Regexp engine in Qt5

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Regular expression engine in Qt5

This page summarizes the research for an alternative regular expression engine to be used in Qt 5.0. The discussion started on the qt5-feedback mailing list, cf.

• http://lists.qt-project.org/pipermail/qt5-feedback/2011-September/001004.html
• https://bugreports.qt.io/browse/QTBUG-20888

Current issues with QRegExp

From http://lists.qt-project.org/pipermail/qt5-feedback/2011-September/001054.html

High level issues

• QRegExp API is broken (see T7 in Low Level)
• QRegExp is used for QtScript though it does not fullfill the ECMAScript specification ECMA-262-1999 . Missing features include
  • Non-greedy quantifiers (see page 141 titled "- 129 -")
  • But current implementation of QtScript uses JSC which uses its own engine anyway, and only use QRegExp in its api as a container.
• Patternist/XPath also needs Regex features not found in QRegExp, including
  • Non-greedy quantifiers ( http://www.w3.org/TR/xpath-functions/#regex-syntax )
• Qt Creator might want to offer multi-line Regex search- and replacing later. This cannot be efficient because of T6 described below. GtkSourceView has exactly that problem …
• Customer complained about QRegExp (though I don't see what's their exact problem):
  • In their code they have RegExp? for matching emoticons. Unfortunately, they cannot use QRegExp? because of poor support for negative/positive lookahead. As a workaround they are using the PCRE (Perl Compatible Regular Expressions) library.
• Public task request:
  • Lookbehind (T4) (bug 217916)
  • Support for POSIX syntax (bug 218604)
  • Removing const modifiers (T7) (bug 219234, bug 209041)
  • Non-greedy quantifiers (T3) (bug 116127)

Low Level issues

• T1: ^ (caret) and $ (dollar) cannot match at each newline
• T2: . (dot) always matches newlines
• T3: lazy/non-greedy/reluctant quantifiers are not supported. this is not to be confused with minimal matching.
• T4: lookbehind is not supported (lookahead is)
• T5: lastIndexIn does not find that last match which indexIn would have found, e.g. lastIndexIn("abcd") for pattern ".*" returns 3, not 0
• T6: only linear input is supported, for a text editor like Kate this does not scale
• T7: QRegExp combines matcher and match object, despite the 1:n relation. As a consequence matching with a const QRegExp instance modifies a const object.

Future

• It must be a solid 3rd party engine — don't want to develop an in-house engine and maintain it.
• QRegExp likely to be moved into its own module in order to keep source compatibility.
• Addresses the above low-level issues (as much as possible).
• (Nice to have) At least the same syntax / features than std::regex

Proposed libraries

• PCRE
• "V8"
• ICU
• Boost.Regex
• std::regex (new in C+11)
• RE2

Feature matrix

	QRegExp	PCRE	V8	ICU	Boost.Regex	std::regex	RE2
See above.
Already being used in Qt?	Yes	Indirectly as a GLIB dependency under Unix. Moreover, a stripped down version of PCRE is available inside WebKit (src/3rdparty/webkit/JavaScriptCore/pcre); all features not required by the JS specification were removed.	Yes (Qt 5)	libicui18n (optionally?) used by Qt 4.8 / Qt 5 in QLocale.	No	No	No
Pros		Widely used, de-facto standard implementation for Perl-like regexps.		Uses UTF-16 natively.			very fast, use a DFA
Cons		Does not run on every platform supported by QtCore / QtBase[2].		Boost does not give guarantees about ABI compatibility.		uses UTF-8, doesn't have the lookbehind neither lookahead
Fixes T1		✔		✔			✔
Fixes T2		✔		✔			✔
Fixes T3		✔		✔			?
Fixes T4		✔		✘			✘
Fixes T5		?		?			✘
Fixes T6		✔ ("by hand", with partial matching)		Maybe yes, see UText .			✔ see StringPiece
Fixes T7		✔		✔			✘

✘✔

Supported syntax: Characters

		QRegExp	PCRE	V8	ICU	Boost.Regex	std::regex	RE2
	BELL	✔	✔		✔
	beginning of input		✔		✔
inside a [set]	BACKSPACE	?	✔
outside a [set]	on a word boundary	✔	✔
	not on a word boundary	✔	✔		✔
	ASCII control character X	✘	✔		✔
	digit	✔	✔		✔
	non digit	✔	✔		✔
	ESCAPE	✘	✔		✔
	end of … quoting	✘	✔		✔
	FORM FEED	✔	✔		✔
	end of previous match	✘	✔		✔
	LINE FEED	✔	✔
{x}	UNICODE CHARACTER NAME x	✘	✘		✔
{x}	UNICODE PROPERTY NAME x	✘	✔		✔
{x}	UNICODE PROPERTY NAME not x	✘	✔		✔
	start of … quoting	✘	✔		✔
	CARRIAGE RETURN	✔	✔		✔
	white space	✔	✔		✔
	non white space	✔	✔		✔
	HORIZONTAL TAB	✔	✔		✔
	U+hhhh (between U+0000 and U+FFFF)	✘	✘
	U+hhhhhhhh (between U+00000000 and U+0010FFFF)	✘	✘
VERTICAL TAB	✔	✔		✘
	word character	✔	✔		✔
	non word character	✔	✔		✔
{hhhh}	U+hhhh	✘	✔ (0-10FFFF)		✔ (0-10FFFF)
	U+hhhh	✔ (0000-FFFF)	✔ (00-FF)		✔ (00-FF)
	grapheme cluster	✘	✘
	end of input (or before the final )	✘	✔		✔
	end of input	✘	✔		✔
	n-th backreference	✔	✔
ooo	ASCII/Latin-1 character 0ooo	✔	✔
.	any character but newlines	✔	✔		✔
^	line beginning	✔	✔		✔
$	line end	✔	✔		✔
quote the following symbol	✔	✔		✔
[pattern]	set	✔	✔		✔

Supported syntax: Operators

Operator		QRegExp	PCRE	V8	ICU	Boost.Regex	std::regex	RE2
*	match 0 or more times	✔	✔		✔	?	?	✔
+	match 1 or more times	✔	✔		✔			✔
?	match 0 or 1 times	✔	✔		✔			✔
{n}	match n times	✔	✔		✔			✔
{n,}	match n or more times	✔	✔		✔			✔
{n,m}	match between n and m times	✔	✔		✔			✔
*?	match 0 or more times, not greedy	✘	✔		✔			✔
?	match 1 or more times, not greedy	✘	✔		✔			✔
??	match 0 or 1 times, not greedy	✘	✔		✔			✔
{n}?	match n times	✘	✔		✔			✔
{n,}?	match n or more times, not greedy	✘	✔		✔			✔
{n,m}?	match between n and m times, not greedy	✘	✔		✔			✔
*+	match 0 or more times, possessive	✘	✔		✔			✘
+	match 1 or more times, possessive	✘	✔		✔			✘
?	match 0 or 1 times, possessive	✘	✔		✔			✘
{n}+	match n times	✘	✔		✔			✘
{n,}+	match n or more times, possessive	✘	✔		✔			✘
{n,m}+	match between n and m times, possessive	✘	✔		✔			✘
( … )	capturing group	✔	✔		✔			✔
(?: … )	group	✔	✔		✔			✔
(?> … )	atomic grouping	✘	✔		✔			✘
(?# … )	comment	✘	✔		✔			✘
(?= … )	look-ahead assertion	✔	✔		✔			✘
(?! … )	negative look-ahead assertion	✔	✔		✔			✘
(?<= … )	look-behind assertion	✘	✔		✔			✘
(?<! … )	negative look-behind assertion	✘	✔		✔			✘
(?flags: … )	flags change	✘	✔					✔
(?flags)	flags change	✘	✔		✔			✔
(?P<name> …)	named capturing group	✘	✔		✘			✔
(?<name> …)	named capturing group	✘	✔		✘			✘
(?'name' …)	named capturing group	✘	✔		✘			✘
…)	branch reset	✘	✔		✘			✘

Supported syntax: flags

Flag		QRegExp	PCRE	V8	ICU	Boost.Regex	std::regex	RE2
/i	case insensitive	✔	✔		✔			✔
/m	multi-line	✘	✔		✔			✔
/s	dot matches anything	~	✔		✔			✔
/x	ignore whitespace and comments	✘	✔		✔			✔
/U	minimal match	✔	✔		✘			✔

Benchmarks

See https://gitorious.org/qt-regexp-benchmarks/qt-regexp-benchmarks for the code and https://gitorious.org/qt-regexp-benchmarks/pages/Home for some results.