鼠标只能输入数字并且只能保留两们小数的函数

                <input type="text" id="txt_purchaseItem_Count" class="textInput" onkeyup="TranTotal();"
                    onkeypress="return CheckKey(this,event);" />
                <span name="txt_purchaseItem_Count" value="数量不能为空！" regexp="empty" style="color: Red">
                    *</span>

//计算值
function TranTotal() {
    var purchaseItem_Count = document.getElementById("txt_purchaseItem_Count").value;
    var purchaseItem_UnitCost = document.getElementById("txt_purchaseItem_UnitCost").value;
    document.getElementById("txt_TotalValue").value = Math.round((purchaseItem_Count * purchaseItem_UnitCost) * 100) / 100;
}
//验证只能输入数字，最多只能两们小数点
function CheckKey(obj, e) {
    var key = window.event ? e.keyCode : e.which;
    var keychar = String.fromCharCode(key);
    reg = /\d|\./;
    var result = reg.test(keychar);
    if (result) {
        if (e.keyCode == 46)
            result = !(obj.value.split('.').length > 1);
        else
            result = !(obj.value.split('.').length > 1 && obj.value.split('.')[1].length > 1);
    }
    if (!result) {
        return false;
    }
    else {
        return true;
    }
}