@strmizo 1stly you dont have to validate. If you do not have username in your route it will throw NotFoundException. And still if you want to access it you can do like this:
$request->username
or
$request->route('username')
But It will be preferable to use route model binding for such things. Have a look at doc http://laravel.com/docs/5.0/routing#route-model-binding