code: protected function contraAccountNumber(array $lines)
      {
          switch ($this->format) {
              case self::FORMAT_CLASSIC:
                  if (preg_match('/(\d{6})((?:C|D)R?)([0-9,]{15})(N\d{3}|NMSC)([0-9P ]{16})/', $lines[0], $match)) {
                      return rtrim(ltrim($match[5], '0P'));
                  }
                  break;
  
              case self::FORMAT_STRUCTURED:
                  $parts = explode("\r\n", $lines[0]);
  
                  if (2 === count($parts)) {
                      return $parts[1];
                  }
                  break;
          }
      }
docs: Get the contra account from a transaction
  
  @param array $lines The transaction text at offset 0 and the description at offset 1
  @return string|null