Asp.net allow only alpha numeric value in password validation

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text.RegularExpressions;

using System.Web;

using System.Web.UI;

using System.Web.UI.WebControls;

namespace passwordformat

{

    public partial class Loginpage : System.Web.UI.Page

    {

        protected void Page_Load(object sender, EventArgs e)

        {

        }

        protected void Button1_Click(object sender, EventArgs e)

        {

            Regex alpha = new Regex("[a-z]");

            Regex numeric = new Regex("[0-9]");

            string password = Password.Text;

            string confirmPass = cnfPassword.Text;

            if (alpha.IsMatch(password) && numeric.IsMatch(password))

            {

                Successmessage.Text = "You are successfully registered";

                Successmessage.Visible = true;

                PasswordErrorMsg.Visible = false;

            }

            else

            {

                PasswordErrorMsg.Text = "Your password must be in alphanumeric";

                PasswordErrorMsg.Visible = true;

                Successmessage.Visible = false;

            }

        }

    }

}