DELPHI 中 字符替换问题

procedure TForm1.btn1Click(Sender: TObject);
var
  SourceString, TempString: string;
  StartIndex, EndIndex: Integer;
begin
  SourceString := '55551234567891555';
  StartIndex := Pos('1', SourceString);
  TempString := Copy(SourceString, StartIndex + 1, MaxInt);
  EndIndex := Pos('1', TempString);
  TempString := Copy(SourceString, StartIndex + 1, EndIndex - 1);
  TempString := StringReplace(SourceString , TempString, '22222222222222', [rfReplaceAll, rfIgnoreCase]);
  edt1.Text := TempString;
end;