Using sre.sub[n], an "unmatched group" error can occur.

The test I used is this pattern:

sre.sub("foo(?:b(ar)|baz)","\\1","foobaz")

This will cause the following backtrace to occur:

Traceback (most recent call last):
  File "<stdin>", line 1, in ?
  File "lib/python2.4/sre.py", line 142, in sub
    return _compile(pattern, 0).sub(repl, string, count)
  File "lib/python2.4/sre.py", line 260, in filter
    return sre_parse.expand_template(template, match)
  File "lib/python2.4/sre_parse.py", line 782, in expand_template
    raise error, "unmatched group"
sre_constants.error: unmatched group

Python Version 2.4.3, Mac OS X (behaviour has been verified on 
Windows 2.4.3 as well).

This behaviour, while by design, is unwanted because this type of 
matching usually requests that a blank match be returned (i.e. the 
example should return '')

The example that I was trying resembles the following:

sre.sub("User: (?:Registered User #(\d+)|Guest)","%USERID|\1%",data)

The intended behaviour is that the function returns "" when the user is 
a guest and the user number if the user is a registered member.

However, when this function encounters a Guest, it raises an exception 
and terminates, which is not what is wanted.

Perl and other regex engines behave as I have described, substituting 
empty strings for unmatched groups. The code fix is relatively simple, 
and would really help out for these types of things.