The
Komodo
programming environment from ActiveState includes a very
interesting dynamic regex debugging feature. You can do
things like: tweak the
regex and the highlighting in your test strings changes to
show what will be matched, etc.
Which is based on
Rx. Dominus
wrote an excellent paper on the development of Rx. It won
a "best paper" award at The Perl Conference this year.
It's included in the proceedings of the conference or on
his web
site.