正則匹配，怎么保留兩位小數(shù)點(diǎn)

js代碼

var old = "";

        function check(ele) {
            var pattern = /^(\+|\-)?((([1-9]([0-9]*))|0)(\.[0-9]*)?)?$/;
            var val = ele.value;
            if(val.match(pattern)) {
                old = val;
                return true;
            } else {
                ele.value = old;
                return false;
            }
        }

html代碼
<input type="text" size="20" onkeyup="check(this)" >

這個正則怎么保留兩位小數(shù)點(diǎn)啊