MDL-61928 formslib: shortforms should work with non-editable forms

[moodle.git] / lib / markdown / Markdown.php

<?php
/**
 * Markdown  -  A text-to-HTML conversion tool for web writers
 *
 * @package   php-markdown
 * @author    Michel Fortin <michel.fortin@michelf.com>
 * @copyright 2004-2016 Michel Fortin <https://michelf.com/projects/php-markdown/>
 * @copyright (Original Markdown) 2004-2006 John Gruber <https://daringfireball.net/projects/markdown/>
 */

namespace Michelf;

/**
 * Markdown Parser Class
 */
class Markdown implements MarkdownInterface {
        /**
         * Define the package version
         * @var string
         */
        const MARKDOWNLIB_VERSION = "1.7.0";

        /**
         * Simple function interface - Initialize the parser and return the result
         * of its transform method. This will work fine for derived classes too.
         *
         * @api
         *
         * @param  string $text
         * @return string
         */
        public static function defaultTransform($text) {
                // Take parser class on which this function was called.
                $parser_class = \get_called_class();

                // Try to take parser from the static parser list
                static $parser_list;
                $parser =& $parser_list[$parser_class];

                // Create the parser it not already set
                if (!$parser) {
                        $parser = new $parser_class;
                }

                // Transform text using parser.
                return $parser->transform($text);
        }

        /**
         * Configuration variables
         */

        /**
         * Change to ">" for HTML output.
         * @var string
         */
        public $empty_element_suffix = " />";

        /**
         * The width of indentation of the output markup
         * @var int
         */
        public $tab_width = 4;
        
        /**
         * Change to `true` to disallow markup or entities.
         * @var boolean
         */
        public $no_markup   = false;
        public $no_entities = false;
        

        /**
         * Change to `true` to enable line breaks on \n without two trailling spaces
         * @var boolean
         */
        public $hard_wrap = false;

        /**
         * Predefined URLs and titles for reference links and images.
         * @var array
         */
        public $predef_urls   = array();
        public $predef_titles = array();

        /**
         * Optional filter function for URLs
         * @var callable
         */
        public $url_filter_func = null;

        /**
         * Optional header id="" generation callback function.
         * @var callable
         */
        public $header_id_func = null;
        
        /**
         * Optional function for converting code block content to HTML
         * @var callable
         */
        public $code_block_content_func = null;

        /**
         * Optional function for converting code span content to HTML.
         * @var callable
         */
        public $code_span_content_func = null;

        /**
         * Class attribute to toggle "enhanced ordered list" behaviour
         * setting this to true will allow ordered lists to start from the index
         * number that is defined first.
         * 
         * For example:
         * 2. List item two
         * 3. List item three
         *
         * Becomes:
         * <ol start="2">
         * <li>List item two</li>
         * <li>List item three</li>
         * </ol>
         *
         * @var bool
         */
        public $enhanced_ordered_list = false;

        /**
         * Parser implementation
         */

        /**
         * Regex to match balanced [brackets].
         * Needed to insert a maximum bracked depth while converting to PHP.
         * @var int
         */
        protected $nested_brackets_depth = 6;
        protected $nested_brackets_re;

        protected $nested_url_parenthesis_depth = 4;
        protected $nested_url_parenthesis_re;

        /**
         * Table of hash values for escaped characters:
         * @var string
         */
        protected $escape_chars = '\`*_{}[]()>#+-.!';
        protected $escape_chars_re;

        /**
         * Constructor function. Initialize appropriate member variables.
         * @return void
         */
        public function __construct() {
                $this->_initDetab();
                $this->prepareItalicsAndBold();
        
                $this->nested_brackets_re = 
                        str_repeat('(?>[^\[\]]+|\[', $this->nested_brackets_depth).
                        str_repeat('\])*', $this->nested_brackets_depth);
        
                $this->nested_url_parenthesis_re = 
                        str_repeat('(?>[^()\s]+|\(', $this->nested_url_parenthesis_depth).
                        str_repeat('(?>\)))*', $this->nested_url_parenthesis_depth);
                
                $this->escape_chars_re = '['.preg_quote($this->escape_chars).']';
                
                // Sort document, block, and span gamut in ascendent priority order.
                asort($this->document_gamut);
                asort($this->block_gamut);
                asort($this->span_gamut);
        }


        /**
         * Internal hashes used during transformation.
         * @var array
         */
        protected $urls        = array();
        protected $titles      = array();
        protected $html_hashes = array();
        
        /**
         * Status flag to avoid invalid nesting.
         * @var boolean
         */
        protected $in_anchor = false;
        
        /**
         * Called before the transformation process starts to setup parser states.
         * @return void
         */
        protected function setup() {
                // Clear global hashes.
                $this->urls        = $this->predef_urls;
                $this->titles      = $this->predef_titles;
                $this->html_hashes = array();
                $this->in_anchor   = false;
        }
        
        /**
         * Called after the transformation process to clear any variable which may
         * be taking up memory unnecessarly.
         * @return void
         */
        protected function teardown() {
                $this->urls        = array();
                $this->titles      = array();
                $this->html_hashes = array();
        }

        /**
         * Main function. Performs some preprocessing on the input text and pass
         * it through the document gamut.
         *
         * @api
         *
         * @param  string $text
         * @return string
         */
        public function transform($text) {
                $this->setup();
        
                # Remove UTF-8 BOM and marker character in input, if present.
                $text = preg_replace('{^\xEF\xBB\xBF|\x1A}', '', $text);

                # Standardize line endings:
                #   DOS to Unix and Mac to Unix
                $text = preg_replace('{\r\n?}', "\n", $text);

                # Make sure $text ends with a couple of newlines:
                $text .= "\n\n";

                # Convert all tabs to spaces.
                $text = $this->detab($text);

                # Turn block-level HTML blocks into hash entries
                $text = $this->hashHTMLBlocks($text);

                # Strip any lines consisting only of spaces and tabs.
                # This makes subsequent regexen easier to write, because we can
                # match consecutive blank lines with /\n+/ instead of something
                # contorted like /[ ]*\n+/ .
                $text = preg_replace('/^[ ]+$/m', '', $text);

                # Run document gamut methods.
                foreach ($this->document_gamut as $method => $priority) {
                        $text = $this->$method($text);
                }
                
                $this->teardown();

                return $text . "\n";
        }

        /**
         * Define the document gamut
         * @var array
         */
        protected $document_gamut = array(
                // Strip link definitions, store in hashes.
                "stripLinkDefinitions" => 20,
                "runBasicBlockGamut"   => 30,
        );

        /**
         * Strips link definitions from text, stores the URLs and titles in
         * hash references
         * @param  string $text
         * @return string
         */
        protected function stripLinkDefinitions($text) {
        
                $less_than_tab = $this->tab_width - 1;

                // Link defs are in the form: ^[id]: url "optional title"
                $text = preg_replace_callback('{
                                                        ^[ ]{0,'.$less_than_tab.'}\[(.+)\][ ]?: # id = $1
                                                          [ ]*
                                                          \n?                           # maybe *one* newline
                                                          [ ]*
                                                        (?:
                                                          <(.+?)>                       # url = $2
                                                        |
                                                          (\S+?)                        # url = $3
                                                        )
                                                          [ ]*
                                                          \n?                           # maybe one newline
                                                          [ ]*
                                                        (?:
                                                                (?<=\s)                 # lookbehind for whitespace
                                                                ["(]
                                                                (.*?)                   # title = $4
                                                                [")]
                                                                [ ]*
                                                        )?      # title is optional
                                                        (?:\n+|\Z)
                        }xm',
                        array($this, '_stripLinkDefinitions_callback'),
                        $text
                );
                return $text;
        }

        /**
         * The callback to strip link definitions
         * @param  array $matches
         * @return string
         */
        protected function _stripLinkDefinitions_callback($matches) {
                $link_id = strtolower($matches[1]);
                $url = $matches[2] == '' ? $matches[3] : $matches[2];
                $this->urls[$link_id] = $url;
                $this->titles[$link_id] =& $matches[4];
                return ''; // String that will replace the block
        }

        /**
         * Hashify HTML blocks
         * @param  string $text
         * @return string
         */
        protected function hashHTMLBlocks($text) {
                if ($this->no_markup) {
                        return $text;
                }

                $less_than_tab = $this->tab_width - 1;

                /**
                 * Hashify HTML blocks:
                 *
                 * We only want to do this for block-level HTML tags, such as headers,
                 * lists, and tables. That's because we still want to wrap <p>s around
                 * "paragraphs" that are wrapped in non-block-level tags, such as
                 * anchors, phrase emphasis, and spans. The list of tags we're looking
                 * for is hard-coded:
                 *
                 * *  List "a" is made of tags which can be both inline or block-level.
                 *    These will be treated block-level when the start tag is alone on 
                 *    its line, otherwise they're not matched here and will be taken as 
                 *    inline later.
                 * *  List "b" is made of tags which are always block-level;
                 */
                $block_tags_a_re = 'ins|del';
                $block_tags_b_re = 'p|div|h[1-6]|blockquote|pre|table|dl|ol|ul|address|'.
                                                   'script|noscript|style|form|fieldset|iframe|math|svg|'.
                                                   'article|section|nav|aside|hgroup|header|footer|'.
                                                   'figure';

                // Regular expression for the content of a block tag.
                $nested_tags_level = 4;
                $attr = '
                        (?>                             # optional tag attributes
                          \s                    # starts with whitespace
                          (?>
                                [^>"/]+         # text outside quotes
                          |
                                /+(?!>)         # slash not followed by ">"
                          |
                                "[^"]*"         # text inside double quotes (tolerate ">")
                          |
                                \'[^\']*\'      # text inside single quotes (tolerate ">")
                          )*
                        )?      
                        ';
                $content =
                        str_repeat('
                                (?>
                                  [^<]+                 # content without tag
                                |
                                  <\2                   # nested opening tag
                                        '.$attr.'       # attributes
                                        (?>
                                          />
                                        |
                                          >', $nested_tags_level).      // end of opening tag
                                          '.*?'.                                        // last level nested tag content
                        str_repeat('
                                          </\2\s*>      # closing nested tag
                                        )
                                  |                             
                                        <(?!/\2\s*>     # other tags with a different name
                                  )
                                )*',
                                $nested_tags_level);
                $content2 = str_replace('\2', '\3', $content);

                /**
                 * First, look for nested blocks, e.g.:
                 *      <div>
                 *              <div>
                 *              tags for inner block must be indented.
                 *              </div>
                 *      </div>
                 *
                 * The outermost tags must start at the left margin for this to match,
                 * and the inner nested divs must be indented.
                 * We need to do this before the next, more liberal match, because the
                 * next match will start at the first `<div>` and stop at the
                 * first `</div>`.
                 */
                $text = preg_replace_callback('{(?>
                        (?>
                                (?<=\n)                 # Starting on its own line
                                |                               # or
                                \A\n?                   # the at beginning of the doc
                        )
                        (                                               # save in $1

                          # Match from `\n<tag>` to `</tag>\n`, handling nested tags 
                          # in between.
                                        
                                                [ ]{0,'.$less_than_tab.'}
                                                <('.$block_tags_b_re.')# start tag = $2
                                                '.$attr.'>                      # attributes followed by > and \n
                                                '.$content.'            # content, support nesting
                                                </\2>                           # the matching end tag
                                                [ ]*                            # trailing spaces/tabs
                                                (?=\n+|\Z)      # followed by a newline or end of document

                        | # Special version for tags of group a.

                                                [ ]{0,'.$less_than_tab.'}
                                                <('.$block_tags_a_re.')# start tag = $3
                                                '.$attr.'>[ ]*\n        # attributes followed by >
                                                '.$content2.'           # content, support nesting
                                                </\3>                           # the matching end tag
                                                [ ]*                            # trailing spaces/tabs
                                                (?=\n+|\Z)      # followed by a newline or end of document
                                        
                        | # Special case just for <hr />. It was easier to make a special 
                          # case than to make the other regex more complicated.
                        
                                                [ ]{0,'.$less_than_tab.'}
                                                <(hr)                           # start tag = $2
                                                '.$attr.'                       # attributes
                                                /?>                                     # the matching end tag
                                                [ ]*
                                                (?=\n{2,}|\Z)           # followed by a blank line or end of document
                        
                        | # Special case for standalone HTML comments:
                        
                                        [ ]{0,'.$less_than_tab.'}
                                        (?s:
                                                <!-- .*? -->
                                        )
                                        [ ]*
                                        (?=\n{2,}|\Z)           # followed by a blank line or end of document
                        
                        | # PHP and ASP-style processor instructions (<? and <%)
                        
                                        [ ]{0,'.$less_than_tab.'}
                                        (?s:
                                                <([?%])                 # $2
                                                .*?
                                                \2>
                                        )
                                        [ ]*
                                        (?=\n{2,}|\Z)           # followed by a blank line or end of document
                                        
                        )
                        )}Sxmi',
                        array($this, '_hashHTMLBlocks_callback'),
                        $text
                );

                return $text;
        }

        /**
         * The callback for hashing HTML blocks
         * @param  string $matches
         * @return string
         */
        protected function _hashHTMLBlocks_callback($matches) {
                $text = $matches[1];
                $key  = $this->hashBlock($text);
                return "\n\n$key\n\n";
        }
        
        /**
         * Called whenever a tag must be hashed when a function insert an atomic 
         * element in the text stream. Passing $text to through this function gives
         * a unique text-token which will be reverted back when calling unhash.
         *
         * The $boundary argument specify what character should be used to surround
         * the token. By convension, "B" is used for block elements that needs not
         * to be wrapped into paragraph tags at the end, ":" is used for elements
         * that are word separators and "X" is used in the general case.
         *
         * @param  string $text
         * @param  string $boundary
         * @return string
         */
        protected function hashPart($text, $boundary = 'X') {
                // Swap back any tag hash found in $text so we do not have to `unhash`
                // multiple times at the end.
                $text = $this->unhash($text);
                
                // Then hash the block.
                static $i = 0;
                $key = "$boundary\x1A" . ++$i . $boundary;
                $this->html_hashes[$key] = $text;
                return $key; // String that will replace the tag.
        }

        /**
         * Shortcut function for hashPart with block-level boundaries.
         * @param  string $text
         * @return string
         */
        protected function hashBlock($text) {
                return $this->hashPart($text, 'B');
        }

        /**
         * Define the block gamut - these are all the transformations that form
         * block-level tags like paragraphs, headers, and list items.
         * @var array
         */
        protected $block_gamut = array(
                "doHeaders"         => 10,
                "doHorizontalRules" => 20,
                "doLists"           => 40,
                "doCodeBlocks"      => 50,
                "doBlockQuotes"     => 60,
        );

        /**
         * Run block gamut tranformations.
         *
         * We need to escape raw HTML in Markdown source before doing anything 
         * else. This need to be done for each block, and not only at the 
         * begining in the Markdown function since hashed blocks can be part of
         * list items and could have been indented. Indented blocks would have 
         * been seen as a code block in a previous pass of hashHTMLBlocks.
         *
         * @param  string $text
         * @return string
         */
        protected function runBlockGamut($text) {
                $text = $this->hashHTMLBlocks($text);
                return $this->runBasicBlockGamut($text);
        }

        /**
         * Run block gamut tranformations, without hashing HTML blocks. This is 
         * useful when HTML blocks are known to be already hashed, like in the first
         * whole-document pass.
         *
         * @param  string $text
         * @return string
         */
        protected function runBasicBlockGamut($text) {
        
                foreach ($this->block_gamut as $method => $priority) {
                        $text = $this->$method($text);
                }
                
                // Finally form paragraph and restore hashed blocks.
                $text = $this->formParagraphs($text);

                return $text;
        }

        /**
         * Convert horizontal rules
         * @param  string $text
         * @return string
         */
        protected function doHorizontalRules($text) {
                return preg_replace(
                        '{
                                ^[ ]{0,3}       # Leading space
                                ([-*_])         # $1: First marker
                                (?>                     # Repeated marker group
                                        [ ]{0,2}        # Zero, one, or two spaces.
                                        \1                      # Marker character
                                ){2,}           # Group repeated at least twice
                                [ ]*            # Tailing spaces
                                $                       # End of line.
                        }mx',
                        "\n".$this->hashBlock("<hr$this->empty_element_suffix")."\n", 
                        $text
                );
        }

        /**
         * These are all the transformations that occur *within* block-level
         * tags like paragraphs, headers, and list items.
         * @var array
         */
        protected $span_gamut = array(
                // Process character escapes, code spans, and inline HTML
                // in one shot.
                "parseSpan"           => -30,
                // Process anchor and image tags. Images must come first,
                // because ![foo][f] looks like an anchor.
                "doImages"            =>  10,
                "doAnchors"           =>  20,
                // Make links out of things like `<https://example.com/>`
                // Must come after doAnchors, because you can use < and >
                // delimiters in inline links like [this](<url>).
                "doAutoLinks"         =>  30,
                "encodeAmpsAndAngles" =>  40,
                "doItalicsAndBold"    =>  50,
                "doHardBreaks"        =>  60,
        );

        /**
         * Run span gamut transformations
         * @param  string $text
         * @return string
         */
        protected function runSpanGamut($text) {
                foreach ($this->span_gamut as $method => $priority) {
                        $text = $this->$method($text);
                }

                return $text;
        }

        /**
         * Do hard breaks
         * @param  string $text
         * @return string
         */
        protected function doHardBreaks($text) {
                if ($this->hard_wrap) {
                        return preg_replace_callback('/ *\n/', 
                                array($this, '_doHardBreaks_callback'), $text);
                } else {
                        return preg_replace_callback('/ {2,}\n/', 
                                array($this, '_doHardBreaks_callback'), $text);
                }
        }

        /**
         * Trigger part hashing for the hard break (callback method)
         * @param  array $matches
         * @return string
         */
        protected function _doHardBreaks_callback($matches) {
                return $this->hashPart("<br$this->empty_element_suffix\n");
        }

        /**
         * Turn Markdown link shortcuts into XHTML <a> tags.
         * @param  string $text
         * @return string
         */
        protected function doAnchors($text) {
                if ($this->in_anchor) {
                        return $text;
                }
                $this->in_anchor = true;
                
                // First, handle reference-style links: [link text] [id]
                $text = preg_replace_callback('{
                        (                                       # wrap whole match in $1
                          \[
                                ('.$this->nested_brackets_re.') # link text = $2
                          \]

                          [ ]?                          # one optional space
                          (?:\n[ ]*)?           # one optional newline followed by spaces

                          \[
                                (.*?)           # id = $3
                          \]
                        )
                        }xs',
                        array($this, '_doAnchors_reference_callback'), $text);

                // Next, inline-style links: [link text](url "optional title")
                $text = preg_replace_callback('{
                        (                               # wrap whole match in $1
                          \[
                                ('.$this->nested_brackets_re.') # link text = $2
                          \]
                          \(                    # literal paren
                                [ \n]*
                                (?:
                                        <(.+?)> # href = $3
                                |
                                        ('.$this->nested_url_parenthesis_re.')  # href = $4
                                )
                                [ \n]*
                                (                       # $5
                                  ([\'"])       # quote char = $6
                                  (.*?)         # Title = $7
                                  \6            # matching quote
                                  [ \n]*        # ignore any spaces/tabs between closing quote and )
                                )?                      # title is optional
                          \)
                        )
                        }xs',
                        array($this, '_doAnchors_inline_callback'), $text);

                // Last, handle reference-style shortcuts: [link text]
                // These must come last in case you've also got [link text][1]
                // or [link text](/foo)
                $text = preg_replace_callback('{
                        (                                       # wrap whole match in $1
                          \[
                                ([^\[\]]+)              # link text = $2; can\'t contain [ or ]
                          \]
                        )
                        }xs',
                        array($this, '_doAnchors_reference_callback'), $text);

                $this->in_anchor = false;
                return $text;
        }

        /**
         * Callback method to parse referenced anchors
         * @param  string $matches
         * @return string
         */
        protected function _doAnchors_reference_callback($matches) {
                $whole_match =  $matches[1];
                $link_text   =  $matches[2];
                $link_id     =& $matches[3];

                if ($link_id == "") {
                        // for shortcut links like [this][] or [this].
                        $link_id = $link_text;
                }
                
                // lower-case and turn embedded newlines into spaces
                $link_id = strtolower($link_id);
                $link_id = preg_replace('{[ ]?\n}', ' ', $link_id);

                if (isset($this->urls[$link_id])) {
                        $url = $this->urls[$link_id];
                        $url = $this->encodeURLAttribute($url);
                        
                        $result = "<a href=\"$url\"";
                        if ( isset( $this->titles[$link_id] ) ) {
                                $title = $this->titles[$link_id];
                                $title = $this->encodeAttribute($title);
                                $result .=  " title=\"$title\"";
                        }
                
                        $link_text = $this->runSpanGamut($link_text);
                        $result .= ">$link_text</a>";
                        $result = $this->hashPart($result);
                } else {
                        $result = $whole_match;
                }
                return $result;
        }

        /**
         * Callback method to parse inline anchors
         * @param  string $matches
         * @return string
         */
        protected function _doAnchors_inline_callback($matches) {
                $whole_match    =  $matches[1];
                $link_text              =  $this->runSpanGamut($matches[2]);
                $url                    =  $matches[3] == '' ? $matches[4] : $matches[3];
                $title                  =& $matches[7];

                // If the URL was of the form <s p a c e s> it got caught by the HTML
                // tag parser and hashed. Need to reverse the process before using
                // the URL.
                $unhashed = $this->unhash($url);
                if ($unhashed != $url)
                        $url = preg_replace('/^<(.*)>$/', '\1', $unhashed);

                $url = $this->encodeURLAttribute($url);

                $result = "<a href=\"$url\"";
                if (isset($title)) {
                        $title = $this->encodeAttribute($title);
                        $result .=  " title=\"$title\"";
                }
                
                $link_text = $this->runSpanGamut($link_text);
                $result .= ">$link_text</a>";

                return $this->hashPart($result);
        }

        /**
         * Turn Markdown image shortcuts into <img> tags.
         * @param  string $text
         * @return string
         */
        protected function doImages($text) {
                // First, handle reference-style labeled images: ![alt text][id]
                $text = preg_replace_callback('{
                        (                               # wrap whole match in $1
                          !\[
                                ('.$this->nested_brackets_re.')         # alt text = $2
                          \]

                          [ ]?                          # one optional space
                          (?:\n[ ]*)?           # one optional newline followed by spaces

                          \[
                                (.*?)           # id = $3
                          \]

                        )
                        }xs', 
                        array($this, '_doImages_reference_callback'), $text);

                // Next, handle inline images:  ![alt text](url "optional title")
                // Don't forget: encode * and _
                $text = preg_replace_callback('{
                        (                               # wrap whole match in $1
                          !\[
                                ('.$this->nested_brackets_re.')         # alt text = $2
                          \]
                          \s?                   # One optional whitespace character
                          \(                    # literal paren
                                [ \n]*
                                (?:
                                        <(\S*)> # src url = $3
                                |
                                        ('.$this->nested_url_parenthesis_re.')  # src url = $4
                                )
                                [ \n]*
                                (                       # $5
                                  ([\'"])       # quote char = $6
                                  (.*?)         # title = $7
                                  \6            # matching quote
                                  [ \n]*
                                )?                      # title is optional
                          \)
                        )
                        }xs',
                        array($this, '_doImages_inline_callback'), $text);

                return $text;
        }

        /**
         * Callback to parse references image tags
         * @param  array $matches
         * @return string
         */
        protected function _doImages_reference_callback($matches) {
                $whole_match = $matches[1];
                $alt_text    = $matches[2];
                $link_id     = strtolower($matches[3]);

                if ($link_id == "") {
                        $link_id = strtolower($alt_text); // for shortcut links like ![this][].
                }

                $alt_text = $this->encodeAttribute($alt_text);
                if (isset($this->urls[$link_id])) {
                        $url = $this->encodeURLAttribute($this->urls[$link_id]);
                        $result = "<img src=\"$url\" alt=\"$alt_text\"";
                        if (isset($this->titles[$link_id])) {
                                $title = $this->titles[$link_id];
                                $title = $this->encodeAttribute($title);
                                $result .=  " title=\"$title\"";
                        }
                        $result .= $this->empty_element_suffix;
                        $result = $this->hashPart($result);
                } else {
                        // If there's no such link ID, leave intact:
                        $result = $whole_match;
                }

                return $result;
        }

        /**
         * Callback to parse inline image tags
         * @param  array $matches
         * @return string
         */
        protected function _doImages_inline_callback($matches) {
                $whole_match    = $matches[1];
                $alt_text               = $matches[2];
                $url                    = $matches[3] == '' ? $matches[4] : $matches[3];
                $title                  =& $matches[7];

                $alt_text = $this->encodeAttribute($alt_text);
                $url = $this->encodeURLAttribute($url);
                $result = "<img src=\"$url\" alt=\"$alt_text\"";
                if (isset($title)) {
                        $title = $this->encodeAttribute($title);
                        $result .=  " title=\"$title\""; // $title already quoted
                }
                $result .= $this->empty_element_suffix;

                return $this->hashPart($result);
        }

        /**
         * Parse Markdown heading elements to HTML
         * @param  string $text
         * @return string
         */
        protected function doHeaders($text) {
                /**
                 * Setext-style headers:
                 *        Header 1
                 *        ========
                 *  
                 *        Header 2
                 *        --------
                 */
                $text = preg_replace_callback('{ ^(.+?)[ ]*\n(=+|-+)[ ]*\n+ }mx',
                        array($this, '_doHeaders_callback_setext'), $text);

                /**
                 * atx-style headers:
                 *   # Header 1
                 *   ## Header 2
                 *   ## Header 2 with closing hashes ##
                 *   ...
                 *   ###### Header 6
                 */
                $text = preg_replace_callback('{
                                ^(\#{1,6})      # $1 = string of #\'s
                                [ ]*
                                (.+?)           # $2 = Header text
                                [ ]*
                                \#*                     # optional closing #\'s (not counted)
                                \n+
                        }xm',
                        array($this, '_doHeaders_callback_atx'), $text);

                return $text;
        }

        /**
         * Setext header parsing callback
         * @param  array $matches
         * @return string
         */
        protected function _doHeaders_callback_setext($matches) {
                // Terrible hack to check we haven't found an empty list item.
                if ($matches[2] == '-' && preg_match('{^-(?: |$)}', $matches[1])) {
                        return $matches[0];
                }
                
                $level = $matches[2]{0} == '=' ? 1 : 2;

                // ID attribute generation
                $idAtt = $this->_generateIdFromHeaderValue($matches[1]);

                $block = "<h$level$idAtt>".$this->runSpanGamut($matches[1])."</h$level>";
                return "\n" . $this->hashBlock($block) . "\n\n";
        }

        /**
         * ATX header parsing callback
         * @param  array $matches
         * @return string
         */
        protected function _doHeaders_callback_atx($matches) {
                // ID attribute generation
                $idAtt = $this->_generateIdFromHeaderValue($matches[2]);

                $level = strlen($matches[1]);
                $block = "<h$level$idAtt>".$this->runSpanGamut($matches[2])."</h$level>";
                return "\n" . $this->hashBlock($block) . "\n\n";
        }

        /**
         * If a header_id_func property is set, we can use it to automatically
         * generate an id attribute.
         *
         * This method returns a string in the form id="foo", or an empty string
         * otherwise.
         * @param  string $headerValue
         * @return string
         */
        protected function _generateIdFromHeaderValue($headerValue) {           
                if (!is_callable($this->header_id_func)) {
                        return "";
                }

                $idValue = call_user_func($this->header_id_func, $headerValue);
                if (!$idValue) {
                        return "";
                }

                return ' id="' . $this->encodeAttribute($idValue) . '"';
        }

        /**
         * Form HTML ordered (numbered) and unordered (bulleted) lists.
         * @param  string $text
         * @return string
         */
        protected function doLists($text) {
                $less_than_tab = $this->tab_width - 1;

                // Re-usable patterns to match list item bullets and number markers:
                $marker_ul_re  = '[*+-]';
                $marker_ol_re  = '\d+[\.]';

                $markers_relist = array(
                        $marker_ul_re => $marker_ol_re,
                        $marker_ol_re => $marker_ul_re,
                        );

                foreach ($markers_relist as $marker_re => $other_marker_re) {
                        // Re-usable pattern to match any entirel ul or ol list:
                        $whole_list_re = '
                                (                                                               # $1 = whole list
                                  (                                                             # $2
                                        ([ ]{0,'.$less_than_tab.'})     # $3 = number of spaces
                                        ('.$marker_re.')                        # $4 = first list item marker
                                        [ ]+
                                  )
                                  (?s:.+?)
                                  (                                                             # $5
                                          \z
                                        |
                                          \n{2,}
                                          (?=\S)
                                          (?!                                           # Negative lookahead for another list item marker
                                                [ ]*
                                                '.$marker_re.'[ ]+
                                          )
                                        |
                                          (?=                                           # Lookahead for another kind of list
                                            \n
                                                \3                                              # Must have the same indentation
                                                '.$other_marker_re.'[ ]+
                                          )
                                  )
                                )
                        '; // mx
                        
                        // We use a different prefix before nested lists than top-level lists.
                        //See extended comment in _ProcessListItems().
                
                        if ($this->list_level) {
                                $text = preg_replace_callback('{
                                                ^
                                                '.$whole_list_re.'
                                        }mx',
                                        array($this, '_doLists_callback'), $text);
                        } else {
                                $text = preg_replace_callback('{
                                                (?:(?<=\n)\n|\A\n?) # Must eat the newline
                                                '.$whole_list_re.'
                                        }mx',
                                        array($this, '_doLists_callback'), $text);
                        }
                }

                return $text;
        }

        /**
         * List parsing callback
         * @param  array $matches
         * @return string
         */
        protected function _doLists_callback($matches) {
                // Re-usable patterns to match list item bullets and number markers:
                $marker_ul_re  = '[*+-]';
                $marker_ol_re  = '\d+[\.]';
                $marker_any_re = "(?:$marker_ul_re|$marker_ol_re)";
                $marker_ol_start_re = '[0-9]+';

                $list = $matches[1];
                $list_type = preg_match("/$marker_ul_re/", $matches[4]) ? "ul" : "ol";

                $marker_any_re = ( $list_type == "ul" ? $marker_ul_re : $marker_ol_re );

                $list .= "\n";
                $result = $this->processListItems($list, $marker_any_re);

                $ol_start = 1;
                if ($this->enhanced_ordered_list) {
                        // Get the start number for ordered list.
                        if ($list_type == 'ol') {
                                $ol_start_array = array();
                                $ol_start_check = preg_match("/$marker_ol_start_re/", $matches[4], $ol_start_array);
                                if ($ol_start_check){
                                        $ol_start = $ol_start_array[0];
                                }
                        }
                }

                if ($ol_start > 1 && $list_type == 'ol'){
                        $result = $this->hashBlock("<$list_type start=\"$ol_start\">\n" . $result . "</$list_type>");
                } else {
                        $result = $this->hashBlock("<$list_type>\n" . $result . "</$list_type>");
                }
                return "\n". $result ."\n\n";
        }

        /**
         * Nesting tracker for list levels
         * @var integer
         */
        protected $list_level = 0;

        /**
         * Process the contents of a single ordered or unordered list, splitting it
         * into individual list items.
         * @param  string $list_str
         * @param  string $marker_any_re
         * @return string
         */
        protected function processListItems($list_str, $marker_any_re) {
                /**
                 * The $this->list_level global keeps track of when we're inside a list.
                 * Each time we enter a list, we increment it; when we leave a list,
                 * we decrement. If it's zero, we're not in a list anymore.
                 *
                 * We do this because when we're not inside a list, we want to treat
                 * something like this:
                 *
                 *              I recommend upgrading to version
                 *              8. Oops, now this line is treated
                 *              as a sub-list.
                 *
                 * As a single paragraph, despite the fact that the second line starts
                 * with a digit-period-space sequence.
                 *
                 * Whereas when we're inside a list (or sub-list), that line will be
                 * treated as the start of a sub-list. What a kludge, huh? This is
                 * an aspect of Markdown's syntax that's hard to parse perfectly
                 * without resorting to mind-reading. Perhaps the solution is to
                 * change the syntax rules such that sub-lists must start with a
                 * starting cardinal number; e.g. "1." or "a.".
                 */             
                $this->list_level++;

                // Trim trailing blank lines:
                $list_str = preg_replace("/\n{2,}\\z/", "\n", $list_str);

                $list_str = preg_replace_callback('{
                        (\n)?                                                   # leading line = $1
                        (^[ ]*)                                                 # leading whitespace = $2
                        ('.$marker_any_re.'                             # list marker and space = $3
                                (?:[ ]+|(?=\n)) # space only required if item is not empty
                        )
                        ((?s:.*?))                                              # list item text   = $4
                        (?:(\n+(?=\n))|\n)                              # tailing blank line = $5
                        (?= \n* (\z | \2 ('.$marker_any_re.') (?:[ ]+|(?=\n))))
                        }xm',
                        array($this, '_processListItems_callback'), $list_str);

                $this->list_level--;
                return $list_str;
        }

        /**
         * List item parsing callback
         * @param  array $matches
         * @return string
         */
        protected function _processListItems_callback($matches) {
                $item = $matches[4];
                $leading_line =& $matches[1];
                $leading_space =& $matches[2];
                $marker_space = $matches[3];
                $tailing_blank_line =& $matches[5];

                if ($leading_line || $tailing_blank_line || 
                        preg_match('/\n{2,}/', $item))
                {
                        // Replace marker with the appropriate whitespace indentation
                        $item = $leading_space . str_repeat(' ', strlen($marker_space)) . $item;
                        $item = $this->runBlockGamut($this->outdent($item)."\n");
                } else {
                        // Recursion for sub-lists:
                        $item = $this->doLists($this->outdent($item));
                        $item = $this->formParagraphs($item, false);
                }

                return "<li>" . $item . "</li>\n";
        }

        /**
         * Process Markdown `<pre><code>` blocks.
         * @param  string $text
         * @return string
         */
        protected function doCodeBlocks($text) {
                $text = preg_replace_callback('{
                                (?:\n\n|\A\n?)
                                (                   # $1 = the code block -- one or more lines, starting with a space/tab
                                  (?>
                                        [ ]{'.$this->tab_width.'}  # Lines must start with a tab or a tab-width of spaces
                                        .*\n+
                                  )+
                                )
                                ((?=^[ ]{0,'.$this->tab_width.'}\S)|\Z) # Lookahead for non-space at line-start, or end of doc
                        }xm',
                        array($this, '_doCodeBlocks_callback'), $text);

                return $text;
        }

        /**
         * Code block parsing callback
         * @param  array $matches
         * @return string
         */
        protected function _doCodeBlocks_callback($matches) {
                $codeblock = $matches[1];

                $codeblock = $this->outdent($codeblock);
                if ($this->code_block_content_func) {
                        $codeblock = call_user_func($this->code_block_content_func, $codeblock, "");
                } else {
                        $codeblock = htmlspecialchars($codeblock, ENT_NOQUOTES);
                }

                # trim leading newlines and trailing newlines
                $codeblock = preg_replace('/\A\n+|\n+\z/', '', $codeblock);

                $codeblock = "<pre><code>$codeblock\n</code></pre>";
                return "\n\n" . $this->hashBlock($codeblock) . "\n\n";
        }

        /**
         * Create a code span markup for $code. Called from handleSpanToken.
         * @param  string $code
         * @return string
         */
        protected function makeCodeSpan($code) {
                if ($this->code_span_content_func) {
                        $code = call_user_func($this->code_span_content_func, $code);
                } else {
                        $code = htmlspecialchars(trim($code), ENT_NOQUOTES);
                }
                return $this->hashPart("<code>$code</code>");
        }

        /**
         * Define the emphasis operators with their regex matches
         * @var array
         */
        protected $em_relist = array(
                ''  => '(?:(?<!\*)\*(?!\*)|(?<!_)_(?!_))(?![\.,:;]?\s)',
                '*' => '(?<![\s*])\*(?!\*)',
                '_' => '(?<![\s_])_(?!_)',
        );

        /**
         * Define the strong operators with their regex matches
         * @var array
         */
        protected $strong_relist = array(
                ''   => '(?:(?<!\*)\*\*(?!\*)|(?<!_)__(?!_))(?![\.,:;]?\s)',
                '**' => '(?<![\s*])\*\*(?!\*)',
                '__' => '(?<![\s_])__(?!_)',
        );

        /**
         * Define the emphasis + strong operators with their regex matches
         * @var array
         */
        protected $em_strong_relist = array(
                ''    => '(?:(?<!\*)\*\*\*(?!\*)|(?<!_)___(?!_))(?![\.,:;]?\s)',
                '***' => '(?<![\s*])\*\*\*(?!\*)',
                '___' => '(?<![\s_])___(?!_)',
        );

        /**
         * Container for prepared regular expressions
         * @var array
         */
        protected $em_strong_prepared_relist;
        
        /**
         * Prepare regular expressions for searching emphasis tokens in any
         * context.
         * @return void
         */
        protected function prepareItalicsAndBold() {
                foreach ($this->em_relist as $em => $em_re) {
                        foreach ($this->strong_relist as $strong => $strong_re) {
                                // Construct list of allowed token expressions.
                                $token_relist = array();
                                if (isset($this->em_strong_relist["$em$strong"])) {
                                        $token_relist[] = $this->em_strong_relist["$em$strong"];
                                }
                                $token_relist[] = $em_re;
                                $token_relist[] = $strong_re;
                                
                                // Construct master expression from list.
                                $token_re = '{(' . implode('|', $token_relist) . ')}';
                                $this->em_strong_prepared_relist["$em$strong"] = $token_re;
                        }
                }
        }

        /**
         * Convert Markdown italics (emphasis) and bold (strong) to HTML
         * @param  string $text
         * @return string
         */
        protected function doItalicsAndBold($text) {
                $token_stack = array('');
                $text_stack = array('');
                $em = '';
                $strong = '';
                $tree_char_em = false;
                
                while (1) {
                        // Get prepared regular expression for seraching emphasis tokens
                        // in current context.
                        $token_re = $this->em_strong_prepared_relist["$em$strong"];
                        
                        // Each loop iteration search for the next emphasis token. 
                        // Each token is then passed to handleSpanToken.
                        $parts = preg_split($token_re, $text, 2, PREG_SPLIT_DELIM_CAPTURE);
                        $text_stack[0] .= $parts[0];
                        $token =& $parts[1];
                        $text =& $parts[2];
                        
                        if (empty($token)) {
                                // Reached end of text span: empty stack without emitting.
                                // any more emphasis.
                                while ($token_stack[0]) {
                                        $text_stack[1] .= array_shift($token_stack);
                                        $text_stack[0] .= array_shift($text_stack);
                                }
                                break;
                        }
                        
                        $token_len = strlen($token);
                        if ($tree_char_em) {
                                // Reached closing marker while inside a three-char emphasis.
                                if ($token_len == 3) {
                                        // Three-char closing marker, close em and strong.
                                        array_shift($token_stack);
                                        $span = array_shift($text_stack);
                                        $span = $this->runSpanGamut($span);
                                        $span = "<strong><em>$span</em></strong>";
                                        $text_stack[0] .= $this->hashPart($span);
                                        $em = '';
                                        $strong = '';
                                } else {
                                        // Other closing marker: close one em or strong and
                                        // change current token state to match the other
                                        $token_stack[0] = str_repeat($token{0}, 3-$token_len);
                                        $tag = $token_len == 2 ? "strong" : "em";
                                        $span = $text_stack[0];
                                        $span = $this->runSpanGamut($span);
                                        $span = "<$tag>$span</$tag>";
                                        $text_stack[0] = $this->hashPart($span);
                                        $$tag = ''; // $$tag stands for $em or $strong
                                }
                                $tree_char_em = false;
                        } else if ($token_len == 3) {
                                if ($em) {
                                        // Reached closing marker for both em and strong.
                                        // Closing strong marker:
                                        for ($i = 0; $i < 2; ++$i) {
                                                $shifted_token = array_shift($token_stack);
                                                $tag = strlen($shifted_token) == 2 ? "strong" : "em";
                                                $span = array_shift($text_stack);
                                                $span = $this->runSpanGamut($span);
                                                $span = "<$tag>$span</$tag>";
                                                $text_stack[0] .= $this->hashPart($span);
                                                $$tag = ''; // $$tag stands for $em or $strong
                                        }
                                } else {
                                        // Reached opening three-char emphasis marker. Push on token 
                                        // stack; will be handled by the special condition above.
                                        $em = $token{0};
                                        $strong = "$em$em";
                                        array_unshift($token_stack, $token);
                                        array_unshift($text_stack, '');
                                        $tree_char_em = true;
                                }
                        } else if ($token_len == 2) {
                                if ($strong) {
                                        // Unwind any dangling emphasis marker:
                                        if (strlen($token_stack[0]) == 1) {
                                                $text_stack[1] .= array_shift($token_stack);
                                                $text_stack[0] .= array_shift($text_stack);
                                        }
                                        // Closing strong marker:
                                        array_shift($token_stack);
                                        $span = array_shift($text_stack);
                                        $span = $this->runSpanGamut($span);
                                        $span = "<strong>$span</strong>";
                                        $text_stack[0] .= $this->hashPart($span);
                                        $strong = '';
                                } else {
                                        array_unshift($token_stack, $token);
                                        array_unshift($text_stack, '');
                                        $strong = $token;
                                }
                        } else {
                                // Here $token_len == 1
                                if ($em) {
                                        if (strlen($token_stack[0]) == 1) {
                                                // Closing emphasis marker:
                                                array_shift($token_stack);
                                                $span = array_shift($text_stack);
                                                $span = $this->runSpanGamut($span);
                                                $span = "<em>$span</em>";
                                                $text_stack[0] .= $this->hashPart($span);
                                                $em = '';
                                        } else {
                                                $text_stack[0] .= $token;
                                        }
                                } else {
                                        array_unshift($token_stack, $token);
                                        array_unshift($text_stack, '');
                                        $em = $token;
                                }
                        }
                }
                return $text_stack[0];
        }

        /**
         * Parse Markdown blockquotes to HTML
         * @param  string $text
         * @return string
         */
        protected function doBlockQuotes($text) {
                $text = preg_replace_callback('/
                          (                                                             # Wrap whole match in $1
                                (?>
                                  ^[ ]*>[ ]?                    # ">" at the start of a line
                                        .+\n                                    # rest of the first line
                                  (.+\n)*                                       # subsequent consecutive lines
                                  \n*                                           # blanks
                                )+
                          )
                        /xm',
                        array($this, '_doBlockQuotes_callback'), $text);

                return $text;
        }

        /**
         * Blockquote parsing callback
         * @param  array $matches
         * @return string
         */
        protected function _doBlockQuotes_callback($matches) {
                $bq = $matches[1];
                // trim one level of quoting - trim whitespace-only lines
                $bq = preg_replace('/^[ ]*>[ ]?|^[ ]+$/m', '', $bq);
                $bq = $this->runBlockGamut($bq); // recurse

                $bq = preg_replace('/^/m', "  ", $bq);
                // These leading spaces cause problem with <pre> content, 
                // so we need to fix that:
                $bq = preg_replace_callback('{(\s*<pre>.+?</pre>)}sx', 
                        array($this, '_doBlockQuotes_callback2'), $bq);

                return "\n" . $this->hashBlock("<blockquote>\n$bq\n</blockquote>") . "\n\n";
        }

        /**
         * Blockquote parsing callback
         * @param  array $matches
         * @return string
         */
        protected function _doBlockQuotes_callback2($matches) {
                $pre = $matches[1];
                $pre = preg_replace('/^  /m', '', $pre);
                return $pre;
        }

        /**
         * Parse paragraphs
         *
         * @param  string $text String to process in paragraphs
         * @param  boolean $wrap_in_p Whether paragraphs should be wrapped in <p> tags
         * @return string
         */
        protected function formParagraphs($text, $wrap_in_p = true) {
                // Strip leading and trailing lines:
                $text = preg_replace('/\A\n+|\n+\z/', '', $text);

                $grafs = preg_split('/\n{2,}/', $text, -1, PREG_SPLIT_NO_EMPTY);

                // Wrap <p> tags and unhashify HTML blocks
                foreach ($grafs as $key => $value) {
                        if (!preg_match('/^B\x1A[0-9]+B$/', $value)) {
                                // Is a paragraph.
                                $value = $this->runSpanGamut($value);
                                if ($wrap_in_p) {
                                        $value = preg_replace('/^([ ]*)/', "<p>", $value);
                                        $value .= "</p>";
                                }
                                $grafs[$key] = $this->unhash($value);
                        } else {
                                // Is a block.
                                // Modify elements of @grafs in-place...
                                $graf = $value;
                                $block = $this->html_hashes[$graf];
                                $graf = $block;
//                              if (preg_match('{
//                                      \A
//                                      (                                                       # $1 = <div> tag
//                                        <div  \s+
//                                        [^>]*
//                                        \b
//                                        markdown\s*=\s*  ([\'"])      #       $2 = attr quote char
//                                        1
//                                        \2
//                                        [^>]*
//                                        >
//                                      )
//                                      (                                                       # $3 = contents
//                                      .*
//                                      )
//                                      (</div>)                                        # $4 = closing tag
//                                      \z
//                                      }xs', $block, $matches))
//                              {
//                                      list(, $div_open, , $div_content, $div_close) = $matches;
//
//                                      // We can't call Markdown(), because that resets the hash;
//                                      // that initialization code should be pulled into its own sub, though.
//                                      $div_content = $this->hashHTMLBlocks($div_content);
//                                      
//                                      // Run document gamut methods on the content.
//                                      foreach ($this->document_gamut as $method => $priority) {
//                                              $div_content = $this->$method($div_content);
//                                      }
//
//                                      $div_open = preg_replace(
//                                              '{\smarkdown\s*=\s*([\'"]).+?\1}', '', $div_open);
//
//                                      $graf = $div_open . "\n" . $div_content . "\n" . $div_close;
//                              }
                                $grafs[$key] = $graf;
                        }
                }

                return implode("\n\n", $grafs);
        }

        /**
         * Encode text for a double-quoted HTML attribute. This function
         * is *not* suitable for attributes enclosed in single quotes.
         * @param  string $text
         * @return string
         */
        protected function encodeAttribute($text) {
                $text = $this->encodeAmpsAndAngles($text);
                $text = str_replace('"', '&quot;', $text);
                return $text;
        }

        /**
         * Encode text for a double-quoted HTML attribute containing a URL,
         * applying the URL filter if set. Also generates the textual
         * representation for the URL (removing mailto: or tel:) storing it in $text.
         * This function is *not* suitable for attributes enclosed in single quotes.
         *
         * @param  string $url
         * @param  string &$text Passed by reference
         * @return string        URL
         */
        protected function encodeURLAttribute($url, &$text = null) {
                if ($this->url_filter_func) {
                        $url = call_user_func($this->url_filter_func, $url);
                }

                if (preg_match('{^mailto:}i', $url)) {
                        $url = $this->encodeEntityObfuscatedAttribute($url, $text, 7);
                } else if (preg_match('{^tel:}i', $url)) {
                        $url = $this->encodeAttribute($url);
                        $text = substr($url, 4);
                } else {
                        $url = $this->encodeAttribute($url);
                        $text = $url;
                }

                return $url;
        }
        
        /**
         * Smart processing for ampersands and angle brackets that need to 
         * be encoded. Valid character entities are left alone unless the
         * no-entities mode is set.
         * @param  string $text
         * @return string
         */
        protected function encodeAmpsAndAngles($text) {
                if ($this->no_entities) {
                        $text = str_replace('&', '&amp;', $text);
                } else {
                        // Ampersand-encoding based entirely on Nat Irons's Amputator
                        // MT plugin: <http://bumppo.net/projects/amputator/>
                        $text = preg_replace('/&(?!#?[xX]?(?:[0-9a-fA-F]+|\w+);)/', 
                                                                '&amp;', $text);
                }
                // Encode remaining <'s
                $text = str_replace('<', '&lt;', $text);

                return $text;
        }

        /**
         * Parse Markdown automatic links to anchor HTML tags
         * @param  string $text
         * @return string
         */
        protected function doAutoLinks($text) {
                $text = preg_replace_callback('{<((https?|ftp|dict|tel):[^\'">\s]+)>}i',
                        array($this, '_doAutoLinks_url_callback'), $text);

                // Email addresses: <address@domain.foo>
                $text = preg_replace_callback('{
                        <
                        (?:mailto:)?
                        (
                                (?:
                                        [-!#$%&\'*+/=?^_`.{|}~\w\x80-\xFF]+
                                |
                                        ".*?"
                                )
                                \@
                                (?:
                                        [-a-z0-9\x80-\xFF]+(\.[-a-z0-9\x80-\xFF]+)*\.[a-z]+
                                |
                                        \[[\d.a-fA-F:]+\]       # IPv4 & IPv6
                                )
                        )
                        >
                        }xi',
                        array($this, '_doAutoLinks_email_callback'), $text);

                return $text;
        }

        /**
         * Parse URL callback
         * @param  array $matches
         * @return string
         */
        protected function _doAutoLinks_url_callback($matches) {
                $url = $this->encodeURLAttribute($matches[1], $text);
                $link = "<a href=\"$url\">$text</a>";
                return $this->hashPart($link);
        }

        /**
         * Parse email address callback
         * @param  array $matches
         * @return string
         */
        protected function _doAutoLinks_email_callback($matches) {
                $addr = $matches[1];
                $url = $this->encodeURLAttribute("mailto:$addr", $text);
                $link = "<a href=\"$url\">$text</a>";
                return $this->hashPart($link);
        }

        /**
         * Input: some text to obfuscate, e.g. "mailto:foo@example.com"
         *
         * Output: the same text but with most characters encoded as either a
         *         decimal or hex entity, in the hopes of foiling most address
         *         harvesting spam bots. E.g.:
         *
         *        &#109;&#x61;&#105;&#x6c;&#116;&#x6f;&#58;&#x66;o&#111;
         *        &#x40;&#101;&#x78;&#97;&#x6d;&#112;&#x6c;&#101;&#46;&#x63;&#111;
         *        &#x6d;
         *
         * Note: the additional output $tail is assigned the same value as the
         * ouput, minus the number of characters specified by $head_length.
         *
         * Based by a filter by Matthew Wickline, posted to BBEdit-Talk.
         * With some optimizations by Milian Wolff. Forced encoding of HTML
         * attribute special characters by Allan Odgaard.
         *
         * @param  string  $text
         * @param  string  &$tail
         * @param  integer $head_length
         * @return string
         */
        protected function encodeEntityObfuscatedAttribute($text, &$tail = null, $head_length = 0) {
                if ($text == "") {
                        return $tail = "";
                }

                $chars = preg_split('/(?<!^)(?!$)/', $text);
                $seed = (int)abs(crc32($text) / strlen($text)); // Deterministic seed.

                foreach ($chars as $key => $char) {
                        $ord = ord($char);
                        // Ignore non-ascii chars.
                        if ($ord < 128) {
                                $r = ($seed * (1 + $key)) % 100; // Pseudo-random function.
                                // roughly 10% raw, 45% hex, 45% dec
                                // '@' *must* be encoded. I insist.
                                // '"' and '>' have to be encoded inside the attribute
                                if ($r > 90 && strpos('@"&>', $char) === false) {
                                        /* do nothing */
                                } else if ($r < 45) {
                                        $chars[$key] = '&#x'.dechex($ord).';';
                                } else {
                                        $chars[$key] = '&#'.$ord.';';
                                }
                        }
                }

                $text = implode('', $chars);
                $tail = $head_length ? implode('', array_slice($chars, $head_length)) : $text;

                return $text;
        }

        /**
         * Take the string $str and parse it into tokens, hashing embeded HTML,
         * escaped characters and handling code spans.
         * @param  string $str
         * @return string
         */
        protected function parseSpan($str) {
                $output = '';
                
                $span_re = '{
                                (
                                        \\\\'.$this->escape_chars_re.'
                                |
                                        (?<![`\\\\])
                                        `+                                              # code span marker
                        '.( $this->no_markup ? '' : '
                                |
                                        <!--    .*?     -->             # comment
                                |
                                        <\?.*?\?> | <%.*?%>             # processing instruction
                                |
                                        <[!$]?[-a-zA-Z0-9:_]+   # regular tags
                                        (?>
                                                \s
                                                (?>[^"\'>]+|"[^"]*"|\'[^\']*\')*
                                        )?
                                        >
                                |
                                        <[-a-zA-Z0-9:_]+\s*/> # xml-style empty tag
                                |
                                        </[-a-zA-Z0-9:_]+\s*> # closing tag
                        ').'
                                )
                                }xs';

                while (1) {
                        // Each loop iteration seach for either the next tag, the next 
                        // openning code span marker, or the next escaped character. 
                        // Each token is then passed to handleSpanToken.
                        $parts = preg_split($span_re, $str, 2, PREG_SPLIT_DELIM_CAPTURE);
                        
                        // Create token from text preceding tag.
                        if ($parts[0] != "") {
                                $output .= $parts[0];
                        }
                        
                        // Check if we reach the end.
                        if (isset($parts[1])) {
                                $output .= $this->handleSpanToken($parts[1], $parts[2]);
                                $str = $parts[2];
                        } else {
                                break;
                        }
                }
                
                return $output;
        }

        /**
         * Handle $token provided by parseSpan by determining its nature and
         * returning the corresponding value that should replace it.
         * @param  string $token
         * @param  string &$str
         * @return string
         */
        protected function handleSpanToken($token, &$str) {
                switch ($token{0}) {
                        case "\\":
                                return $this->hashPart("&#". ord($token{1}). ";");
                        case "`":
                                // Search for end marker in remaining text.
                                if (preg_match('/^(.*?[^`])'.preg_quote($token).'(?!`)(.*)$/sm', 
                                        $str, $matches))
                                {
                                        $str = $matches[2];
                                        $codespan = $this->makeCodeSpan($matches[1]);
                                        return $this->hashPart($codespan);
                                }
                                return $token; // Return as text since no ending marker found.
                        default:
                                return $this->hashPart($token);
                }
        }

        /**
         * Remove one level of line-leading tabs or spaces
         * @param  string $text
         * @return string
         */
        protected function outdent($text) {
                return preg_replace('/^(\t|[ ]{1,' . $this->tab_width . '})/m', '', $text);
        }


        /**
         * String length function for detab. `_initDetab` will create a function to
         * handle UTF-8 if the default function does not exist.
         * @var string
         */
        protected $utf8_strlen = 'mb_strlen';

        /**
         * Replace tabs with the appropriate amount of spaces.
         *
         * For each line we separate the line in blocks delemited by tab characters.
         * Then we reconstruct every line by adding the  appropriate number of space
         * between each blocks.
         * 
         * @param  string $text
         * @return string
         */
        protected function detab($text) {       
                $text = preg_replace_callback('/^.*\t.*$/m',
                        array($this, '_detab_callback'), $text);

                return $text;
        }

        /**
         * Replace tabs callback
         * @param  string $matches
         * @return string
         */
        protected function _detab_callback($matches) {
                $line = $matches[0];
                $strlen = $this->utf8_strlen; // strlen function for UTF-8.
                
                // Split in blocks.
                $blocks = explode("\t", $line);
                // Add each blocks to the line.
                $line = $blocks[0];
                unset($blocks[0]); // Do not add first block twice.
                foreach ($blocks as $block) {
                        // Calculate amount of space, insert spaces, insert block.
                        $amount = $this->tab_width - 
                                $strlen($line, 'UTF-8') % $this->tab_width;
                        $line .= str_repeat(" ", $amount) . $block;
                }
                return $line;
        }

        /**
         * Check for the availability of the function in the `utf8_strlen` property
         * (initially `mb_strlen`). If the function is not available, create a 
         * function that will loosely count the number of UTF-8 characters with a
         * regular expression.
         * @return void
         */
        protected function _initDetab() {
        
                if (function_exists($this->utf8_strlen)) {
                        return;
                }

                $this->utf8_strlen = function($text) {
                        return preg_match_all('/[\x00-\xBF]|[\xC0-\xFF][\x80-\xBF]*/', $text, $m);
                };
        }

        /**
         * Swap back in all the tags hashed by _HashHTMLBlocks.
         * @param  string $text
         * @return string
         */
        protected function unhash($text) {
                return preg_replace_callback('/(.)\x1A[0-9]+\1/', 
                        array($this, '_unhash_callback'), $text);
        }

        /**
         * Unhashing callback
         * @param  array $matches
         * @return string
         */
        protected function _unhash_callback($matches) {
                return $this->html_hashes[$matches[0]];
        }
}