The Opposite of re match

[Matt Gerrans]

Is there some re trick to find out specificaly why an re *doesn't* match?

I have a script which tests user input on a collection of regular
expressions; if it fails all of them the input is rejected, but I'd like to
be able to display a useful message like "You entered 'abc_1q23.ext' which
is pretty close to the 'aaa_nnn.ext' format, but you cannot have a letter in
the number field."    Since the re test will simply return None for all the
available format choices, there is no object to work with, of course.
Trying to anticipate every possible zany form of input that may arrive could
be a pretty big task, too.