正则表达式验证小数是否非法

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN">
<HTML>
<HEAD>
<TITLE>正则表达式验证小数 </TITLE>
</HEAD>

<BODY>
<script language="JavaScript" type="text/javascript">
    function checkNum(obj)
    {
    var re = /^-?[1-9]*(\.\d*)?$|^-?d^(\.\d*)?$/;
     if (!re.test(obj.value))
     {
    if(isNaN(obj.value)){
        alert("非法数字");
        obj.value="";
        obj.focus();
        return false;
       }
     }
    }
    </script>
    <input name="rate" type="text"onkeyup="checkNum(this)" />
</BODY>
</HTML>