js 邮政编码验证

function ischeck(){

var postcode = document.getElementById("postcode").value;
            if (postcode != "") {   //邮政编码判断
                var pattern = /^[0-9]{6}$/;
                flag = pattern.test(postcode);
                if (!flag) {
                    alert("非法的邮政编码！")
                    document.getElementById("postcode").focus();
                    return false;
                }
            }

}