Checking string for invalid characters...

Hello, I was wondering if some some had a routine which would check a string for invalid characters. I figure the logic would be something of the following: The user passes me a string and I compare character by character against my array of "allowed" strings. but this is a while looop with a while loop and it can get kind of cumbersome. If any one already has a good routine or solution I would apreciate any help. Thanks Marcelo PS: Please reply to my email address since my mail system cannot handle me being on this list.