re.search much slower then grep on some regular expressions

[J. Cliff Dyer]

On Wed, 2008-07-09 at 12:29 -0700, samwyse wrote:
> On Jul 8, 11:01 am, Kris Kennaway <k... at FreeBSD.org> wrote:
> > samwyse wrote:
> 
> > > 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. "
> 
> > 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).
> 
> That's funny, the compilation is almost instantaneous for me.
> However, I just tested it to several files, the first containing
> 4875*'a', the rest each twice the size of the previous.  And you're
> right, for each doubling of the file size, the match take four times
> as long, meaning O(n^2).  156000*'a' would probably take 8 hours.
> Here are my results:
> 
> compile_lexicon() took 0.0236021580595 secs
> test('file-0.txt') took 24.8322969831 secs
> test('file-1.txt') took 99.3956799681 secs
> test('file-2.txt') took 398.349623132 secs

Sounds like a good strategy would be to find the smallest chunk of the
file that matches can't cross, and iterate your search on units of those
chunks.  For example, if none of your regexes cross line boundaries,
search each line of the file individually.  That may help turn around
the speed degradation you're seeing.

Cheers,
Cliff