Out of the box the laravel router doesn't support this. You can use the Enhanced Router package from Jason Lewis or a fork that enables support for Laravel 4.2
Alternatively you can do it yourself. You could basically add the where condition to every route inside the group:
Route::group(['prefix' => 'foo/{bar}'], function() {
Route::get('/', function(){
// ...
})->where('bar', '[0-9a-Z]+');
});
Or do it a bit more dynamic and add this at the bottom of your route group:
Route::group(['prefix' => 'foo/{bar}'], function($group) {
// ...
foreach($group->getRoutes() as $route){
$route->where('bar', '[0-9a-Z]+');
}
});