I just want to thank everyone for the feedback so far, I am looking into the issues that have been brought up (they'll have to wait until this evening to be fixed though...I have a day job).
A graphical tool to create them, built of units instead of just strings, and the output could still be a regex string.
A question though: a lot of people seem to encounter issues/50x errors with the server-side calculation of the regexp. Why not have a client-side script to interpret the regexp?
Would save server ressources and wouldn't limit the user to server capacity to interpret more complex regexps.