{-------------------------------------------------

 Returns:  

  0  : Card is invalid or unknown

  1  : Card is a valid AmEx

  2  : Card is a valid Visa

  3  : Card is a valid MasterCard

-------------------------------------------------}  

function CheckCC(c: string): Integer;

var  

 card: string[21];  

 Vcard: array[0..21] of Byte absolute card;  

 Xcard: Integer;  

 Cstr: string[21];

 y, x: Integer;  

begin  

 Cstr := '';  

 FillChar(Vcard, 22, #0);

 card := c;  

 for x := 1 to 20 do  

   if (Vcard[x] in [48..57]) then  

     Cstr := Cstr + chr(Vcard[x]);  

 card := '';

 card := Cstr;

 Xcard := 0;  

 if not odd(Length(card)) then  

   for x := (Length(card) - 1) downto 1 do  

   begin  

     if odd(x) then

       y := ((Vcard[x] - 48) * 2)

     else  

       y := (Vcard[x] - 48);  

     if (y >= 10) then  

       y := ((y - 10) + 1);  

     Xcard := (Xcard + y)  

   end  

 else

   for x := (Length(card) - 1) downto 1 do

   begin

     if odd(x) then  

       y := (Vcard[x] - 48)  

     else  

       y := ((Vcard[x] - 48) * 2);  

     if (y >= 10) then  

       y := ((y - 10) + 1);

     Xcard := (Xcard + y)  

   end;  

 x := (10 - (Xcard mod 10));  

 if (x = 10) then

   x := 0;  

 if (x = (Vcard[Length(card)] - 48)) then

   Result := Ord(Cstr[1]) - Ord('2')

 else  

   Result := 0

end;

procedure TForm1.Button1Click(Sender: TObject);

begin

 case CheckCC(Edit1.Text) of

   0: Label1.Caption := 'Card is invalid or unknown';

   1: Label1.Caption := 'Card is a valid AmEx';

   2: Label1.Caption := 'Card is a valid Visa';

   3: Label1.Caption := 'Card is a valid MasterCard';

 end;

end;