How to create dynamically controllers function name in codeigniter

[eluser]beeLoop[/eluser]
Hello,
I am creating a fashion oriented site in codeigniter.
Now my requirement is: i need url in form of "http://example.com/user_name"

For-example: http://example.com/beeLoop.

So , what are the possible ways to create dynamically controllers function name in codeigniter.