protected void loginbutton_Click(object sender, EventArgs e)
    {
        string UsernameRegex = "[a-zA-Z]+";
        string PasswordRegex = "[a-zA-Z0-9]+";

        if (!Regex.IsMatch(usernametextbox.Text, UsernameRegex))
        {
            string UsernameCheck = "valid";
        }
        else
        {
            string UsernameCheck = "invalid";
        }

        if (!Regex.IsMatch(passwordtextbox.Text, PasswordRegex))
        {
            string PasswordCheck = "valid";
        }
        else
        {
            string PasswordCheck = "invalid";
        }


        if(UsernameCheck = "valid") //i will include password here after i solved the problem
        {
            //do something
        }
            SqlConnection conn = new SqlConnection(ConfigurationManager.ConnectionStrings["DefaultConnection"].ConnectionString);
            conn.Open();
            string checkuser = "select count(*) from Users where Username = @username and Password = @password";

            SqlCommand com = new SqlCommand(checkuser, conn);
            com.Parameters.Add("@username", SqlDbType.NVarChar).Value = usernametextbox.Text;
            com.Parameters.Add("@password", SqlDbType.NVarChar).Value = passwordtextbox.Text;

            int temp = Convert.ToInt32(com.ExecuteScalar().ToString());

            if (temp > 0)
            {
                Response.Redirect("Cars.aspx");
            }
            else
            {
                loginfaillabel.Text = "Your Username or Password doesn't match our records";
            }
        }