Add video tag to youtube link fron content

You can try doing this:

$string =  "Some text before the link http://www.youtube.com/?123 some text after the link";
preg_match_all('@(https?://www\.?youtube.com/.[^\s.,"\']+)@i', $string, $matches);
var_dump($matches);

In your case would be something similar to this:

preg_match_all('@(https?://www\.?youtube.com/.[^\s.,"\']+)@i', $post->post_content, $matches);
foreach($matches as $match){
    echo "<video>$match</video>";
}