<script type="text/javascript">
    function test(){
        var o = "23655";
        var a = 14;
        var b = 5;
        //var pt = /^\d{1,iLength}(\.\d{1,dLength})?$/;
        var pt = /"\d{1,"+a+"}"/;
        if(!pt.test(o)){   
            alert("a");
        }else{
            alert("b");
        }

    }
</script>

------解决方案--------------------

上面的写错了。应该是这样
JScript code

var a = 14;
          var b = 5;
          eval("var pt = \/^\\d{1,"+a+"}(\\.\\d{1,"+b+"})?$\/");
          if(!pt.test(o)){   
              return false;
          }