how to put username after domain name

[eluser]cartalot[/eluser]
if you can deal with one extra word in the URL it will greatly simplify what you want to do
something like 'users'

www.xyz.com/users/abc

and then set up a route like
$route['users/(:any)'] = "users/display";

where your controller is called users, with a method called display, that searches for the user and displays them.
note that - whats cool is that the route can direct to any controller and method, it doesn't need to be the same name,

$route['users/(:any)'] = "customers/find";