Esempio:
codice:
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN">
<html>
<head>
<title>test</title> 
<script type="text/javascript">

function Numeri(){
var imput = document.getElementById("Valore").value
if(imput.length>2){
alert("Inserire solo due numeri");
document.getElementById("Valore").value=imput.substring(0,imput.length-1)
}
else if(imput!="" && !/^([0-9])+$/.test(imput)){
alert('Inserire solo 2 numeri')
document.getElementById("Valore").value='';
}}
</script>
</head>
<body>
<form id="form1" method="post" action=""> 
<input name="Valore" id="Valore" type="text" value="" onkeyup="Numeri()" /> 
</form>
</body>
</html>