function checkForm() {
  
  var flag;
  var str = document.contact_form.email.value;
  var pattern = /^([a-zA-Z0-9])+([\.a-zA-Z0-9_-])*@([a-zA-Z0-9_-])+(\.[a-zA-Z0-9_-]+)+/;
  var flag = pattern.test(str);
  
  if(!document.contact_form.email.value=='') {
    if(!flag) {
      alert ("Ongeldig email adres");
      document.contact_form.email.focus();
      return false;
    }
  } else {
    alert("Vul uw email in aub");
    document.contact_form.email.focus();
    return false;
  }
  
  if(document.contact_form.mssg.value=='') {
    alert("Vul een bericht in aub");
    document.contact_form.mssg.focus();
    return false;
  }
  
  var agree = confirm("Verzenden?");
  if(agree) {
    return true;
    } else {
    return false;
  }
  
}