当文本域获得焦点时,字体自动消失代码

1.
protected void Page_Load(object sender, EventArgs e)
    {
        string JS = "document.getElementById('" + this.txt_a_etc.ClientID + "').value=''";
        this.txt_a_etc.Attributes.Add("onClick", JS);
        ............
   }
2.
<asp:TextBox CssClass="LineTxtArea" ID="txt_LimitRemark" runat="server"  TextMode="MultiLine" onmousedown="if(this.innerText=='请输入其他限制事项'){this.innerText='';}">请输入其他限制事项</asp:TextBox>

3.判断输入为数字
  <asp:TextBox CssClass="LineTxt175" ID="txt_Rent" runat="server" onblur="if(/^(\d)+$/.test(this.value)){}else{alert('请输入数字!')}"></asp:TextBox>
4.输入不管多长,一旦停输入自动消失
<asp:TextBox CssClass="LineTxt175" ID="txt_Rent" runat="server"　onkeyup="this.value=this.value.replace(/[^\d]/ig,'');"></asp:TextBox>