Routes and Placeholders

In Config\Routes.php I created this roule

PHP Code:

$routes->group(SEGMENTURL_ADMIN, ['filter' => 'admin_role:superadmin,admin'], function($routes) {
    $routes->get('(:segment)', '$1_admin');
}); 


but the script return a 404 error (\App\Controllers$1_admin::index).

If I add any char before the placeholder, example

PHP Code:

$routes->group(SEGMENTURL_ADMIN, ['filter' => 'admin_role:superadmin,admin'], function($routes) {
    $routes->get('(:segment)', '_$1_admin');
}); 


the script run ok!!!


No problem without a group routes, for example 

PHP Code:

$routes->get('{locale}/(:segment)', '$1_frontend'); 


is there a bug or am I wrong something?

Thanks!