JS判断IP合法性

<title>IP地址合法性验证js代码 - </title>
<script language="javascript">
function checkIP()
{
obj=document.getElementByIdx_x("ip").value
//ip地址
var exp=/^(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])\.(\d{1,2}|1\d\d|2[0-4]\d|25[0-5])$/;
var reg = obj.match(exp);
if(reg==null)
{
alert("IP地址不合法！");
}
else
{
alert("IP地址合法！");
}
}
</script>

请输入IP地址：<input type="text" id="ip">
<input type="button" onclick="checkIP()" value="验证">