Well i will leave it to smarter people then me whether this is a "bug" or not. But for me it is exactly what i would expect. If i'm assigning a route, and that route is supposed to have a number, then if the url does not have a number it should be a 404.
If however its a case of - it could have a number - or not - then assign a route for the other case. Then it will work for either condition:
PHP Code:
$route["admin/foos/(:num)"] = "admin/hello/$1";
$route["admin/foos"] = "admin/hello";
