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Introduction to Library Functions PCRECOMPAT(3)

NAME

PCRE - Perl-compatible regular expressions

DIFFERENCES BETWEEN 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 SunOS 5.10 Last change: 1 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 (*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. 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 PCRE_DOLLAR_ENDONLY is set and PCRE_MULTILINE is not

set, the $ meta-character matches only at the very end of

the string. SunOS 5.10 Last change: 2 Introduction to Library Functions PCRECOMPAT(3)

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 repeti-

tion 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.

(f) The PCRE_NOTBOL, PCRE_NOTEOL, PCRE_NOTEMPTY, and

PCRE_NO_AUTO_CAPTURE options for 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.

(k) The alternative matching function (pcre_dfa_exec())

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

ATTRIBUTES

See attributes(5) for descriptions of the following attri-

butes: SunOS 5.10 Last change: 3 Introduction to Library Functions PCRECOMPAT(3)

_______________________________________

| ATTRIBUTE TYPE | ATTRIBUTE VALUE|

|____________________|__________________|_

| Availability | library/pcre |

|____________________|__________________|_

| Interface Stability| Uncommitted |

|____________________|_________________|

NOTES Source for PCRE is available on http://opensolaris.org. SunOS 5.10 Last change: 4