How to validate website textbox?

Match with RegEx ^(http://)?\w+\.\w+\.\w+$ which satisfies your format of URL. You can customize it as per need.
Ref: http://msdn.microsoft.com/en-us/library/ms228595(v=vs.80).aspx[^]

Use regular expression validator
and set expression website url and controltovalidate to textbox id.

use this, it solves your problem.

you can add a regular expression validator like

<asp:TextBox ID="txtUrl" runat="server"></asp:TextBox>
    
<asp:RegularExpressionValidator 
EnableClientScript="false" 
ID="validatorUrl" 
runat="server"
ValidationExpression="(http|https)://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?"
ControlToValidate="txtUrl" 
SetFocusOnError="true" 
ErrorMessage="* Url not valid!" />

code behind :

if (this.validatorUrl.IsValid)
{
  // do stuff
}

this reg-ex is quite syple, there are a lot more complicated

or you can use Uri.TryCreate Method

public static bool TryCreate(Uri baseUri,Uri relativeUri,out Uri result)

Return Value :A Boolean value that is true if the Uri was successfully created; otherwise, false.

Hi here's the solution-

C# Code To validate site URL using Regular Expression:

using System;
using System.Text.RegularExpressions;

namespace MyApp
{
    public class Class1
    {
        public static void Main(string[] args)
        {
            string Site_URL = "http://IndiHub.com";
            Regex myRegex = new Regex(@"http(s)?://([\w-]+\.)+[\w-]+(/[\w- ./?%&=]*)?", RegexOptions.Compiled);
            bool bResult = myRegex.IsMatch(Site_URL);

            Console.WriteLine(Site_URL  + " Valid URL? " + bResult);

            Site_URL = "TestingWiz.com";

            bResult = myRegex.IsMatch(Site_URL);

            Console.WriteLine(Site_URL + " Valid URL? " + bResult);

            Console.ReadLine();
        }
    }
} 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.Resources;
using System.Net;
using System.Net.Sockets;

namespace WebApplication2
{
    public partial class _Default : System.Web.UI.Page
    {
        protected void Page_Load(object sender, EventArgs e)
        {

        }

        protected void Button1_Click(object sender, EventArgs e)
        {
          if (UrlIsValid(TextBox1.Text.ToString()))
          {
            Label1.Text = "Valid";
          }
            else
          {
            Label1.Text = "InValid";
          }

        }
        public static bool UrlIsValid(string smtpHost)
        {
            bool br = false;
            try
            {
                IPHostEntry ipHost = Dns.Resolve(smtpHost);
                br = true;
            }
            catch (SocketException se)
            {
                br = false;
            }
            return br;
        }

    }
}