How to make request to non-existent controller mapped to given controller?

[eluser]Morgan Cheng[/eluser]
I noticed that, in twitter, each person's home page has only one segment in path, such as "http://twitter.com/morgancheng". I'm wondering how to implement this in CodeIgniter. Obviously, I cannot make controller for each user. All I want is to map non-existent controller to a given controller. In that controller I will parse the URL path and fetch data for given user.

How to do it in CodeIgniter?