Código HTML:
Ver original<input type="text" name="txt" id="txt" onkeypress="return valida(event);"/>
Código Javascript
:
Ver originalfunction valida(e){
var r = /[8]/;
var evt = e.keyCode ? e.keyCode : e.which;
var txt = document.getElementById('txt');
var value = txt.value + String.fromCharCode(evt);
return r.test(value);
}