in reply to
Re: How will my regular expression match?
in thread How will my regular expression match?
I use this a lot, usually with the shorter command-line invocation ('-Dr', available if perl was built with debugging enabled). However the output was designed primarily for internals debugging, and is likely to be more scary than helpful to the casual user.
Various people have taken advantage of the information it provides to offer a friendlier interface to regexp debugging. Though I haven't tried any of them, I'm aware of ReBug (I believe this is open source, but I can't get to the website: the author gave a talk on it at the 2001 OSCON), Komodo (commercial, with "try before you buy"), and The Regex Coach ("donationware", donate if you like it) which was recently mentioned on PM.