<script>

function youtubeId(url) {

    var tag = "";

    if(url)  {

        var regExp = /^.*((youtu.be\/)|(v\/)|(\/u\/\w\/)|(embed\/)|(watch\?))\??v?=?([^#\&\?]*).*/;

        var matchs = url.match(regExp);

        if (matchs) {

            tag += "유튜브 아이디 : "+matchs[7];

        }

        return tag;

    }

}

    var s1 = "https://www.youtube.com/watch?v=pAWk-yh2ysc";

    var s2 = "http://youtu.be/WUzwkGmnAaw";

    document.write(youtubeId(s1));

    document.write("<br />");

    document.write(youtubeId(s2));

</script>