Hacking at 0300

Extending Parsedown involves adding elements to the $InlineTypes and $BlockTypes arrays, then adding methods to handle them.

See the actual code.

Italics

I use <i> a lot, to indicate transliterated words. So I use could use "/" to indicate that:
/Shabbat/ is a Hebrew word becomes <i>Shabbat</i> is a Hebrew word. To do that:
do

class myParsedown extends Parsedown{
  function __construct(){
    $this->InlineTypes['/'] []= 'Italic';
    // after adding all the new inline types, create the list of characters
    $this->inlineMarkerList = implode ('', array_keys($this->InlineTypes));
    // allow the character to be escaped by '\'
    $this->specialCharacters []= '/';
  }

  protected function inlineItalic($excerpt){
    if (preg_match('#^/(.+?)/#', $excerpt['text'], $matches)) {
      return array(
        'extent' => strlen($matches[0]), 
        'element' => array(
          'name' => 'i',
          'handler' => array(
            'function' => 'lineElements',
            'argument' => $matches[1],
            'destination' => 'elements'
          )
        )
      );
    }
}

Now, my transliterated words are almost always Hebrew, so I can automatically add the lang=he attribute:

  protected function inlineItalic($excerpt){
    if (preg_match('#^/(.+?)/#', $excerpt['text'], $matches)) {
      return array(
        'extent' => strlen($matches[0]), 
        'element' => array(
          'name' => 'i',
          'handler' => array(
            'function' => 'lineElements',
            'argument' => $matches[1],
            'destination' => 'elements'
          ),
          'attributes' => array('lang' => 'he') // Add attributes
        )
      );
    }
}

and now /Shabbat/ is a Hebrew word becomes <i lang=he>Shabbat</i> is a Hebrew word.

Cite

I also use the <cite>. I'm running out of single characters to indicate elements, so I'm going to redefine "-". I don't need two different markers for <em>.

  function __construct(){
    $this->InlineTypes['_'] = ['Cite']; // redefinition; I am replacing the old array (which was ['Emphasis'])
    // ... rest of the constructor as above
  }

  protected function inlineCite($excerpt){
    if (preg_match('#^_(.+?)_#', $excerpt['text'], $matches)) {
      return array(
        'extent' => strlen($matches[0]), 
        'element' => array(
          'name' => 'cite',
          'handler' => array(
            'function' => 'lineElements',
            'argument' => $matches[1],
            'destination' => 'elements'
          )
        )
      );
    }

And now _A Tale of Two Cities_ becomes <cite>A Tale of Two Cities</cite>