Nested Regex Conditionals

[Paul Dale]

Hi All,

I know that several of you will probably want to reply "you should write 
a parser", and I may. For that matter any tips on theory in that 
direction would be appreciated.

However, if you would indulge me in my regex question I would also be 
most grateful.

I'm writing an edi parser and to be in compliance with the specification 
I need to have conditionals that are dependent on conditionals. In some 
regular expression implementations this is possible. The code ...

#!/usr/bin/env python
import re
pattern = re.compile(r"""
    (?P<first>(first))
      (?(first)
        (?P<second>(second))
      )
      (?(second)
        (?P<third>(third))
      )
      """, re.VERBOSE)
string = 'firstsecondthird'
match = re.match(pattern,string)
print match.group('first','second','third')

Prints ('first', 'second', None)

and I haven't found any way to have a second conditional, nor any 
reference to it in any documentation I've found.

Am I missing something, and it is possible? Or is it not possible in python?

It seems like it might be a bug, as it knows there is a group (None, 
instead of an IndexError), but it doesn't match ...

Thanks for any help :)

Paul