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I think your interpretation is correct, though when I've benchmarked it, now and previously, I have had variable results as you'll see below.

It seems to consistantly slow the regex down (even excluding the study time), if you only match against a single regex.

There can be some considerable speed up when matching against multiple regexes, but it's not consistant in how much you get. In the example below, matching against 2 regexes on a study'd string sometimes shows upto 50% improvement relative to an unstudy'd one, but less so when matching 3 regexes against the same two strings. It seems to be a function of the constant content of the regexs. Ie. what characters are involved. Those containing rare characters showing greater benefit than those not, but that's really educated speculation.

That said, there always seems to be some benefit to studying the string if you intend to match against more than once with 2 or more different regexes.

There doesn't seem to be any benefit (actually normally a small penalty) for multiple matches with a single regex, as with the /g modifier.

perl> use Benchmark qw[cmpthese]; perl> $s = "I say it's the opposite, because the FAQ says it takes mo +re time, and that it works on the scalar to be matched rather than the regex, but I might as well ask. In what circumstances would I + study() a string before hitting it with a regex?" perl> $t = $s perl> study $s perl> cmpthese( -1, { studied=>'$_=()=$s=~m[FAQ]o', slacker=>'$_=()=$ +t=~m[FAQ]o' } ) Benchmark: running slacker, studied, each for at least 1 CPU seconds. +.. slacker: 1 wallclock secs ( 1.01 usr + 0.00 sys = 1.01 CPU) @ 56 +776.24/s (n=57344) studied: 2 wallclock secs ( 1.12 usr + 0.00 sys = 1.12 CPU) @ 54 +807.31/s (n=61439) Rate studied slacker studied 54807/s -- -3% slacker 56776/s 4% -- perl> cmpthese( -1,{\ studied=>'$a = ($s = ~m[FAQ]o && $s =~ m[regex]o)',\ slacker=>'$b = ($t =~ m[FAQ]o && $t =~ m[regex]o)',\ }) Benchmark: running slacker, studied, each for at least 1 CPU seconds. +.. slacker: 2 wallclock secs ( 1.08 usr + 0.00 sys = 1.08 CPU) @ 98 +754.16/s (n=106852) studied: 2 wallclock secs ( 1.20 usr + 0.00 sys = 1.20 CPU) @ 65 +054.91/s (n=78196) Rate studied slacker studied 65055/s -- -34% slacker 98754/s 52% -- perl> cmpthese( -1,{\ studied=>'$a = ($s =~ m[FAQ]o && $s =~ m[regex]o) && $s =~ m[circumsta +nces]o',\ slacker=>'$b = ($t =~ m[FAQ]o && $t =~ m[regex]o) && $t =~ m[circumsta +nces]o',\ }) Benchmark: running slacker, studied, each for at least 1 CPU seconds. +.. slacker: 2 wallclock secs ( 1.00 usr + 0.00 sys = 1.00 CPU) @ 71 +679.00/s (n=71679) studied: 2 wallclock secs ( 1.17 usr + 0.00 sys = 1.17 CPU) @ 56 +503.84/s (n=66166) Rate studied slacker studied 56504/s -- -21% slacker 71679/s 27% -- perl> cmpthese( -10,{\ studied=>'$a = ($s =~ m[FAQ]o && $s =~ m[regex]o)',\ slacker=>'$b = ($t =~ m[FAQ]o && $t =~ m[regex]o)',\ }) Benchmark: running slacker, studied, each for at least 10 CPU second +s... slacker: 10 wallclock secs (11.21 usr + 0.01 sys = 11.22 CPU) @ 1 +11296.69/s (n=1248415) studied: 11 wallclock secs (10.00 usr + 0.00 sys = 10.00 CPU) @ 1 +41637.98/s (n=1417088) Rate slacker studied slacker 111297/s -- -21% studied 141638/s 27% --

Examine what is said, not who speaks.

The 7th Rule of perl club is -- pearl clubs are easily damaged. Use a diamond club instead.


In reply to Re: Re: Re: Compiling Regular Expressions by BrowserUk
in thread Compiling Regular Expressions by Anonymous Monk

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