<script type="text/javascript">
var ary = ["\\d", "\\d", ":"];
function checkval(val) {
var num = val.length;
if (num < 8) {
var pat = new RegExp(ary[num % 3]);
var inputval = String.fromCharCode(window.event.keyCode);
return pat.test(inputval);
} else return false;
}
</script>
<input onkeypress="return checkval(this.value);" />