How to add custom username validation logic?

add_filter('nsl_validate_username', function ($isValid, $username, $errors) {
    if (strlen($username) < 6) { $errors->add('invalid_username', '' . __('ERROR') . ':' . __('Sorry, username must contain at least 6 characters.'));
        $isValid = false;
    }
    return $isValid;
}, 10, 3);