How to use the route mydomain.com/parameter

Hello,

I am stuck with a routing problem,

I would like to be able using the route http://mydomain.com/parameter

Like facebook use it for profiles http://facebook.com/username

I tried using the route 
$route['(.*)'] 

But i don't really like this solution because i have to declare all my usefull routes before it into routes.php
Because if i don't all routing request will correspond to this rule

Is anybody has already use a good solution for this problem ?

Thankk you very much for your help