Multiple regex match idiom

[Hrvoje Niksic]

I often have the need to match multiple regexes against a single
string, typically a line of input, like this:

if (matchobj = re1.match(line)):
  ... re1 matched; do something with matchobj ...
elif (matchobj = re2.match(line)):
  ... re2 matched; do something with matchobj ...
elif (matchobj = re3.match(line)):
....

Of course, that doesn't work as written because Python's assignments
are statements rather than expressions.  The obvious rewrite results
in deeply nested if's:

matchobj = re1.match(line)
if matchobj:
  ... re1 matched; do something with matchobj ...
else:
  matchobj = re2.match(line)
  if matchobj:
    ... re2 matched; do something with matchobj ...
  else:
    matchobj = re3.match(line)
    if matchobj:
      ...

Normally I have nothing against nested ifs, but in this case the deep
nesting unnecessarily complicates the code without providing
additional value -- the logic is still exactly equivalent to the
if/elif/elif/... shown above.

There are ways to work around the problem, for example by writing a
utility predicate that passes the match object as a side effect, but
that feels somewhat non-standard.  I'd like to know if there is a
Python idiom that I'm missing.  What would be the Pythonic way to
write the above code?