[Dilettante_Pro]

<input id="inp">
<input placeholder= "next field">
<script>
   inp.oninput = function() {
       this.value = this.value.replace(/\D/g,'').substr(0,13);
   }
   document.oninput= function() {
           if(inp.value.length < 13) inp.focus();
   }
</script>