Add Submenu class

class My_Walker_Nav_Menu extends Walker_Nav_Menu {
  function start_lvl(&$output, $depth = 0, $args = Array() ) {
    $indent = str_repeat("\t", $depth);
    $output = preg_replace( "/(.*)(\<li.*?class\=\")([^\"]*)(\".*?)$/", "$1$2$3 dropdown$4", $output );
        $output .= "\n$indent<ul class=\"sub-menu\">\n";
  }
}