tinyfugue rocks
TinyFugue
It doesn't look like it has been updated since '99, but it was always considered the gold standard of mud clients when I played (early-mid nineties). It has good regular expression support. You have to code in its own mini-language, but back when I was in school people would write MEGS of source that we would swap around.

I've only ever run it on a *NIX system, but it looks like it has windows ports too.