Javascript获取youtube视频ID（正则表达式问题）

I use:

<script type="text/javascript">
    document.addEventListener("DOMContentLoaded", function(event) {
        var field1 = document.getElementById("field_wy4dm0");
        field1.addEventListener("change", combineFields);

        function combineFields() {
            var val1 = field1.value;
            var val1re = val1.match("/(.*)?")[1];
            var str = document.getElementById("changeThisMovie").innerHTML;
            var res = str.replace("yeC3AisTs2Y", val1re);
            document.getElementById("changeThisMovie").innerHTML = res;
        }
    });
</script>
<script type="text/javascript">
    do