code: protected function contraAccountNumber(array $lines)
      {
          if (!preg_match('/\n(\d{8})\d{7}(\d{8})/', $lines[0], $match)) {
              return null;
          }
  
          $date = substr($match[1], 2);
          $number = $match[2];
  
          if (preg_match(sprintf('/%sCH%s/', $date, $number), $lines[1])) {
              return $number;
          }
  
          return null;
      }
docs: Get the contra account number from a transaction
  
  @param array $lines The transaction text at offset 0 and the description at offset 1
  @return string|null