$content = '<a href="https://www.youtu.be/llGF8G0BpfA.youtube" target="blank" rel="nofollow">PUT ANYTHING HERE</a><a href="https://www.youtu.be/llGF8G0Bpf.youtube" target="blank" rel="nofollow">PUT ANYTHING HERE</a><a href="https://www.youtu.be/llGF8G0BpfA.youtube" target="blank" rel="nofollow">PUT ANYTHING HERE</a>';
    $search = '/\<a(.+?)href="https:\/\/(www\.){0,1}((youtu\.be\/|youtube\.com(\/embed\/|\/v\/|\/watch(.+)v=){0,1})(.{10,12}))\.youtube"(.+?)<\/a>/i';
    $replace = '<iframe width="250" height="150" src="http://www.youtube-nocookie.com/embed/$7"></iframe>';
    $content = preg_replace($search, $replace, $content);
    print $content;