function ValidateCreditCardNumber(CreditCardNumber:string):boolean;
Const
  DigitsAllowed = ['0'..'9'];
  MaxCCSize = 19;
  MinCCSize = 13;
var
  i : integer;
  CleanCardNumber: string;
  digit: Integer;
  CheckSum: Integer; { Holds the value of the operation }
  Flag: Boolean; { used to indicate when ready }
  Counter: Integer; { index counter }
  PartNumber: string; { used to extract each digit of number }
  Number: Integer; { used to convert each digit to integer}
Begin
  CleanCardNumber:='';
  digit:=0;

  // Remove any non numeric value
  for I := 1 to Length(CreditCardNumber) do
  Begin
    if CreditCardNumber[i] in DigitsAllowed then
      CleanCardNumber:= CleanCardNumber + CreditCardNumber[i];
  End;

  // Check for valid card length number
  if (Length(CleanCardNumber) > MaxCCSize) then
  Begin
    Result:= False;
    Exit;
  End;

  // get the starting value for our counter
  Counter := Length(CleanCardNumber);
  CheckSum := 0;
  PartNumber := '';
  Number := 0;
  Flag := false;

  while (Counter >= 1) do
  begin
    // get the current digit
    PartNumber := Copy(CleanCardNumber, Counter, 1);
    Number := StrToInt(PartNumber); // convert to integer
    if (Flag) then // only do every other digit
    begin
      Number := Number * 2;
      if (Number >= 10) then
        Number := Number - 9;
    end;
    CheckSum := CheckSum + Number;

    Flag := not (Flag);

    Counter := Counter - 1;
  end;

  result := ((CheckSum mod 10) = 0);
End;