function checkContactForm() {
  
  var form = document.getElementById('contact_form');
  
  var flag;
  var str = 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(form.fname.value == 'Voornaam') {
    //form.fname.className = "form_error";
    alert("Vult u uw voornaam in aub");
    form.fname.focus();
    
  } else if(form.lname.value == 'Achternaam') {
    //form.lname.className = "form_error";
    alert("Vult u uw achternaam in aub");
    form.lname.focus();
    
  } else if(form.email.value == 'E-mailadres') {
    //form.email.className = "form_error";
    alert("Vult u uw email adres in aub");
    form.email.focus();
  
  } else if(!flag) {
    //form.email.className = "email_error";
    alert("Het email adres is incorrect");
    form.email.focus();
    
  } else if(form.mssg.value == 'Uw vraag') {
    //form.mssg.className = "form_error";
    alert("Vult u een bericht in aub");
    form.mssg.focus();
    
  } else {
  
    var agree = confirm("Verzenden?");
    if(agree) {
      form.submit()
    }
  }
}