Security Hazard!

Are you sure you want to evaluate a user regexp? The user can insert a (?{...}) construct to include some Perl executable code, which can in turn include a call to system, or backticks, which can contain very bad things.

And there is more than one way to do nasty things. The user can write a Denial-of-Service regexp, a regexp which backtracks a lot and locks your machine. See Mastering Regular Expressions page 140+ (the book with two owls on the cover)

Update

Yes, Perlplexer, you are right. The security hazards are mostly irrelevant in a Perl/Tk program. Yet, my warning still holds for any monk who would use the answers in this thread for a CGI program (or any client-server app for that matter).