How to filter a set of urls by a constraint

index.php

<?php

$app = new Silex\Application();

$app->mount('/admin', new MyCode\MyControllerProvider());

MyControllerProvider.php

<?php

namespace MyCode;

use Silex\Application;
use Silex\ControllerProviderInterface;
use Silex\ControllerCollection;
use Symfony\Component\HttpFoundation\Response;

class MyControllerProvider implements ControllerProviderInterface
{
    public function connect(Application $app)
    {
        // creates a new controller based on the default route

        /** @var $controllers ControllerCollection */
        $controllers = $app['controllers_factory'];

        $controllers->get('/', function (Application $app) {
            return 'hello';
        }):

        $controllers->before(function() { throw new \Exception(); });

        return $controllers;
    }
}