<%!
//--------------------------------
// Filter out non-digit characters
//--------------------------------
private static String getDigitsOnly (String s) {
StringBuffer digitsOnly = new StringBuffer ();
char c;
for (int i = 0; i < s.length (); i++) {
c = s.charAt (i);
if (Character.isDigit (c)) {
digitsOnly.append (c);
}
}
return digitsOnly.toString ();
}
//-------------------
// Perform Luhn check
//-------------------
public static boolean isValid (String cardNumber) {
String digitsOnly = getDigitsOnly (cardNumber);
int sum = 0;
int digit = 0;
int addend = 0;
boolean timesTwo = false;
for (int i = digitsOnly.length () - 1; i >= 0; i--) {
digit = Integer.parseInt (digitsOnly.substring (i, i + 1));
if (timesTwo) {
addend = digit * 2;
if (addend > 9) {
addend -= 9;
}
}
else {
addend = digit;
}
sum += addend;
timesTwo = !timesTwo;
}
int modulus = sum % 10;
return modulus == 0;
}
%>
Kreditkartennummer 12345678 ist
<%= isValid("12345678") ? "gültig" : "ungültig" %>
Kreditkartennummer 3750 952354 91005 ist
<%= isValid("3750 952354 91005") ? "gültig" : "ungültig" %>