codice:
<html> <head> 
<script type="text/javascript"> 
function numero(evt) {
//se c'è un errore
document.getElementById("errorBox").style.display="inline";
}    
</script> 
</head>  
   <body> 
      <form action='prova.php' method="GET"> 
         <input type="text" name="codice" onkeypress="return numero(event);"> 
         <input type="submit"> 
<div id="errorBox" style="display:none">Errore</div>
      </form> 
   </body> 
</html>