Server Side Validation to allow only numbers (Numeric TextBox) in ASP.Net

You can do with Custom Validator

    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>
    <asp:CustomValidator ID="CustomValidator1" runat="server" ErrorMessage="Only numbers" OnServerValidate = "Validate_Numeric" ControlToValidate = "TextBox1"></asp:CustomValidator>
    <asp:Button ID="Button1" runat="server" Text="Button" />

  protected void Validate_Numeric(object sender, ServerValidateEventArgs e)
    {
        System.Text.RegularExpressions.Regex r = new System.Text.RegularExpressions.Regex("^[0-9]+$");
        e.IsValid = r.IsMatch(TextBox1.Text);
    }