Remove Controller

I have to remove controller name from the URL in CodeIgniter for my website but my others controller is not working .
its showing this message " The page you requested was not found ".

I used following code for remove my website controller name from routers.php:

$default_controller = "webportal";
$route['default_controller'] = $default_controller;
$controller_exceptions = array('admin','email');

$route['default_controller'] = $default_controller;
$route["^((?!\b".implode('\b|\b', $controller_exceptions)."\b).*)$"] = $default_controller.'/$1';
$route['404_override'] = '';