naming a directory after a user-submitted string

Thanks for the thoughts on .sit files. I still don't quite know what I'll do (maybe I'll just force'em to make .zips -- it's not like it's IMPOSSIBLE for a Mac to make .zips). Here's another question, possibly easier. Possibly even bone-headed. What kind of checking/filtering/changing do I need to do on a user-submitted string before I can feel comfortable using it to name a new directory in the web root on Linux/Apache? Anybody have a quick Regular Expression they can toss at me? If so, I'd be muchly appreciative. Or is this just a Terrible Idea That Should Never Be Contemplated? Thanks! Joey www.moderntales.com