Custom routes

You can try something like this:

Code:

$route['login/(.+)'] = 'auth/login/$1';

This is will capture all segments after 'login'. 
I would be a bit hesitant to use it since if you have route that starts with the '/login' it will be overwritten. 

So the following construction will be automatically redirected to 'auth/login/$1'

Code:

$route['login/app] = 'auth/app/$1';