iOS keyboard - limit number of keyboard characters

Im wanting to limit the number of characters the user can put in the keyboard input box. I know how to delete the extra letters once the user has typed them, however i do not want them to be able to type them in the first place as then they just disappear. here is the relevant code i have used:

 var text : String;
 private var keyboard : iPhoneKeyboard;
 
 function Update(){
 if (keyboard.text.length > 9){
         keyboard.text = keyboard.text.Substring(0, 9); 
     }
 }

any help would be greatly appreciated, thanks