Check IBAN for correctness

function ChangeAlpha(input: string): string;
  // A -> 10, B -> 11, C -> 12 ...
var
  a: Char;
begin
  Result := input;
  for a := 'A' to 'Z' do
  begin
    Result := StringReplace(Result, a, IntToStr(Ord(a) - 55), [rfReplaceAll]);
  end;
end;

function CalculateDigits(iban: string): Integer;
var
  v, l: Integer;
  alpha: string;
  number: Longint;
  rest: Integer;
begin
  iban := UpperCase(iban);
  if Pos('IBAN', iban) > 0 then
    Delete(iban, Pos('IBAN', iban), 4);
  iban := iban + Copy(iban, 1, 4);
  Delete(iban, 1, 4);
  iban := ChangeAlpha(iban);
  v := 1;
  l := 9;
  rest := 0;
  alpha := '';
  try
    while v <= Length(iban) do
    begin
      if l > Length(iban) then
        l := Length(iban);
      alpha := alpha + Copy(iban, v, l);
      number := StrToInt(alpha);
      rest := number mod 97;
      v := v + l;
      alpha := IntToStr(rest);
      l := 9 - Length(alpha);
    end;
  except
    rest := 0;
  end;
  Result := rest;
end;

function CheckIBAN(iban: string): Boolean;
begin
  iban := StringReplace(iban, ' ', '', [rfReplaceAll]);
  if CalculateDigits(iban) = 1 then
    Result := True
  else
    Result := False;
end;

procedure TForm1.Button1Click(Sender: TObject);
begin
  if not CheckIBAN('CH10002300A1023502601') = true then
  ShowMessage('IBAN is wrong')
  else
  ShowMessage('IBAN is right');
end;