re.search much slower then grep on some regular expressions
kris at FreeBSD.org
Tue Jul 8 18:01:39 CEST 2008
> On Jul 4, 6:43 am, Henning_Thornblad <Henning.Thornb... at gmail.com>
>> What can be the cause of the large difference between re.search and
>> 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.
> "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 is None:
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).
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