code: public function processReferencedLink(Text $text, array $options = array())
      {
          if (!$text->contains('[')) {
              return;
          }
  
          /** @noinspection PhpUnusedParameterInspection */
          $text->replace('{
              #(                   # wrap whole match in $1
                \[
                  (' . $this->getNestedBrackets() . ')    # link text = $2
                \]
  
                [ ]?              # one optional space
                (?:\n[ ]*)?       # one optional newline followed by spaces
  
                \[
                  (.*?)       # id = $3
                \]
              #)
          }xs', function (Text $whole, Text $linkText, Text $id = null) use ($options) {
              if (is_null($id) || (string) $id == '') {
                  $id = new Text($linkText);
              }
  
              $id->lower();
  
              if ($this->markdown->getUrlRegistry()->exists($id)) {
                  $url = new Text($this->markdown->getUrlRegistry()->get($id));
                  $url->escapeHtml()->replace('/(?<!\\\\)_/', '\\\\_');
                  $this->markdown->emit('escape.special_chars', [$url]);
  
                  $linkOptions = [
                      'href' => $url->getString(),
                  ];
  
                  if ($this->markdown->getTitleRegistry()->exists($id)) {
                      $title = new Text($this->markdown->getTitleRegistry()->get($id));
                      $linkOptions['title'] = $title->escapeHtml()->getString();
                  }
  
                  return $this->getRenderer()->renderLink($linkText->getString(), $linkOptions);
              } else {
                  if ($options['strict']) {
                      throw new SyntaxError(
                          sprintf('Unable to find id "%s" in Reference-style link', $id),
                          $this, $whole, $this->markdown
                      );
                  }
  
                  return $whole;
              }
          });
      }
docs: Handle reference-style links: [link text] [id]
  
  @param Text  $text
  @param array $options