On Sat, Oct 26, 2013 at 11:38 AM, Peter Bex peter@xs4all.nl wrote:
On Sat, Oct 26, 2013 at 10:37:36AM -0700, Matt Welland wrote:
This regex is so slow that you don't need a timer to see the impact (at
least not on my machine with chicken 4.8.0):
(string-match [a-z][a-z0-9\\-_.]{0,20}
a012345678901234567890123456789)
Changing the {0,20} to + makes it run normally fast so I just replaced
the
regex with a string-length and modified the {0,20} to + . I don't
necessarily need a fix for this but it seems like a possible symptom of a
deeper problem so I thought I'd report it.
Hi Matt,
Thanks for your report. I'm afraid this is a known problem with
Irregex - to avoid producing a state machine with too many states,
it will always use a backtracking implementation for all repetition
counts.
I think it's best to take a look at how to fix this upstream first.
Maybe Alex has an idea of how to do that.
Pre-compiling the regex didn't seem to make any difference.
That's because the backtracker matches really slowly.
Just for completeness I compared with Ruby and the ruby equivalent
is (in human terms) instant.
a012345678901234567890123456789.match(/[a-z][a-z0-9]{0,20}/)
Thanks for that! We need more of such real-world test cases.
Ideally I'd love to have a complete library of regex examples, which
we can use to optimize irregex further.
By the way, I note that your Ruby regex isn't exactly the same as the
one you used in CHICKEN. Does it make a difference if you use the
same regex?
Ah, yes, I did initially compare the exact same regex and saw no
discernible speed impact in Ruby. I did a few variations to see if there
was any sensitivity to ^ or $ at the beginning or end of the regex or
variations to the contents of the character group. This seems to be pretty
much a function of the match count {0,20}.
Some coding for performance hints on the regex page would likely be helpful.
Cheers,
Peter
--
http://www.more-magic.net
--
Matt
-=-
90% of the nations wealth is held by 2% of the people. Bummer to be in the
majority...
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