function isEmpty(str){
	return (str == null) || (str.length == 0);
}
function isEmail(str){
	if(isEmpty(str)) return false;
	var re = /^[^\s()<>@,;:\/]+@\w[\w\.-]+\.[a-z]{2,}$/i
	return re.test(str);
}
function validate(f){
	var errors = "";
	if(isEmpty(f.naam.value)) errors += "Please fill in your name\n";
	if(!isEmail(f.email.value)) errors += "Please fill in a valid emailaddress\n";
	if(isEmpty(f.opmerkingen.value)) errors += "Please fill in a comment\n";
	if(errors != "") alert(errors);
	return errors == "";
}