code: protected function contraAccountNumber(array $lines)
      {
          foreach ($lines as $line) {
              if (preg_match('/REK\: ([a-zA-Z]{2}[0-9]{2}[a-zA-Z0-9]{4}[0-9]{7}([a-zA-Z0-9]?){0,16})/', $line, $match)) {
                  return rtrim(ltrim($match[1], '0P'));
              }
          }
      }
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