function checkInvoer(){
    
    var message = "De volgende gegevens ontbreken\n\n";
    var check = true;
    var field = document.getElementsByTagName("input");
    var textareas = document.getElementsByTagName("textarea");
    
    for(var i=0;i<field.length;i++){
    
        if(field[i].className == "mandatory") {
            if (field[i].value == ""){
                message = message + "\n- " +field[i].name.replace("_"," ");
                var check = false;
            }

        if( ( (field[i].name == "email") || (field[i].name == "eigenaar_email") )&& (field[i].value != "")){

            var filter = /^([a-zA-Z0-9_.-])+@(([a-zA-Z0-9-])+.)+([a-zA-Z0-9]{2,4})+$/;
            if(!filter.test(field[i].value)){
                message = message + "\n- " +field[i].name.replace("_"," ") +" : Dit is geen geldig email adres";
                var check = false;
            }
        }
        if((field[i].name == "telefoonnummer")||(field[i].name == "telefoon")||(field[i].name == "fax")||(field[i].name == "tel_nummer")||(field[i].name == "eigenaar_telefoon") || (field[i].name == "eigenaar_contactpersoon_tel_nummer")) {
            if (field[i].value != parseFloat(field[i].value)){
                message = message + "\n- " +field[i].name.replace("_"," ") +" : Dit is geen nummer";
                var check = false;
            }
        }
    }
}
for(var i=0;i<textareas.length;i++){
    if(textareas[i].className == "mandatory") {
        if (textareas[i].value == ""){
            message = message + "\n- " +textareas[i].name;
            var check = false;
        }
    }
}
if (check == false){
alert (message);
}
return check;
}