function validate_address() {
  var validity = true;

  validity = check_empty(document.forms[0].orchester);
  validity = check_empty(document.forms[0].traeger) && validity;
  validity = check_empty(document.forms[0].stadt) && validity;
  validity = check_country(document.forms[0].land) && validity;
  validity = check_empty(document.forms[0].name) && validity;
  validity = check_empty(document.forms[0].strasse) && validity;
  validity = check_empty(document.forms[0].plz_ort) && validity;
  validity = check_email(document.forms[0].email) && validity;
  validity = check_empty(document.forms[0].jahr) && validity;
  validity = check_empty(document.forms[0].anzahl) && validity;
	
	if (!validity) {
    alert("Bitte überprüfen Sie Ihre Eingabedaten!\nPlease check your data!");
  }
	return validity;
}

function check_empty(element) {
  if (element.value.length == 0) {
	  mark_error(element);
		return false;
	}
	else {
	  unmark_error(element);
	  return true;
	}
}

function check_country(element) {
  if (element.options[0].selected) {
	  mark_error(element);
		return false;
	}
	else {
	  unmark_error(element);
	  return true;
	}
}

function check_email(element) {
  var address = element.value;
  if ((address == "")
      || (address.indexOf ('@') == -1)
      || (address.indexOf ('.') == -1)) {
		mark_error(element);
    return false;
	}
	else {
	  unmark_error(element);
    return true;
	}
}

function mark_error(element) {	
	element.className= "notValid";
}

function unmark_error(element) {	
	element.className= "";
}