[nicce]

Här har du en för javascript också....

	function checkcardnumber(CardNumber)
	{
 var RegularExpression = /^[0-9]{13,16}$/i;
  
 if (! ValidateField(RegularExpression, CardNumber))
 {
 	alert('Malformed Card number.');
 	return false;
 }
 	
 sum = 0; mul = 1; l = CardNumber.length;
 for (i = 0; i < l; i++) 
 {
 	digit = CardNumber.substring(l-i-1,l-i);
 	tproduct = parseInt(digit ,10)*mul;
 	if (tproduct >= 10)
  sum += (tproduct % 10) + 1;
 	else
  sum += tproduct;
 	if (mul == 1)
  mul++;
 	else
  mul--;
 }
 if (!(sum % 10) == 0)
 {
 	alert('Malformed Card number.');
 	return false;
 }
	}
	function ValidateField(pattern, FieldStr)
	{
 return pattern.test(FieldStr);
	}