keycode

<input type="number" id="myid"/>
<script>
    var parentValue;
    var myid=document.getElementById("myid");
    myid.onkeyup=function(e){
        var keyCode=e.keyCode;
        var value=this.value;
        if(!value){
            value=(keyCode==8)?"":parentValue;
        }else{
            //正则
        }
        parentValue=value;
        myid.value=value;
        alert(this.value);
    }
</script>