function checkContactForm( form ) {
  if (form.naam.value == "") {
    alert( "Pleas fill in your name." );
    form.naam.focus();
    return false ;
  }

  if (form.achternaam.value == "") {
    alert( "Pleas fill in your lastname." );
    form.achternaam.focus(); 
    return false ;
  }
  if (form.adres.value == "") {
    alert( "Pleas fill in your address." );
    form.adres.focus(); 
    return false ;
  }
  if (form.postcode.value == "") {
    alert( "Pleas fill in your zipcode." );
    form.postcode.focus();
    return false ;
  }
  var postcode = form.postcode.value;
  valid = new RegExp("^[0-9]{4}[a-zA-Z]{2}$").exec(postcode);
  if(!valid) { 
    alert( "Pleas fill in a valid zipcode.\n ex. 1000AA " );
    form.postcode.focus();
    return false;
  }
if (form.plaats.value == "") {
    alert( "Pleas fill in a city." );
    form.plaats.focus();
    return false ;
  }
	if (form.land.value == "") {
    alert( "Pleas fill in a country." );
    form.land.focus();
    return false ;
  }
	if (form.tel.value == "") {
    alert( "Pleas fill in your phone number." );
    form.tel.focus();
    return false ;
  }
	var tel = form.tel.value;
  valid = new RegExp("^[0-9]{10}$").exec(tel);
  if(!valid) { 
    alert( "Pleas fill in a valid phone number.\n ex. 0206666666" );
    form.tel.focus();
    return false;
  }

	if (form.email.value == "") {
    alert( "Pleas fill in your email." );
    form.email.focus();
    return false ;
  }
  var emailAdres = form.email.value;
  valid = new RegExp("^[a-zA-Z]+([.]?[a-zA-Z0-9_-]+)*[@]([a-zA-Z0-9_-]+[.])+([a-zA-Z]{2,4})$").exec(emailAdres);
  if(!valid) { 
    alert( "Pleas fill in a valid email." );
    form.email.focus();
    return false;
  }

  if (form.bericht.value == "") {
    alert( "Pleas fill in your message." );
    form.bericht.focus();
    return false ;
  }
  return true;
 }