re.search much slower then grep on some regular expressions

Kris Kennaway kris at FreeBSD.org
Tue Jul 8 18:01:39 CEST 2008


samwyse wrote:
> On Jul 4, 6:43 am, Henning_Thornblad <Henning.Thornb... at gmail.com>
> wrote:
>> What can be the cause of the large difference between re.search and
>> grep?
> 
>> While doing a simple grep:
>> grep '[^ "=]*/' input                  (input contains 156.000 a in
>> one row)
>> doesn't even take a second.
>>
>> Is this a bug in python?
> 
> You might want to look at Plex.
> http://www.cosc.canterbury.ac.nz/greg.ewing/python/Plex/
> 
> "Another advantage of Plex is that it compiles all of the regular
> expressions into a single DFA. Once that's done, the input can be
> processed in a time proportional to the number of characters to be
> scanned, and independent of the number or complexity of the regular
> expressions. Python's existing regular expression matchers do not have
> this property. "
> 
> I haven't tested this, but I think it would do what you want:
> 
> from Plex import *
> lexicon = Lexicon([
>     (Rep(AnyBut(' "='))+Str('/'),  TEXT),
>     (AnyBut('\n'), IGNORE),
> ])
> filename = "my_file.txt"
> f = open(filename, "r")
> scanner = Scanner(lexicon, f, filename)
> while 1:
>     token = scanner.read()
>     print token
>     if token[0] is None:
>         break

Hmm, unfortunately it's still orders of magnitude slower than grep in my 
own application that involves matching lots of strings and regexps 
against large files (I killed it after 400 seconds, compared to 1.5 for 
grep), and that's leaving aside the much longer compilation time (over a 
minute).  If the matching was fast then I could possibly pickle the 
lexer though (but it's not).

Kris

Kris



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