Cannot strip JW Player shortcode?

JW Player Plugin for WordPress does not register its shortcode like all other shortcodes, so strip_shortcodes() will not know about it and not strip it. In the code there is a note that this is because it uses argument names with a . in it, and WordPress does not support this.

There are probably multiple ways to solve this, but I would copy the relevant line from strip_shortcodes() and integrate the regex from the plugin:

function custom_excerpt( $length, $more_text ) {
    global $post;
    $text = get_the_content();
    $text = strip_shortcodes( $text );
    // Strip JW Player shortcode too
    $text = preg_replace( '/(.?)\[(jwplayer)\b(.*?)(?:(\/))?\](?:(.+?)\[\/\2\])?(.?)/s', '$1$6', $text );
    $text = apply_filters('the_content', $text);
    // Continue with the rest of your function ...