` element.
	 * The default behavior is to put them on the `` element.
	 */
	public $codeAttributesOnPre = false;

	/**
	 * @inheritDoc
	 */
	protected $escapeCharacters = [
		// from Markdown
		'\\', // backslash
		'`', // backtick
		'*', // asterisk
		'_', // underscore
		'{', '}', // curly braces
		'[', ']', // square brackets
		'(', ')', // parentheses
		'#', // hash mark
		'+', // plus sign
		'-', // minus sign (hyphen)
		'.', // dot
		'!', // exclamation mark
		'<', '>',
		// added by MarkdownExtra
		':', // colon
		'|', // pipe
	];

	private $_specialAttributesRegex = '\{(([#\.][A-z0-9-_]+\s*)+)\}';

	// TODO allow HTML intended 3 spaces

	// TODO add markdown inside HTML blocks

	// TODO implement definition lists

	// TODO implement footnotes

	// TODO implement Abbreviations


	// block parsing

	protected function identifyReference($line)
	{
		return ($line[0] === ' ' || $line[0] === '[') && preg_match('/^ {0,3}\[(.+?)\]:\s*([^\s]+?)(?:\s+[\'"](.+?)[\'"])?\s*('.$this->_specialAttributesRegex.')?\s*$/', $line);
	}

	/**
	 * Consume link references
	 */
	protected function consumeReference($lines, $current)
	{
		while (isset($lines[$current]) && preg_match('/^ {0,3}\[(.+?)\]:\s*(.+?)(?:\s+[\(\'"](.+?)[\)\'"])?\s*('.$this->_specialAttributesRegex.')?\s*$/', $lines[$current], $matches)) {
			$label = strtolower($matches[1]);

			$this->references[$label] = [
				'url' => $this->replaceEscape($matches[2]),
			];
			if (isset($matches[3])) {
				$this->references[$label]['title'] = $matches[3];
			} else {
				// title may be on the next line
				if (isset($lines[$current + 1]) && preg_match('/^\s+[\(\'"](.+?)[\)\'"]\s*$/', $lines[$current + 1], $matches)) {
					$this->references[$label]['title'] = $matches[1];
					$current++;
				}
			}
			if (isset($matches[5])) {
				$this->references[$label]['attributes'] = $matches[5];
			}
			$current++;
		}
		return [false, --$current];
	}

	/**
	 * Consume lines for a fenced code block
	 */
	protected function consumeFencedCode($lines, $current)
	{
		// consume until ```
		$block = [
			'code',
		];
		$line = rtrim($lines[$current]);
		if (($pos = strrpos($line, '`')) === false) {
			$pos = strrpos($line, '~');
		}
		$fence = substr($line, 0, $pos + 1);
		$block['attributes'] = substr($line, $pos);
		$content = [];
		for($i = $current + 1, $count = count($lines); $i < $count; $i++) {
			if (rtrim($line = $lines[$i]) !== $fence) {
				$content[] = $line;
			} else {
				break;
			}
		}
		$block['content'] = implode("\n", $content);
		return [$block, $i];
	}

	protected function renderCode($block)
	{
		$attributes = $this->renderAttributes($block);
		return ($this->codeAttributesOnPre ? "" : "
")
			. htmlspecialchars($block['content'] . "\n", ENT_NOQUOTES | ENT_SUBSTITUTE, 'UTF-8')
			. "
\n";
	}

	/**
	 * Renders a headline
	 */
	protected function renderHeadline($block)
	{
		foreach($block['content'] as $i => $element) {
			if ($element[0] === 'specialAttributes') {
				unset($block['content'][$i]);
				$block['attributes'] = $element[1];
			}
		}
		$tag = 'h' . $block['level'];
		$attributes = $this->renderAttributes($block);
		return "<$tag$attributes>" . rtrim($this->renderAbsy($block['content']), "# \t") . "\n";
	}

	protected function renderAttributes($block)
	{
		$html = [];
		if (isset($block['attributes'])) {
			$attributes = preg_split('/\s+/', $block['attributes'], -1, PREG_SPLIT_NO_EMPTY);
			foreach($attributes as $attribute) {
				if ($attribute[0] === '#') {
					$html['id'] = substr($attribute, 1);
				} else {
					$html['class'][] = substr($attribute, 1);
				}
			}
		}
		$result = '';
		foreach($html as $attr => $value) {
			if (is_array($value)) {
				$value = trim(implode(' ', $value));
			}
			if (!empty($value)) {
				$result .= " $attr=\"$value\"";
			}
		}
		return $result;
	}


	// inline parsing


	/**
	 * @marker {
	 */
	protected function parseSpecialAttributes($text)
	{
		if (preg_match("~$this->_specialAttributesRegex~", $text, $matches)) {
			return [['specialAttributes', $matches[1]], strlen($matches[0])];
		}
		return [['text', '{'], 1];
	}

	protected function renderSpecialAttributes($block)
	{
		return '{' . $block[1] . '}';
	}

	protected function parseInline($text)
	{
		$elements = parent::parseInline($text);
		// merge special attribute elements to links and images as they are not part of the final absy later
		$relatedElement = null;
		foreach($elements as $i => $element) {
			if ($element[0] === 'link' || $element[0] === 'image') {
				$relatedElement = $i;
			} elseif ($element[0] === 'specialAttributes') {
				if ($relatedElement !== null) {
					$elements[$relatedElement]['attributes'] = $element[1];
					unset($elements[$i]);
				}
				$relatedElement = null;
			} else {
				$relatedElement = null;
			}
		}
		return $elements;
	}

	protected function renderLink($block)
	{
		if (isset($block['refkey'])) {
			if (($ref = $this->lookupReference($block['refkey'])) !== false) {
				$block = array_merge($block, $ref);
			} else {
				return $block['orig'];
			}
		}
		$attributes = $this->renderAttributes($block);
		return '' . $this->renderAbsy($block['text']) . '';
	}

	protected function renderImage($block)
	{
		if (isset($block['refkey'])) {
			if (($ref = $this->lookupReference($block['refkey'])) !== false) {
				$block = array_merge($block, $ref);
			} else {
				return $block['orig'];
			}
		}
		$attributes = $this->renderAttributes($block);
		return 'html5 ? '>' : ' />');
	}
}