使用正则表达式和replace替换一个字符串中截取的一字符串

aspx中：

<table>
                <tr>
                    <td style=" 178px" colspan="4">
                    <asp:Label id="Label1" runat="server" Text="输入要截取的字符串" CssClass="input_border1"></asp:Label>
                    </td>
                   
                </tr>
                <tr>
                   
                    <td colspan="2" >
   <asp:TextBox id="txtString" runat="server" CssClass="input_border1"></asp:TextBox></td>
                    <td style=" 621px" colspan="2">
   <asp:Button id="btnSubmit" runat="server" Text="截取" onclick="btnSubmit_Click" CssClass="btn_pic1"></asp:Button>&nbsp;
                    </td>
                </tr>
                <tr>
                  
                    <td style=" 203px" colspan="4">
   <asp:Label id="lblMessage" runat="server"></asp:Label></td>
                   
                </tr>
            </table>

aspx.cs中：
 public static Regex RX = new Regex(@"^-?[1-9]\d*|[[\u4e00-\u9fa5]+]$",RegexOptions.IgnoreCase);
  protected void btnSubmit_Click(object sender, System.EventArgs e)
  {
            this.lblMessage.Text = "";
            string str = this.txtString.Text;
            if (RX.IsMatch(str))
            {
                int indexBegin = str.IndexOf("[");
                int indexEnd = str.LastIndexOf("]");
                string endstr = str.Substring(indexBegin + 1, indexEnd - indexBegin - 1);//截取“[ ]”里的文本
                Regex Rx = new Regex(@"[\u4e00-\u9fa5]+", RegexOptions.IgnoreCase);
                String resultStr = "<font color=red>" + endstr + "</font>";
                string ResultAll = Rx.Replace(endstr, resultStr);
                // lblMessage.Text = ResultAll;
                Regex Rex = new Regex(@"[[\u4e00-\u9fa5]+]");
                this.lblMessage.Text = Rex.Replace(str, "[" + ResultAll + "]");
            }
            else
            {
                //return;
                lblMessage.Text = str;
            }
  }