Backreference within a character class

[taashlo at sandia.gov]

"Fredrik Lundh" <effbot at telia.com> writes:

> taashlo at sandia.gov wrote:
> > 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?
> 
> this might work: r"(.)(?!\1).\1"
> 
> </F>

This works perfectly.  Now can somebody point me to where this is
documented so that:

1) I can understand what this is doing, and
2) I can learn more about these regex extensions.

Thanks,
Tad