in reply to Re^3: Question on Regex
in thread Question on Regex
Which shows that sub athanasius is up to 5 times faster than sub karlgoethebier. Even easier to see when supplying a positive COUNT value to timethese:
#! perl use strict; use warnings; use Benchmark qw( :hireswallclock cmpthese timethese ); my $string = ".co.uk/Jobs/Company-Sector/C8A6446X4PND86M9WYJ/Tradewind +/?APath=2.21.0.0.0"; cmpthese( timethese ( 1_000_000, { 'karlgoethebier' => sub { $string =~ m/.+\/.+\/.+\/.+\/(.+)\/.+/; retu +rn $1; }, 'athanasius' => sub { return (split '/', $string)[4] }, } ) );
Output:
13:09 >perl 390_SoPW.pl Benchmark: timing 1000000 iterations of athanasius, karlgoethebier... athanasius: 5.19894 wallclock secs ( 5.15 usr + 0.00 sys = 5.15 CPU) + @ 194250.19/s (n=1000000) karlgoethebier: 20.2485 wallclock secs (20.08 usr + 0.00 sys = 20.08 +CPU) @ 49808.24/s (n=1000000) Rate karlgoethebier athanasius karlgoethebier 49808/s -- -74% athanasius 194250/s 290% -- 13:16 >
Not really surprising, since regexen with quantifiers can be expensive:
Avoid regular expressions with many quantifiers.... Such patterns can result in exponentially slow backtracking behavior unless the quantified subpatterns match on their first “pass”.
— The Camel Book, 4th Edition, p. 693.
So, what happens if we limit the backtracking?
#! perl use strict; use warnings; use Benchmark qw( :hireswallclock cmpthese timethese ); my $string = ".co.uk/Jobs/Company-Sector/C8A6446X4PND86M9WYJ/Tradewind +/?APath=2.21.0.0.0"; cmpthese( timethese ( 1_000_000, { 'karlgoethebier' => sub { $string =~ m/.+\/.+\/.+\/.+\/(.+)\/.+/; retu +rn $1; }, 'karlgoethebier2' => sub { $string =~ m/.+?\/.+?\/.+?\/.+?\/(.+?)\/.+/; + return $1; }, 'athanasius' => sub { return (split '/', $string)[4] }, } ) );
Result:
13:28 >perl 390_SoPW.pl Benchmark: timing 1000000 iterations of athanasius, karlgoethebier, ka +rlgoethebier2... athanasius: 4.91799 wallclock secs ( 4.88 usr + 0.00 sys = 4.88 CPU) + @ 204792.14/s (n=1000000) karlgoethebier: 20.1721 wallclock secs (20.05 usr + 0.00 sys = 20.05 +CPU) @ 49885.26/s (n=1000000) karlgoethebier2: 2.55908 wallclock secs ( 2.53 usr + 0.00 sys = 2.53 + CPU) @ 395569.62/s (n=1000000) Rate karlgoethebier athanasius karlgoethebie +r2 karlgoethebier 49885/s -- -76% -8 +7% athanasius 204792/s 311% -- -4 +8% karlgoethebier2 395570/s 693% 93% +-- 13:29 >perl 390_SoPW.pl
The regex is now significantly faster than split-with-subscript. Interesting!
Athanasius <°(((>< contra mundum
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Re^5: Question on Regex
by karlgoethebier (Abbot) on Nov 19, 2012 at 14:15 UTC | |
by karlgoethebier (Abbot) on Nov 19, 2012 at 20:24 UTC | |
by Athanasius (Archbishop) on Nov 20, 2012 at 02:48 UTC | |
by karlgoethebier (Abbot) on Nov 20, 2012 at 07:52 UTC |
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