function checkForm(){
    document.getElementById('error').innerHTML = "";
    var error = 0;
    
    if(!validateName()){
       document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Er is geen naam ingevuld.<br />';
       error++; 
    }
    
    if(!validateFirstName()){
       document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Er is geen voornaam ingevuld.<br />';
       error++; 
    }
    
    if(!validateEmail()){
       document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Er is geen e-mail ingevuld, of deze is niet correct.<br />';
       error++; 
    }
    
    if(!validateAddress()){
       document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Er is geen straat en huisnummer ingevuld.<br />';
       error++; 
    }
    
    if(!validatePostal()){
       document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Er is geen postcode ingevuld, of deze is niet correct.<br />';
       error++; 
    }
    
    if(!validateMunicipality()){
       document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Er is geen gemeente ingevuld.<br />';
       error++; 
    }
    
    if(!validateTelephone()){
       document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Er is geen telefoonnummer ingevuld.<br />';
       error++; 
    }
    
    if(error > 0){
        return false;
    }else{
        return true;
    }
}

function validateEmail() {
   var reg = /^([A-Za-z0-9_\-\.])+\@([A-Za-z0-9_\-\.])+\.([A-Za-z]{2,4})$/;
   var address = document.tender.email.value;
   if(reg.test(address) == false) {
      document.getElementById('email').setAttribute("class", "e");
      <!--document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'Het e-mailadres is niet correct.<br />';-->
      return false;
   }else{
        document.getElementById('email').setAttribute("class", "ok");
    return true;
   }
}

function validateName(){
    if(document.tender.name.value == ""){
        document.getElementById('name').setAttribute("class", "e");
        return false;
    }else{
        document.getElementById('name').setAttribute("class", "ok");
        return true;
    }
}

function validateFirstName(){
    if(document.tender.firstname.value == ""){
        document.getElementById('firstname').setAttribute("class", "e");
        return false;
    }else{
        document.getElementById('firstname').setAttribute("class", "ok");
        return true;
    }
}

function validateAddress(){
    if(document.tender.address.value == ""){
        document.getElementById('address').setAttribute("class", "e");
        return false;
    }else{
        document.getElementById('address').setAttribute("class", "ok");
        return true;
    }
}

function validatePostal() {
   var reg = /^([0-9_\-\.])+([0-9_\-\.])+([0-9_\-\.])+([0-9_\-\.])$/;
   var Postal = document.tender.postal.value;
   if(reg.test(Postal) == false) {
      document.getElementById('postal').setAttribute("class", "e");
      <!--document.getElementById('error').innerHTML = document.getElementById('error').innerHTML+'U mag maar 4 cijfers gebruiken<br />';-->
      return false;
   }else{
        document.getElementById('postal').setAttribute("class", "ok");
    return true;
   }
}

function validateMunicipality(){
    if(document.tender.municipality.value == ""){
        document.getElementById('municipality').setAttribute("class", "e");
        return false;
    }else{
        document.getElementById('municipality').setAttribute("class", "ok");
        return true;
    }
}

function validateTelephone(){
    if(document.tender.telephone.value == ""){
        document.getElementById('telephone').setAttribute("class", "e");
        return false;
    }else{
        document.getElementById('telephone').setAttribute("class", "ok");
        return true;
    }
}