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Documentation update.

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Philip.Hazel 2017-03-29 18:10:55 +00:00
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@ -1,4 +1,4 @@
.TH PCRE2COMPAT 3 "18 October 2016" "PCRE2 10.23" .TH PCRE2COMPAT 3 "29 March 2017" "PCRE2 10.30"
.SH NAME .SH NAME
PCRE2 - Perl-compatible regular expressions (revised API) PCRE2 - Perl-compatible regular expressions (revised API)
.SH "DIFFERENCES BETWEEN PCRE2 AND PERL" .SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
@ -6,7 +6,8 @@ PCRE2 - Perl-compatible regular expressions (revised API)
.sp .sp
This document describes the differences in the ways that PCRE2 and Perl handle This document describes the differences in the ways that PCRE2 and Perl handle
regular expressions. The differences described here are with respect to Perl regular expressions. The differences described here are with respect to Perl
versions 5.10 and above. versions 5.24, but as both Perl and PCRE2 are continually changing, the
information may sometimes be out of date.
.P .P
1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does 1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
have are given in the have are given in the
@ -15,16 +16,17 @@ have are given in the
.\" .\"
page. page.
.P .P
2. PCRE2 allows repeat quantifiers only on parenthesized assertions, but they 2. Like Perl, PCRE2 allows repeat quantifiers on parenthesized assertions, but
do not mean what you might think. For example, (?!a){3} does not assert that they do not mean what you might think. For example, (?!a){3} does not assert
the next three characters are not "a". It just asserts that the next character that the next three characters are not "a". It just asserts that the next
is not "a" three times (in principle: PCRE2 optimizes this to run the assertion character is not "a" three times (in principle: PCRE2 optimizes this to run the
just once). Perl allows repeat quantifiers on other assertions such as \eb, but assertion just once). Perl allows some repeat quantifiers on other assertions,
these do not seem to have any use. for example, \eb* (but not \eb{3}), but these do not seem to have any use.
.P .P
3. Capturing subpatterns that occur inside negative lookahead assertions are 3. Capturing subpatterns that occur inside negative lookaround assertions are
counted, but their entries in the offsets vector are never set. Perl sometimes counted, but their entries in the offsets vector are set only if the assertion
(but not always) sets its numerical variables from inside negative assertions. is a condition. Perl has changed its behaviour in this regard from time to
time.
.P .P
4. The following Perl escape sequences are not supported: \el, \eu, \eL, 4. The following Perl escape sequences are not supported: \el, \eu, \eL,
\eU, and \eN when followed by a character name or Unicode value. (\eN on its \eU, and \eN when followed by a character name or Unicode value. (\eN on its
@ -35,13 +37,13 @@ generated by default. However, if the PCRE2_ALT_BSUX option is set,
\eU and \eu are interpreted as ECMAScript interprets them. \eU and \eu are interpreted as ECMAScript interprets them.
.P .P
5. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is 5. The Perl escape sequences \ep, \eP, and \eX are supported only if PCRE2 is
built with Unicode support. The properties that can be tested with \ep and \eP built with Unicode support (the default). The properties that can be tested
are limited to the general category properties such as Lu and Nd, script names with \ep and \eP are limited to the general category properties such as Lu and
such as Greek or Han, and the derived properties Any and L&. PCRE2 does support Nd, script names such as Greek or Han, and the derived properties Any and L&.
the Cs (surrogate) property, which Perl does not; the Perl documentation says PCRE2 does support the Cs (surrogate) property, which Perl does not; the Perl
"Because Perl hides the need for the user to understand the internal documentation says "Because Perl hides the need for the user to understand the
representation of Unicode characters, there is no need to implement the internal representation of Unicode characters, there is no need to implement
somewhat messy concept of surrogates." the somewhat messy concept of surrogates."
.P .P
6. PCRE2 does support the \eQ...\eE escape for quoting substrings. Characters 6. PCRE2 does support the \eQ...\eE escape for quoting substrings. Characters
in between are treated as literals. This is slightly different from Perl in in between are treated as literals. This is slightly different from Perl in
@ -60,29 +62,16 @@ Note the following examples:
The \eQ...\eE sequence is recognized both inside and outside character classes. The \eQ...\eE sequence is recognized both inside and outside character classes.
.P .P
7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code}) 7. Fairly obviously, PCRE2 does not support the (?{code}) and (??{code})
constructions. However, there is support for recursive patterns. This is not constructions. However, there is support PCRE2's "callout" feature, which
available in Perl 5.8, but it is in Perl 5.10. Also, the PCRE2 "callout" allows an external function to be called during pattern matching. See the
feature allows an external function to be called during pattern matching. See
the
.\" HREF .\" HREF
\fBpcre2callout\fP \fBpcre2callout\fP
.\" .\"
documentation for details. documentation for details.
.P .P
8. Subroutine calls (whether recursive or not) are treated as atomic groups. 8. Subroutine calls (whether recursive or not) were treated as atomic groups up
Atomic recursion is like Python, but unlike Perl. Captured values that are set to PCRE2 release 10.23, but from release 10.30 this changed, and backtracking
outside a subroutine call can be referenced from inside in PCRE2, but not in into subroutine calls is now supported, as in Perl.
Perl. There is a discussion that explains these differences in more detail in
the
.\" HTML <a href="pcre2pattern.html#recursiondifference">
.\" </a>
section on recursion differences from Perl
.\"
in the
.\" HREF
\fBpcre2pattern\fP
.\"
page.
.P .P
9. If any of the backtracking control verbs are used in a subpattern that is 9. 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 called as a subroutine (whether or not recursively), their effect is confined
@ -130,13 +119,13 @@ certainly user mistakes.
16. In PCRE2, the upper/lower case character properties Lu and Ll are not 16. In PCRE2, the upper/lower case character properties Lu and Ll are not
affected when case-independent matching is specified. For example, \ep{Lu} affected when case-independent matching is specified. For example, \ep{Lu}
always matches an upper case letter. I think Perl has changed in this respect; always matches an upper case letter. I think Perl has changed in this respect;
in the release at the time of writing (5.16), \ep{Lu} and \ep{Ll} match all in the release at the time of writing (5.24), \ep{Lu} and \ep{Ll} match all
letters, regardless of case, when case independence is specified. letters, regardless of case, when case independence is specified.
.P .P
17. PCRE2 provides some extensions to the Perl regular expression facilities. 17. PCRE2 provides some extensions to the Perl regular expression facilities.
Perl 5.10 includes new features that are not in earlier versions of Perl, some Perl 5.10 includes new features that are not in earlier versions of Perl, some
of which (such as named parentheses) have been in PCRE2 for some time. This of which (such as named parentheses) were in PCRE2 for some time before. This
list is with respect to Perl 5.10: list is with respect to Perl 5.24:
.sp .sp
(a) Although lookbehind assertions in PCRE2 must match fixed length strings, (a) Although lookbehind assertions in PCRE2 must match fixed length strings,
each alternative branch of a lookbehind assertion can match a different length each alternative branch of a lookbehind assertion can match a different length
@ -190,6 +179,6 @@ Cambridge, England.
.rs .rs
.sp .sp
.nf .nf
Last updated: 18 October 2016 Last updated: 29 March 2017
Copyright (c) 1997-2016 University of Cambridge. Copyright (c) 1997-2017 University of Cambridge.
.fi .fi