<html>
<head>
<script type="text/javascript">
function validate(){
x=document.myForm
input=x.myInput.value
if (input.length>5){
alert("The field cannot contain more than 5 characters!")
return false
}else {
return true
}
}
</script>
</head>
<body>
<form name="myForm" action="http://www.java2s.com" onsubmit="return validate()">
Enter some text (less than 5 characters):
<input type="text" name="myInput" size="20">
<input type="submit" value="Submit">
</form>
</body>
</html>