Routing problem, always return a fixed value in handle

I have a regex that matches any slug likes this:

dan-huyen
dan-huyen-123
123-dan-huyen

and not match the following:

123
123456

Here is the regex:

$theregex = '/^(?=.*[a-z])[a-z0-9-]+$/';

The regex has been tested, and I make sure that it worked perfectly. Demo test: https://regex101.com/r/fQ1iD5/17

Then I use this regex into my Micro App to catch any request. This is one of my routing:

$v1->get("/{class:$people_restricted}/{slug:(?=.*[a-z])[a-z0-9-]+}", 'getOneBySlug');

This is the function that handled the request if routing matched:

    public function getOneBySlug($class, $slug) {

        $object = get_result($class, 'findFirstBySlug', $slug);

        if ($object) {
            $clean_obj = $this->filter($class, $object);
            return array('status' => 'ok', 'data' => $clean_obj);
        } else {
            $messages = array();
            $messages[] = 'Object not found';
            return array('status' => 'error', 'messages' => $messages);
        }
}

I've tried the routing above and it does match the request like this:

/composer/dan-huyen?fields=id,title,content,image,views&extra_fields={song:fields=id,title,slug},{document:fields=id,title,slug}

But instead of return the right slug in function getOneBySlug ($slug = 'dan-huyen'), it always returns the value '2' of $slug variable ($slug = 2); The $class variable return the right value ('composer')

So may I miss something where?

Update: I changed my question title from "How to route use complex regex" to "Routing problem, always return a fixed value in handle" because I missed something on cheking the result.