I can't test this unfortunately, I don't use the code as my WP theme has the YouTube feature integrated. 😉 

OR the code is ok and wpForo somehow adds that for some reason.

I hope that stray tag doesn't cause any issue (i think it doesn't).

And thanks again for your great community help here.

the code (without wpForo content) looks like this

function convertYoutube($string) {
    return preg_replace(
        "/\s**youtu(be.com\/watch\?v=|.be\/)(+)(*)/i",
        "<iframe src=\"//www.youtube.com/embed/$2\" allowfullscreen></iframe>",
        $string
    );
}

remove

<a+>

and use

/\s**youtube.com\/watch\?v=(+)(*)/i

/\s**youtu(be.com\/watch\?v=|.be\/)( +)(*)/i

I see a stray

</a>

in the result at the end. I mean using the unaltered code, just like Robert posted it.

ok, I use the snippet plugin that adds "<?php" automatically

you have to use the CSS code from the link above and the CSS class from the link too

> jquery wrap an iframe in div

Also it doesn't seem to work at all.

But thanks !!!