<html>
<meta charset="utf-8">
<input id="i">
<div id="out"></div>
<script>
parse=function(s){
var arr=s.split(/(?=..$)/)
if(arr.length<2) return "0 руб "+arr[0]+"коп"
return arr[0]+"руб "+arr[1]+"коп"
}
i.oninput=function(){
out.innerHTML=parse(this.value)
}
</script>
</html>