dist2/doc/pcre2compat.3 - nest-cam/4320010/pcre2 - Git at Google

 .TH PCRE2COMPAT 3 "15 March 2015" "PCRE2 10.20"
 .SH NAME
 PCRE2 - Perl-compatible regular expressions (revised API)
 .SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
 .rs
 .sp
 This document describes the differences in the ways that PCRE2 and Perl handle
 regular expressions. The differences described here are with respect to Perl
 versions 5.10 and above.
 .P
 1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
 have are given in the
 .\" HREF
 \fBpcre2unicode\fP
 .\"
 page.
 .P
 2. PCRE2 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: PCRE2 optimizes this to run the assertion
 just once). Perl allows repeat quantifiers on other assertions such as \eb, but
 these do not seem to have any use.
 .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
 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
 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 PCRE2, an error is
 generated by default. However, if the PCRE2_ALT_BSUX option is set,
 \eU and \eu are interpreted as ECMAScript interprets them.
 .P
 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
 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&. PCRE2 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
 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
 that $ and @ are also handled as literals inside the quotes. In Perl, they
 cause variable interpolation (but of course PCRE2 does not have variables).
 Note the following examples:
 .sp
     Pattern            PCRE2 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
 7. Fairly obviously, PCRE2 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 PCRE2 "callout"
 feature allows an external function to be called during pattern matching. See
 the
 .\" HREF
 \fBpcre2callout\fP
 .\"
 documentation for details.
 .P
 8. Subroutine calls (whether recursive or not) are treated as atomic groups.
 Atomic recursion is like Python, but unlike Perl. Captured values that are set
 outside a subroutine call can be referenced from inside in PCRE2, but not in
 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
 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
 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
 10. 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 PCRE2, but there are examples where it differs.
 .P
 11. Most backtracking verbs in assertions have their normal actions. They are
 not confined to the assertion.
 .P
 12. 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 PCRE2 it is set to
 "b".
 .P
 13. PCRE2'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 PCRE2
 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
 14. Perl recognizes comments in some places that PCRE2 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 PCRE2 never does, even if the PCRE2_EXTENDED option is set.
 .P
 15. Perl, when in warning mode, gives warnings for character classes such as
 [A-\ed] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE2 has no
 warning features, so it gives an error in these cases because they are almost
 certainly user mistakes.
 .P
 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}
 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
 letters, regardless of case, when case independence is specified.
 .P
 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
 of which (such as named parentheses) have been in PCRE2 for some time. This
 list is with respect to Perl 5.10:
 .sp
 (a) Although lookbehind assertions in PCRE2 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 PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $
 meta-character matches only at the very end of the string.
 .sp
 (c) A backslash followed by a letter with no special meaning is faulted. (Perl
 can be made to issue a warning.)
 .sp
 (d) If PCRE2_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) PCRE2_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 PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, and
 PCRE2_NO_AUTO_CAPTURE options have no Perl equivalents.
 .sp
 (g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
 by the PCRE2_BSR_ANYCRLF option.
 .sp
 (h) The callout facility is PCRE2-specific.
 .sp
 (i) The partial matching facility is PCRE2-specific.
 .sp
 (j) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a
 different way and is not Perl-compatible.
 .sp
 (k) PCRE2 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
 .rs
 .sp
 .nf
 Philip Hazel
 University Computing Service
 Cambridge, England.
 .fi
 .
 .
 .SH REVISION
 .rs
 .sp
 .nf
 Last updated: 15 March 2015
 Copyright (c) 1997-2015 University of Cambridge.
 .fi
	.TH PCRE2COMPAT 3 "15 March 2015" "PCRE2 10.20"
	.SH NAME
	PCRE2 - Perl-compatible regular expressions (revised API)
	.SH "DIFFERENCES BETWEEN PCRE2 AND PERL"
	.rs
	.sp
	This document describes the differences in the ways that PCRE2 and Perl handle
	regular expressions. The differences described here are with respect to Perl
	versions 5.10 and above.
	.P
	1. PCRE2 has only a subset of Perl's Unicode support. Details of what it does
	have are given in the
	.\" HREF
	\fBpcre2unicode\fP
	.\"
	page.
	.P
	2. PCRE2 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: PCRE2 optimizes this to run the assertion
	just once). Perl allows repeat quantifiers on other assertions such as \eb, but
	these do not seem to have any use.
	.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
	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
	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 PCRE2, an error is
	generated by default. However, if the PCRE2_ALT_BSUX option is set,
	\eU and \eu are interpreted as ECMAScript interprets them.
	.P
	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
	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&. PCRE2 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
	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
	that $ and @ are also handled as literals inside the quotes. In Perl, they
	cause variable interpolation (but of course PCRE2 does not have variables).
	Note the following examples:
	.sp
	Pattern PCRE2 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
	7. Fairly obviously, PCRE2 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 PCRE2 "callout"
	feature allows an external function to be called during pattern matching. See
	the
	.\" HREF
	\fBpcre2callout\fP
	.\"
	documentation for details.
	.P
	8. Subroutine calls (whether recursive or not) are treated as atomic groups.
	Atomic recursion is like Python, but unlike Perl. Captured values that are set
	outside a subroutine call can be referenced from inside in PCRE2, but not in
	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
	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
	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
	10. 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 PCRE2, but there are examples where it differs.
	.P
	11. Most backtracking verbs in assertions have their normal actions. They are
	not confined to the assertion.
	.P
	12. 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 PCRE2 it is set to
	"b".
	.P
	13. PCRE2'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 PCRE2
	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
	14. Perl recognizes comments in some places that PCRE2 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 PCRE2 never does, even if the PCRE2_EXTENDED option is set.
	.P
	15. Perl, when in warning mode, gives warnings for character classes such as
	[A-\ed] or [a-[:digit:]]. It then treats the hyphens as literals. PCRE2 has no
	warning features, so it gives an error in these cases because they are almost
	certainly user mistakes.
	.P
	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}
	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
	letters, regardless of case, when case independence is specified.
	.P
	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
	of which (such as named parentheses) have been in PCRE2 for some time. This
	list is with respect to Perl 5.10:
	.sp
	(a) Although lookbehind assertions in PCRE2 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 PCRE2_DOLLAR_ENDONLY is set and PCRE2_MULTILINE is not set, the $
	meta-character matches only at the very end of the string.
	.sp
	(c) A backslash followed by a letter with no special meaning is faulted. (Perl
	can be made to issue a warning.)
	.sp
	(d) If PCRE2_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) PCRE2_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 PCRE2_NOTBOL, PCRE2_NOTEOL, PCRE2_NOTEMPTY, PCRE2_NOTEMPTY_ATSTART, and
	PCRE2_NO_AUTO_CAPTURE options have no Perl equivalents.
	.sp
	(g) The \eR escape sequence can be restricted to match only CR, LF, or CRLF
	by the PCRE2_BSR_ANYCRLF option.
	.sp
	(h) The callout facility is PCRE2-specific.
	.sp
	(i) The partial matching facility is PCRE2-specific.
	.sp
	(j) The alternative matching function (\fBpcre2_dfa_match()\fP matches in a
	different way and is not Perl-compatible.
	.sp
	(k) PCRE2 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
	.rs
	.sp
	.nf
	Philip Hazel
	University Computing Service
	Cambridge, England.
	.fi
	.
	.
	.SH REVISION
	.rs
	.sp
	.nf
	Last updated: 15 March 2015
	Copyright (c) 1997-2015 University of Cambridge.
	.fi