Problem with Route_To

I have programmed the following route which works correctly, it uses a regex

Code:

$routes->addPlaceholder('slug_alfa', '[a-z0-9]+[\-]?[a-z0-9]+([\-][a-z0-9]+)*');
$routes->get('(:slug_alfa)/sitios-interes', 'Web_Empresa::sitio_interes/$1',["as"=>"web_empresa_servicios_interes"]);

But when using route_to function it fails, for example

PHP Code:

route_to("web_empresa_servicios_interes",'slug') 


preg_match(): Compilation failed: missing closing parenthesis at offset 41
SYSTEMPATH/Router/RouteCollection.php at line 1351