which re a|l|t|e|r|n|a|t|i|v|e matched?

[anton muhin]

Skip Montanaro wrote:

> I have a long regular expression with the top-level form
> 
>     pat = 'A|B|C|D|...'
> 
> where there are a couple hundred alternatives, each one being a fairly
> simple regular expression (typically just the name of a machine).  Assuming
> I've compiled that and match against it:
> 
>     matcher = re.compile(pat)
>     match = matcher.match(foo)
>     if match is not None:
>         ...
> 
> is there a way to know what alternative was matched?  Note that I'm not
> looking for match.group(1).  I want to know which pattern among the various
> was matched.  (I realize there might be more than one, but returning just
> one is okay.)
> 
> If it helps, the regular expression is formed from the keys of a dictionary
> like so:
> 
>     pat = '('+'|'.join(d.keys())+')'
> 
> I'm concatenating them like this so I don't need to make as many re.match()
> calls.  I could narrow things down by doing a binary search of the keys(),
> but I was hoping for a simple way to do it in one shot.
> 
> Thx,
> 
> Skip
> 

I'm not sure that you are looking for, but this example might be of 
intereset for you:

from __future__ import generators

def enumerate(seq):
     n = 0
     for e in seq:
         yield n, e
         n += 1

import re

chars = "abcdefghij"

pat = "|".join(["(?P<m%d>%s)" % (no, c) for no, c in enumerate(chars)])
r = re.compile(pat)

match = r.match("def")
if match:
     for name, val in match.groupdict().iteritems():
         if val:
             print "matched: %s; (no = %d)" % (name, int(name[1:]))

(sorry, Python 2.2)

hth,
anton.