Backreference within a character class

[taashlo at sandia.gov]

For the Python RE gurus:

Using the re module, lets say that I want to match "XIX" but not "XXX"
or "WOW" but not "WWW".  In my first attempt I used r"(.)([^\1])\1".
This, of course, did't work because the "\1" in character class isn't
interpreted as a backreference.

So is it possible to specify a regular expression to match these
patterns?

Thanks,
Tad Ashlock <taashlo at sandia.gov>