WP_Customize_Manager::_validate_external_header_video PHP Method

    public function _validate_external_header_video($validity, $value)
    {
        $video = esc_url_raw($value);
        if ($video) {
            if (!preg_match('#^https?://(?:www\\.)?(?:youtube\\.com/watch|youtu\\.be/)#', $video)) {
                $validity->add('invalid_url', __('Please enter a valid YouTube URL.'));
            }
        }
        return $validity;
    }