function isNumeriekeToets(evt){
	var charCode = (evt.which) ? evt.which : event.keyCode
	if (charCode > 31 && (charCode < 48 || charCode > 57))
		return false;

	return true;
}

function tekensOver(tekstveld, teruggave, maxTekens){
	
	if(tekstveld.value.length > maxTekens){
		tekstveld.value = tekstveld.value.substring(0, maxTekens);
	}
	
	document.getElementById(teruggave).innerHTML = maxTekens - tekstveld.value.length;
	
}

function setAnoniem(checkbox){
	
	document.getElementById('afzender').disabled = checkbox.checked;
	
}

function popup(url, breedte, hoogte, extra){
	
	var left = (screen.width-breedte)/2; 
	var top = (screen.height-hoogte)/2; 
	window.open(url, 'Popup'+Math.round(Math.random()*500), 'width='+breedte+',height='+hoogte+',left='+left+',top='+top + ','+ extra);
	
}