<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN">
<html>
<head>
<title>Required Field</title>
<SCRIPT>
function checkRequired() {
var num=document.card.cardnum.value;
alert("your enter the value:"+num);
document.card.cardnum.value=37;
event.returnValue=false;
}
</SCRIPT>
</head>
<body>
<form action="clear.html" name="card" id="card" onSubmit="checkRequired();">
Enter your name:<br>
<input type="text" name="cardnum" size=10><br>
<input type="submit">
</form>
</body>
</html>