input text如何通过js实现限制输入啊

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<script type="text/javascript">
window.onload=function(){
var i=document.getElementById("test");
var reg=/^\d+\.\d*$/;
i.onkeydown=function(e){
var a=e||window.event;
if((a.keyCode>=48&&a.keyCode<=57)||(a.keyCode>=96&&a.keyCode<=105)){
}else if(a.keyCode==190||a.keyCode==110){
prevent(a);
if(!reg.test(this.value)&&this.value!=''){
this.value+='.';
}
}else if(a.keyCode==8){

}else{
prevent(a);
}
}
}
function prevent(x){
if(x.preventDefault){
x.preventDefault();
}else{
x.returnValue=false;
}
}
</script>
</head><body>
<input type="text" id="test" />
</body>
</html>
其他情况自己加些判断试试