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[[Category:Developing_Qt::Qt Planning::Qt Public Roadmap]]<br />[toc align_right=&quot;yes&amp;quot; depth=&quot;3&amp;quot;]
[[Category:Developing_Qt::Qt Planning::Qt Public Roadmap]]
[toc align_right="yes" depth="3"]


= Regular expression engine in Qt5 =
= Regular expression engine in Qt5 =


This page summarizes the research for an alternative regular expression engine to be used in [[Qt_5.0 | Qt 5.0]]. The discussion started on the qt5-feedback mailing list, cf.<br />* http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-September/001004.html<br />* https://bugreports.qt.nokia.com/browse/QTBUG-20888
This page summarizes the research for an alternative regular expression engine to be used in [[Qt_5.0 | Qt 5.0]]. The discussion started on the qt5-feedback mailing list, cf.
* http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-September/001004.html
* https://bugreports.qt.nokia.com/browse/QTBUG-20888


== Current issues with QRegExp ==
== Current issues with QRegExp ==
Line 12: Line 15:


* QRegExp API is broken (see '''T7''' in Low Level)
* QRegExp API is broken (see '''T7''' in Low Level)
* QRegExp is used for QtScript though it does not fullfill the ECMAScript specification &quot;ECMA-262-1999&amp;quot;:http://www.ecma-international.org/publications/standards/Ecma-262.htm . Missing features include
* QRegExp is used for QtScript though it does not fullfill the ECMAScript specification "ECMA-262-1999":http://www.ecma-international.org/publications/standards/Ecma-262.htm . Missing features include
** Non-greedy quantifiers (see page 141 titled &quot;- 129 <s>&quot;)<br />'''''' But current implementation of QtScript uses JSC which uses its own engine anyway, and only use QRegExp in its api as a container.<br />* Patternist/XPath also needs Regex features not found in QRegExp, including<br />'''''' Non-greedy quantifiers ( http://www.w3.org/TR/xpath-functions/#regex-syntax )<br />* Qt Creator might want to offer multi-line Regex search</s> and replacing later. This cannot be efficient because of '''T6''' described below. GtkSourceView has exactly &quot;that problem&amp;quot;:http://bugzilla.gnome.org/show_bug.cgi?id=134674#c1 …
** 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":http://bugzilla.gnome.org/show_bug.cgi?id=134674#c1 …
* Customer complained about QRegExp (though I don't see what's their exact 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.''
** ''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:
* Public task request:
** Lookbehind ('''T4''') (&quot;bug 217916&amp;quot;:http://trolltech.com/developer/task-tracker/index_html?id=217916&amp;amp;method=entry)
** Lookbehind ('''T4''') ("bug 217916":http://trolltech.com/developer/task-tracker/index_html?id=217916&amp;amp;method=entry)
** Support for POSIX syntax (&quot;bug 218604&amp;quot;:http://trolltech.com/developer/task-tracker/index_html?id=218604&amp;amp;method=entry)
** Support for POSIX syntax ("bug 218604":http://trolltech.com/developer/task-tracker/index_html?id=218604&amp;amp;method=entry)
** Removing const modifiers ('''T7''') (&quot;bug 219234&amp;quot;:http://trolltech.com/developer/task-tracker/index_html?id=219234&amp;amp;method=entry, &quot;bug 209041&amp;quot;:http://trolltech.com/developer/task-tracker/index_html?id=209041&amp;amp;method=entry)
** Removing const modifiers ('''T7''') ("bug 219234":http://trolltech.com/developer/task-tracker/index_html?id=219234&amp;amp;method=entry, "bug 209041":http://trolltech.com/developer/task-tracker/index_html?id=209041&amp;amp;method=entry)
** Non-greedy quantifiers ('''T3''') (&quot;bug 116127&amp;quot;:http://trolltech.com/developer/task-tracker/index_html?id=116127&amp;amp;method=entry)
** Non-greedy quantifiers ('''T3''') ("bug 116127":http://trolltech.com/developer/task-tracker/index_html?id=116127&amp;amp;method=entry)


=== Low Level issues ===
=== Low Level issues ===
Line 28: Line 35:
* '''T3''': lazy/non-greedy/reluctant quantifiers are not supported. this is not to be confused with minimal matching.
* '''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)
* '''T4''': lookbehind is not supported (lookahead is)
* '''T5''': lastIndexIn does not find that last match which indexIn would have found, e.g. lastIndexIn(&quot;abcd&amp;quot;) for pattern &quot;.'''&quot; returns 3, not 0<br />''' '''T6''': only linear input is supported, for a text editor like Kate this does not scale
* '''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.
* '''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.


Line 40: Line 48:
== Proposed libraries ==
== Proposed libraries ==


* &quot;PCRE&amp;quot;:http://www.pcre.org/
* "PCRE":http://www.pcre.org/
* &quot;V8&amp;quot;
* "V8"
* &quot;ICU&amp;quot;:http://userguide.icu-project.org/strings/regexp
* "ICU":http://userguide.icu-project.org/strings/regexp
* &quot;Boost.Regex&amp;quot;:http://www.boost.org/doc/libs/1_47_0/libs/regex/doc/html/index.html
* "Boost.Regex":http://www.boost.org/doc/libs/1_47_0/libs/regex/doc/html/index.html
* std::regex (new in C+''11)<br />* &quot;RE2&amp;quot;:http://code.google.com/p/re2/
* std::regex (new in C+''11)
<br />h2. Feature matrix
* "RE2":http://code.google.com/p/re2/
<br />|''. |''. QRegExp |''. PCRE|''. V8|''. ICU|''. Boost.Regex|''. std::regex|''. RE2|<br />|''. General comments|See above.| | | | | | |<br />|''. 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 |<br />|''. Pros | |Widely used, de-facto standard implementation for Perl-like regexps.| |Uses UTF-16 natively. | | |very fast, use a DFA|<br />|''. Cons | |Uses UTF-8, thus requiring string and indexes conversion from/to UTF-16 (QString). UTF-16 support is being developed.|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 |<br />|''. Fixes T1 | |✔| | ✔| | |✔ |<br />|''. Fixes T2 | |✔| | ✔| | |✔ |<br />|''. Fixes T3 | |✔| |✔| | |? |<br />|''. Fixes T4 | |✔| |✘| | | ✘|<br />|''. Fixes T5 | |?| |?| | |✘ |<br />|''. Fixes T6 | |✔ (&quot;by hand&amp;quot;, with partial matching)| |Maybe yes, see &quot;UText&amp;quot;:http://userguide.icu-project.org/strings/utext .| | |✔ see StringPiece |<br />|''. Fixes T7 | |✔| |✔| | | ✘|
<br />✘✔


h2. Feature matrix


<br />h3. Supported syntax: Characters
|''. |''. QRegExp |''. PCRE|''. V8|''. ICU|''. Boost.Regex|''. std::regex|''. RE2|
<br />|''. |''. |''. QRegExp |''. PCRE|''. V8|''. ICU|''. Boost.Regex|''. std::regex|''. RE2|<br />|''. |BELL|✔|✔| |✔| | | |<br />|''. |beginning of input| |✔| |✔| | | |<br />|''. inside a [set]|BACKSPACE|?|✔| | | | | |<br />|''. outside a [set]|on a word boundary|✔|✔| | | | | |<br />|''. |not on a word boundary|✔|✔| |✔| | | |<br />|''. |ASCII control character X|✘|✔| |✔| | | |<br />|''. |digit|✔|✔| |✔| | | |<br />|''. |non digit|✔|✔| |✔| | | |<br />|''. |ESCAPE|✘|✔| |✔| | | |<br />|''. |end of … quoting|✘|✔| |✔| | | |<br />|''. |FORM FEED|✔|✔| |✔| | | |<br />|''. |end of previous match|✘|✔| |✔| | | |<br />|''. |LINE FEED|✔|✔| | | | | |<br />|''. {x}|UNICODE CHARACTER NAME x|✘|✘| |✔| | | |<br />|''. {x}|UNICODE PROPERTY NAME x|✘|✔| |✔| | | |<br />|''. {x}|UNICODE PROPERTY NAME not x|✘|✔| |✔| | | |<br />|''. |start of … quoting|✘|✔| |✔| | | |<br />|''. |CARRIAGE RETURN|✔|✔| |✔| | | |<br />|''. |white space|✔|✔| |✔| | | |<br />|''. |non white space|✔|✔| |✔| | | |<br />|''. |HORIZONTAL TAB|✔|✔| |✔| | | |<br />|''. |U+hhhh (between U+0000 and U+FFFF)|✘|✘| | | | | |<br />|''. |U+hhhhhhhh (between U+00000000 and U+0010FFFF)|✘|✘| | | | | |<br />|''. VERTICAL TAB|✔|✔| |✘| | | |<br />|''. |word character|✔|✔| |✔| | | |<br />|''. |non word character|✔|✔| |✔| | | |<br />|''. {hhhh}|U+hhhh|✘|✔ (0-10FFFF)| |✔ (0-10FFFF)| | | |<br />|''. |U+hhhh|✔ (0000-FFFF)|✔ (00-FF)| |✔ (00-FF)| | | |<br />|''. |grapheme cluster|✘|✘| | | | | |<br />|''. |end of input (or before the final )|✘|✔| |✔| | | |<br />|''. |end of input|✘|✔| |✔| | | |<br />|''. |n-th backreference|✔|✔| | | | | |<br />|''. ooo|ASCII/Latin-1 character 0ooo|✔|✔| | | | | |<br />|''. .|any character but newlines|✔|✔| |✔| | | |<br />|''. ^|line beginning|✔|✔| |✔| | | |<br />|''. $|line end|✔|✔| |✔| | | |<br />|''. quote the following symbol|✔|✔| |✔| | | |<br />|''. [pattern]|set|✔|✔| |✔| | | |
|''. General comments|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 | |Uses UTF-8, thus requiring string and indexes conversion from/to UTF-16 (QString). UTF-16 support is being developed.|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":http://userguide.icu-project.org/strings/utext .| | |✔ see StringPiece |
|''. Fixes T7 | |✔| |✔| | | ✘|


<br />h3. Supported syntax: Operators
✘✔
<br />|''. Operator |''. |''. QRegExp |''. PCRE|''. V8|''. ICU|''. Boost.Regex|''. std::regex|''. RE2|<br />|''. * |match 0 or more times|✔|✔| |✔|?|?|✔|<br />|''. + |match 1 or more times|✔|✔| |✔| | |✔|<br />|''. ? |match 0 or 1 times|✔|✔| |✔| | |✔|<br />|''. {n} |match n times|✔|✔| |✔| | |✔|<br />|''. {n,} |match n or more times|✔|✔| |✔| | |✔|<br />|''. {n,m} |match between n and m times|✔|✔| |✔| | |✔|<br />|''. *? |match 0 or more times, not greedy|✘|✔| |✔| | |✔|<br />|''. ''? |match 1 or more times, not greedy|✘|✔| |✔| | |✔|<br />|''. ?? |match 0 or 1 times, not greedy|✘|✔| |✔| | |✔|<br />|''. {n}? |match n times|✘|✔| |✔| | |✔|<br />|''. {n,}? |match n or more times, not greedy|✘|✔| |✔| | |✔|<br />|''. {n,m}? |match between n and m times, not greedy|✘|✔| |✔| | |✔|<br />|''. *+ |match 0 or more times, possessive|✘|✔| |✔| | |✘|<br />|''.''+ |match 1 or more times, possessive|✘|✔| |✔| | |✘|<br />|''. ?'' |match 0 or 1 times, possessive|✘|✔| |✔| | |✘|<br />|''. {n}+ |match n times|✘|✔| |✔| | |✘|<br />|''. {n,}+ |match n or more times, possessive|✘|✔| |✔| | |✘|<br />|''. {n,m}+ |match between n and m times, possessive|✘|✔| |✔| | |✘|<br />|''. ( … ) |capturing group|✔|✔| |✔| | |✔|<br />|''. (?: … ) |group|✔|✔| |✔| | |✔|<br />|''. (?&amp;gt; … ) |atomic grouping|✘|✔| |✔| | |✘|<br />|''. (?# … ) |comment|✘|✔| |✔| | |✘|<br />|''. (?= … ) |look-ahead assertion|✔|✔| |✔| | |✘|<br />|''. (?! … ) |negative look-ahead assertion|✔|✔| |✔| | |✘|<br />|''. (?&lt;= … ) |look-behind assertion|✘|✔| |✔| | |✘|<br />|''. (?&lt;! … ) |negative look-behind assertion|✘|✔| |✔| | |✘|<br />|''. (?flags: … ) |flags change|✘|✔| | | | |✔|<br />|''. (?flags) |flags change|✘|✔| |✔| | |✔|<br />|''. (?P&amp;lt;name&amp;gt; …) |named capturing group|✘|✔| |✘| | |✔|<br />|''. (?&lt;name&amp;gt; …) |named capturing group|✘|✔| |✘| | |✘|<br />|''. (?'name' …) |named capturing group|✘|✔| |✘| | |✘|<br />|''. (?&amp;#x7c; …) |branch reset|✘|✔| |✘| | |✘|<br />|''. | | | | | | | | |<br />|''. | | | | | | | | |


<br />h3. Supported syntax: flags
 
<br />|''. Flag |''. |''. QRegExp |''. PCRE|''. V8|''. ICU|''. Boost.Regex|''. std::regex|''. RE2|<br />|''. /i |case insensitive|✔|✔| |✔| | |✔|<br />|''. /m |multi-line|✘|✔| |✔| | |✔|<br />|''. /s |dot matches anything|~<ref>It's actually not possible to UNSET /s for QRegExp, i.e. making the dot not to match a newline.</ref> |✔| |✔| | |✔|<br />|''. /x |ignore whitespace and comments|✘|✔| |✔| | |✔|<br />|_. /U |minimal match|✔|✔| |✘| | |✔|
 
h3. 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|✔|✔| |✔| | | |
 
 
h3. 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|✘|✔| |✘| | |✘|
|''. (?&amp;#x7c; …) |branch reset|✘|✔| |✘| | |✘|
|''. | | | | | | | | |
|''. | | | | | | | | |
 
 
h3. Supported syntax: flags
 
|''. Flag |''. |''. QRegExp |''. PCRE|''. V8|''. ICU|''. Boost.Regex|''. std::regex|''. RE2|
|''. /i |case insensitive|✔|✔| |✔| | |✔|
|''. /m |multi-line|✘|✔| |✔| | |✔|
|''. /s |dot matches anything|~<ref>It's actually not possible to UNSET /s for QRegExp, i.e. making the dot not to match a newline.</ref> |✔| |✔| | |✔|
|''. /x |ignore whitespace and comments|✘|✔| |✔| | |✔|
|_. /U |minimal match|✔|✔| |✘| | |✔|


= Benchmarks =
= Benchmarks =

Revision as of 09:48, 25 February 2015

[toc align_right="yes" depth="3"]

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.

Current issues with QRegExp

From http://lists.qt.nokia.com/pipermail/qt5-feedback/2011-September/001054.html

High level issues

' 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 )

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

h2. Feature matrix

|. |. QRegExp |. PCRE|. V8|. ICU|. Boost.Regex|. std::regex|. RE2| |. General comments|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 | |Uses UTF-8, thus requiring string and indexes conversion from/to UTF-16 (QString). UTF-16 support is being developed.|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":http://userguide.icu-project.org/strings/utext .| | |✔ see StringPiece | |. Fixes T7 | |✔| |✔| | | ✘|

✘✔


h3. 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|✔|✔| |✔| | | |


h3. 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|✘|✔| |✘| | |✘| |. (?&#x7c; …) |branch reset|✘|✔| |✘| | |✘| |. | | | | | | | | | |. | | | | | | | | |


h3. Supported syntax: flags

|. Flag |. |. QRegExp |. PCRE|. V8|. ICU|. Boost.Regex|. std::regex|. RE2| |. /i |case insensitive|✔|✔| |✔| | |✔| |. /m |multi-line|✘|✔| |✔| | |✔| |. /s |dot matches anything|~[1] |✔| |✔| | |✔| |. /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.

  1. ↑ It's actually not possible to UNSET /s for QRegExp, i.e. making the dot not to match a newline.
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