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Re^3: Apply A Set Of Regexes To A String

by tachyon (Chancellor)
on Oct 12, 2004 at 06:11 UTC ( #398377=note: print w/replies, xml ) Need Help??

in reply to Re^2: Apply A Set Of Regexes To A String
in thread Apply A Set Of Regexes To A String

One of the main benefits of alternation is that you can compile the RE. Essentially an alternation RE is very similar to the loop, but the loop code has been optimised to the task and is in C with alternation, but generalised and less efficient if you do it in perl. The difference is significant, at least that is what this Benchmark shows.....

use Benchmark 'cmpthese'; $iterations = 1000000; %re = ( foo => 'foo1', bar => 'bar1', ); $re = join '|', keys %re; $re = qr/($re)/; $name1 = "RE"; $code1 = << 'END_CODE1'; $_ = 'foo bar'; s/$re/$re{$1}/g; END_CODE1 $name2 = "Loop"; $code2 = << 'END_CODE2'; $_ = 'foo bar'; for my $sub( keys %re ) { s/$sub/$re{$sub}/g; } END_CODE2 cmpthese( $iterations, {$name1 => $code1, $name2 => $code2} ); __END__ Benchmark: timing 1000000 iterations of Loop, RE... Loop: 24 wallclock secs (24.77 usr + 0.00 sys = 24.77 CPU) @ 40 +377.94/s (n=1000000) RE: 8 wallclock secs ( 7.27 usr + 0.00 sys = 7.27 CPU) @ 13 +7551.58/s (n=1000000) Rate Loop RE Loop 40378/s -- -71% RE 137552/s 241% --

But if I change that to a more real world situation by making the string a 14Kb one (approximately a web page size)

$_ = 'foo bar' x 2000; Benchmark: timing 10000 iterations of Loop, RE... Loop: 20 wallclock secs (19.90 usr + 0.00 sys = 19.90 CPU) @ 50 +2.56/s (n=10000) RE: 30 wallclock secs (29.44 usr + 0.00 sys = 29.44 CPU) @ 33 +9.64/s (n=10000) Rate RE Loop RE 340/s -- -32% Loop 503/s 48% -

And now the loop is faster. In fact try this case:

$_ = 'fo ba' x 1000 . 'foo bar'; Benchmark: timing 10000 iterations of Loop, RE... Loop: 1 wallclock secs ( 0.71 usr + 0.00 sys = 0.71 CPU) @ 14 +064.70/s (n=10000) RE: 12 wallclock secs (11.10 usr + 0.00 sys = 11.10 CPU) @ 90 +1.23/s (n=10000) Rate RE Loop RE 901/s -- -94% Loop 14065/s 1461% --

~This is a purpose designed worst case for alternation as it requires continouous back tracking. So I have shattered my own delusions! Perl loops are faster than RE alternation.



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[atcroft]: .oO(It is a well-known, well-tried module that has probably encountered more corner-cases and oddities than the average mortal programmer can imagine when trying to roll their own...)
[atcroft]: .oO(*Blah* It, I meant....)
[james28909]: omg
[james28909]: im just finding out that july and august have 31 days?
[atcroft]: stevieb: Apparently it was contagious....
[atcroft]: james28909: Make both hands into fists, place them together (with thumbs concealed), and every knuckle is a month (starting with Jan.) has 31 days, every dip (between knuckles) does not....
[atcroft]: (And Feb. is the odd case, because it is 28, unless it is a year divisible by 4, or if it is divisible by both 100 and 400 (at which point it is 29).)
[james28909]: i know but scroll through your calendar on your computer.

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