function IsISBN(ISBN: string): Boolean;
var
Number, CheckDigit: string;
CheckValue, CheckSum, Err: Integer;
i, Cnt: Word;
begin
// Get check digit
CheckDigit := Copy(ISBN, Length(ISBN), 1);
// Get rest of ISBN, minus check digit and its hyphen
Number := Copy(ISBN, 1, Length(ISBN) - 2);
// Length of ISBN remainder must be 11 and check digit between 9 and 9 or X
if (Length(Number) = 11) and (Pos(CheckDigit, '0123456789X') > 0) then
begin
// Get numeric value for check digit
if (CheckDigit = 'X') then
CheckSum := 10
else
Val(CheckDigit, CheckSum, Err);
// Iterate through ISBN remainder, applying decode algorithm
Cnt := 1;
for i := 1 to 12 do
begin
// Act only if current character is between "0" and "9" to exclude hyphens
if (Pos(Number[i], '0123456789') > 0) then
begin
Val(Number[i], CheckValue, Err);
// Algorithm for each character in ISBN remainder, Cnt is the nth character
// so processed
CheckSum := CheckSum + CheckValue * (11 - Cnt);
Inc(Cnt);
end;
end;
// Verify final value is evenly divisible by 11
if (CheckSum mod 11 = 0) then
IsISBN := True
else
IsISBN := False;
end
else
IsISBN := False;
end;
// Kullanımı
procedure TForm1.Button1Click(Sender: TObject);
begin
// '975-93527-0-2': Delphi Programcılığı ve SQL ISBN
if IsISBN(Edit1.Text) then
ShowMessage('Geçerli')
else
ShowMessage('Geçersiz');
end;