NSFormatter usage

Hi guys,
I'm trying to read as much as I can before posting, but sometimes I  
just can't find what I'm looking for...
So actually I'm wondering if I could use NSFormatter on a WOTextField  
to apply certain rules:
let's say I have a WOTextField for the user to enter his email. I  
want to make sure it will be lowercase (just an example). Assuming I  
already have in my code a method that take a string as an argument  
and return it lowercased, how I would "create" the correct NSFormatter?

I'm asking this because the other way I can think of is to create an  
instance variable to store the email entered, then pass this object  
to my toLowerCase method and finally call the setEmail method.
Which is of course not that funny;)

So I was wondering if I could call my toLowerCase method using a  
NSFormatter.

toLowerCase is of course just to illustrate my meaning;)

In fact, what I'd like to do is to "format" the text input before  
making any validation on it. I think I could use many stuff to  
achieve that, and most probably things more advanced/powerful like  
stuff from Wonder, but I don't feel like I'm ready for it;) still too  
beginner:)

If someone could give me his opinion, regarding both the NSFormatter  
part or the "way of thinking";)

Thanks a lot

Xavier