Replace special chars

[eluser]Shella[/eluser]
Hi,
I'm just seeking for some tips about how to manage a chars replacement.
My point is that in my system, when an user is registering, I create a directory.

Let's say... http://www.domain.com/user/nickname

where nickname is the nickname user had choose.

What I want is to grant that user do NOT insert characters as ? * à ... any kind of chars that you too would avoid.

Is there any Codeigniter function that can help me here?

Thanks

Shella