How to validate 10 digit mobile number in asp.net with regular expression control.

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"

    CodeFile="Default.aspx.cs" Inherits="_Default" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">

</asp:Content>

<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">

    <div>

        <table>

            <tr>

                <td>

                    <asp:Label ID="Label4" runat="server" Text="Mobile No:" Font-Bold="true"></asp:Label>

                </td>

                <td>

                    <asp:TextBox ID="txt_mobile" CssClass="textform" runat="server"></asp:TextBox>

                </td>

                <td>

                    <asp:RequiredFieldValidator ID="RequiredFieldValidator4" runat="server" ForeColor="Red"

                        ControlToValidate="txt_mobile" Font-Size="X-Small" ErrorMessage="Please enter mobile number"

                        ValidationGroup="valid"></asp:RequiredFieldValidator>

                    <asp:RegularExpressionValidator ID="rfvMobil1" Font-Size="XX-Small" ForeColor="Red" ControlToValidate="txt_mobile"

                        ValidationExpression="^[7-9][0-9](\s){0,1}(\-){0,1}(\s){0,1}[0-9]{1}[0-9]{7}$"

                        runat="server" ErrorMessage="Please enter Valid MobileNo" ValidationGroup="valid"

                        Display="Dynamic"></asp:RegularExpressionValidator>

                </td>

            </tr>

            <tr>

                <td class="barhds">

                </td>

                <td align="left">

                    <asp:Button ID="btnsubmit" runat="server" Text="Submit" ValidationGroup="valid" />

                </td>

            </tr>

        </table>

    </div>

</asp:Content>