function checkEmail(address)
{
  var filter = /^([\w-]+(?:\.[\w-]+)*)@((?:[\w-]+\.)*\w[\w-]{0,66})\.([a-z]{2,6}(?:\.[a-z]{2})?)$/i;
  if (filter.test(address)) return true;
  else return false;
}

function checkForm(formName,isBrochure)
{
  var form = document.getElementById(formName);
  var error = false;
  var errorMsg = "De volgende velden zijn niet (correct) ingevuld:\n";
  if (form.firstName.value == null || form.firstName.value == "") { error = true; errorMsg += "\n- voornaam"; }
  if (form.lastName.value  == null || form.lastName.value  == "") { error = true; errorMsg += "\n- achternaam"; }
  if (form.address.value   == null || form.address.value   == "") { error = true; errorMsg += "\n- straat + nummer"; }
  if (form.zipcode.value   == null || form.zipcode.value   == "") { error = true; errorMsg += "\n- postcode"; }
  if (form.city.value      == null || form.city.value      == "") { error = true; errorMsg += "\n- plaats"; }
  if (form.phone.value     == null || form.phone.value     == "" || isNaN(form.phone.value)) { error = true; errorMsg += "\n- telefoonnummer"; }
  if (form.email.value != null && form.email.value != "" && !checkEmail(form.email.value)) { error = true; errorMsg += "\n- e-mail"; }
  if (isBrochure == "0") { if (form.info.value == null || form.info.value == "") { error = true; errorMsg += "\n- informatie over"; } }
  if (error) alert(errorMsg);
  else document.getElementById(formName).submit();
}