Leaving aside the interesting discussion of censorship (I wrote something rather like this for a kids' website myself -- it's different when you're dealing with kids and their parents... one complained to me that their kid had seen "the C-word" on our website. I was horrified and checked my code before realising they meant "crap"...) why has nobody said "Use A Module"?

There are a bunch of solutions if you search CPAN, and they have all kinds of features like storing the rude words in ROT-13 so that you see "shpx" instead of "fuck", plus using soundalike detection so that more creative miscreants can't get away with "phuk" either.