From: Charles Mills <cmills@...>
Date: 2005-02-16T11:39:58+09:00
Subject: Re: Curious regexp behavior

Derek Lewis wrote:
> On a whim, I just decided to try an experiment with regexps, to see
how
> they perform in two slightly different cases.  I wanted to see how
using
> a single regexp object for many many evaluations performed compared
to
> using the regexp within the loop.
>
> The scripts I wrote searched through a words file that is 234937
lines
> long.
>
> Here's the scripts I wrote, to clarify:
> First one:
>
> total = 0
> File.open( 'words', 'r' ) { |file|
>    file.each_line { |line|
> 		word = line.chomp
> 		total +=1 if word =~ /[a-df-h][aeiou]{2}/
> 	}
> }
> puts total
>
> Second one:
>
> rexp = /[a-df-h][aeiou]{2}/
> total = 0
> File.open( 'words', 'r' ) { |file|
>    file.each_line { |line|
> 		word = line.chomp
> 		total +=1 if word =~ rexp
> 	}
> }
> puts total
>
>
> I expected the second one to be slightly faster, but was surprised to
> see that it was actually slightly slower.  I ran each one about 10-15
> times, and eyeballed an average.  The results from each run after the
> first were pretty consistant.
>
> It's just a curiosity, but does anyone know what might cause them to
be
> 'backwards' like that? :)
>
I'll wager a guess.  In the first version Ruby knows that
'/[a-df-h][aeiou]{2}/' is a regexp.  In the second one Ruby doesn't
know if 'rexp' is a variable or method, so it has to do 1 maybe 2 look
ups on every interation before it dispatches String#=~.
Also regexp's are immutable so Ruby doesn't allocate a new regexp on
every interation and storing the regexp has no effect in that regard.

-Charlie