preg_replace('/[-]+/', '-', preg_replace('/[^a-zA-Z0-9_-]/s', '-', strtolower($string)));

[^a-zA-Z0-9_-]

[^a-zA-Z0-9_-]+
              ^- + = one or more

[^a-zA-Z0-9_]+

preg_replace('/[^a-zA-Z0-9_]+/s', '-', strtolower($string));

preg_replace('/[^a-z0-9_]+/s', '-', strtolower($string));

$urlSlug = preg_replace('/[^a-z0-9_]+/', '-', strtolower($string));

preg_replace('/[^a-zA-Z0-9_-]+/s', '-', strtolower($string));