code: protected function statementNumber($text)
      {
          if ($line = $this->getLine('60F', $text)) {
              if (preg_match('/(C|D)(\d{6})([A-Z]{3})([0-9,]{1,15})/', $line, $match)) {
                  return $match[2];
              }
          }
  
          return null;
      }
docs: Rabobank does not use statement numbers. Use the opening balance
  date as statement number instead.
  
  @param string $text Statement body text
  @return string|null