|
</P>
|
|
<P>
|
|
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:
|
|
<pre>
|
|
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
|
|
</pre>
|
|
The \Q...\E sequence is recognized both inside and outside character classes.
|
|
</P>
|
|
<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 it is in Perl 5.10. Also, the PCRE "callout"
|
|
feature allows an external function to be called during pattern matching. See
|
|
the
|
|
<a href="pcrecallout.html"><b>pcrecallout</b></a>
|
|
documentation for details.
|
|
</P>
|
|
<P>
|
|
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
|
|
<a href="pcrepattern.html#recursiondifference">section on recursion differences from Perl</a>
|
|
in the
|
|
<a href="pcrepattern.html"><b>pcrepattern</b></a>
|
|
page.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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 differs.
|
|
</P>
|
|
<P>
|
|
12. Most backtracking verbs in assertions have their normal actions. They are
|
|
not confined to the assertion.
|
|
</P>
|
|
<P>
|
|
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".
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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.
|
|
</P>
|
|
<P>
|
|
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:
|
|
<br>
|
|
<br>
|
|
(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.
|
|
<br>
|
|
<br>
|
|
(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.
|
|
<br>
|
|
<br>
|
|
(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.)
|
|
<br>
|
|
<br>
|
|
(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.
|
|
<br>
|
|
<br>
|
|
(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.
|
|
<br>
|
|
<br>
|
|
(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, PCRE_NOTEMPTY_ATSTART, and
|
|
PCRE_NO_AUTO_CAPTURE options for <b>pcre_exec()</b> have no Perl equivalents.
|
|
<br>
|
|
<br>
|
|
(g) The \R escape sequence can be restricted to match only CR, LF, or CRLF
|
|
by the PCRE_BSR_ANYCRLF option.
|
|
<br>
|
|
<br>
|
|
(h) The callout facility is PCRE-specific.
|
|
<br>
|
|
<br>
|
|
(i) The partial matching facility is PCRE-specific.
|
|
<br>
|
|
<br>
|
|
(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.
|
|
<br>
|
|
<br>
|
|
(k) The alternative matching functions (<b>pcre_dfa_exec()</b>,
|
|
<b>pcre16_dfa_exec()</b> and <b>pcre32_dfa_exec()</b>,) match in a different way
|
|
and are not Perl-compatible.
|
|
<br>
|
|
<br>
|
|
(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.
|
|
</P>
|
|
<br><b>
|
|
AUTHOR
|
|
</b><br>
|
|
<P>
|
|
Philip Hazel
|
|
<br>
|
|
University Computing Service
|
|
<br>
|
|
Cambridge CB2 3QH, England.
|
|
<br>
|
|
</P>
|
|
<br><b>
|
|
REVISION
|
|
</b><br>
|
|
<P>
|
|
Last updated: 10 November 2013
|
|
<br>
|
|
Copyright © 1997-2013 University of Cambridge.
|
|
<br>
|
|
<p>
|
|
Return to the <a href="index.html">PCRE index page</a>.
|
|
</p>
|