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 | Posted by Dmitry Olshansky |  Permalink Reply |
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Dmitry Olshansky 
| http://d.puremagic.com/issues/show_bug.cgi?id=5169
Summary: Add(?:) Non-capturing parentheses group support to
std.regex
Product: D
Version: D2
Platform: All
OS/Version: All
Status: NEW
Severity: enhancement
Priority: P2
Component: Phobos
AssignedTo: nobody@puremagic.com
ReportedBy: dmitry.olsh@gmail.com
--- Comment #0 from Dmitry Olshansky <dmitry.olsh@gmail.com> 2010-11-05 09:35:15 PDT ---
Intro: Non-capturing parentheses group the regex so you can apply regex operators, but do not capture anything and do not create backreferences.
Examples:
//A very dumb example, matches abcabcabc, no backrefs created
(?:abc){3}
//A decent attempt to snatch href field of <a> html tag, without unnessary
//backrefs:
<(?:a|A)(?:[^<>]*)href *= *"?([^"<> ]*)"?(?:[^<>]*)>
Rationale: ECMA262 standart mentioned on
http://www.digitalmars.com/d/2.0/phobos/std_regex.html
requires support of such construct. Sooner or later we should get rid of
"however, some of the very advanced forms may behave slightly differently",
also given the fact that sometimes it's simple. See attached patch.
Backtracking is also costly, see benchmark code/results
(uses the proposed patch):
//===bench.d===
import std.regex, std.stdio,std.datetime;
void main(){
auto r1 = regex(`(?:a|A)(?:[^<>]*)href *= *"?([^"<> ]*)"?(?:[^<>]*)>`,"g");
auto r2 = regex(`(a|A)([^<>]*)href *= *"?([^"<> ]*)"?([^<>]*)>`,"g");
void nobackref(){
match(`<a href = http://www.google.com id="G"/>`,r1).hit;
}
void backref(){
match(`<a href = http://www.google.com id="G"/>`,r2).hit;
}
auto bench = benchmark!(nobackref,backref)(1_000);
writeln("No backref: ",bench[0].milliseconds);
writeln("With backref: ",bench[1].milliseconds);
}
//======
Results on my machine, min .. max of 10
No backref: 256.955 .. 267.341
With backref: 580.636 .. 587.187
P.S. I have rebuilt phobos (on Windows), and run unitestes, output:
C:\dmd2\src\phobos>unittest.exe
--- std.socket(660) broken test ---
(std.socket.HostException: Address family mismatch)
9abc5a5a12345678
args.length = 1
args[0] = 'C:\dmd2\src\phobos\unittest.exe~T'
Vendor string: AuthenticAMD
Processor string: AMD Phenom(tm) II X4 940 Processor
Signature: Family=16 Model=4 Stepping=2
Features: MMX FXSR SSE SSE2 SSE3 3DNow! 3DNow!+ MMX+ AMD64 HTT
Multithreading: 4 threads / 4 cores
Success!
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