From: Robert Dober <robert.dober@...>
Date: 2006-07-27T15:37:04+09:00
Subject: Re: gsub

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On 7/26/06, Robert Dober <robert.dober@gmail.com> wrote:
>
> Sorry about yet another post, but  I think this  is noteworthy...


and yet another

Well that is nice ;)
> > actually that would be something like
> >
> > s.sub( %r{#{Regexp.escape(r)}(?!.*#{Regexp.escape(r)})},"")
> >
> > but I am afraid that this is much more expensive than the
> > reverse.sub.reverse trick and much less readable
> >
> > Robert
> >
> Variable length lookahead assertions are a killer, just look at this, I
> have
> replaced the last < in this page with ***
> that is a string of length 2369, look at the benchmark please:
>
> robert@roma:~/log/ruby/theory/regex$ cat test2.rb && ./test2.rb
> #!/usr/bin/env ruby
> #
> #
>
> require 'benchmark'
> string = nil
> File.open("gsub.html"){ | file | string = file.read }
> s1=s2=nil
> r1 = Regexp.new( "<" )
> r2 = Regexp.new( "<(?!.*<)", Regexp::MULTILINE )


that is stupid
use non greedy
r2 = Regexp.new( "<(?!.*?<)", Regexp::MULTILINE )

n = 50000
> Benchmark.bm do |x|
>       x.report("reverse.sub.reverse") {
>         n.times do
>                 s1 = string.reverse.sub(r1,"***").reverse
>         end
>         }
>       x.report("lookahead") {
>         n.times do
>                 s2 = string.sub(r2,"***")
>         end
>         }
> end
>       user     system      total        real
> reverse.sub.reverse  1.470000   0.010000   1.480000 (  1.483691)
> lookahead 48.100000   0.060000  48.160000 ( 52.608063)


better but not god enough
generous lookahead 14.900000   0.010000  14.910000 ( 15.481074)

As far as I know fixed with lookahead is about ok, though.
> Cheers
> Robert
>

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