public void Page_Load()
{
  Membership.ValidatingPassword +=
    new MembershipValidatePasswordEventHandler(OnValidatePassword);
}

public void OnValidatePassword(object sender,
                              ValidatePasswordEventArgs args)
{
  System.Text.RegularExpressions.Regex r =
    new System.Text.RegularExpressions.Regex(@"(?=.{6,})(?=(.*\d){1,})(?=(.*\W){1,})");


  if (!r.IsMatch(args.Password))
  {
    args.FailureInformation =
      new HttpException("Password must be at least 6 characters long and " +
                        "contain at least one number and one special character.");
    args.Cancel = true;
  }
}