code: protected function parseContent($content, &$position, $pattern, $optional, $type)
      {
          $this->typeCheck->parseContent(func_get_args());
  
          $subject = substr($content, $position - 1);
          if (
              !preg_match($pattern, $subject, $matches) ||
              '' === $matches[1]
          ) {
              if ($optional) {
                  return null;
              }
  
              throw new Exception\UnexpectedContentException(
                  $type,
                  $position
              );
          }
  
          $position += strlen($matches[0]);
  
          return $matches[1];
      }
docs: @param string $content
  @param integer &$position
  @param string  $pattern
  @param boolean $optional
  @param string  $type
  
  @return string|null