Merge branch 'MDL-45849-selfenrol' of https://github.com/Peterburnett/moodle

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

<?php
/**
 * Markdown Extra - A text-to-HTML conversion tool for web writers
 *
 * @package   php-markdown
 * @author    Michel Fortin <michel.fortin@michelf.com>
 * @copyright 2004-2018 Michel Fortin <https://michelf.com/projects/php-markdown/>
 * @copyright (Original Markdown) 2004-2006 John Gruber <https://daringfireball.net/projects/markdown/>
 */

namespace Michelf;

/**
 * Markdown Extra Parser Class
 */
class MarkdownExtra extends \Michelf\Markdown {
        /**
         * Configuration variables
         */

        /**
         * Prefix for footnote ids.
         * @var string
         */
        public $fn_id_prefix = "";

        /**
         * Optional title attribute for footnote links and backlinks.
         * @var string
         */
        public $fn_link_title     = "";
        public $fn_backlink_title = "";

        /**
         * Optional class attribute for footnote links and backlinks.
         * @var string
         */
        public $fn_link_class     = "footnote-ref";
        public $fn_backlink_class = "footnote-backref";

        /**
         * Content to be displayed within footnote backlinks. The default is '↩';
         * the U+FE0E on the end is a Unicode variant selector used to prevent iOS
         * from displaying the arrow character as an emoji.
         * @var string
         */
        public $fn_backlink_html = '&#8617;&#xFE0E;';

        /**
         * Class name for table cell alignment (%% replaced left/center/right)
         * For instance: 'go-%%' becomes 'go-left' or 'go-right' or 'go-center'
         * If empty, the align attribute is used instead of a class name.
         * @var string
         */
        public $table_align_class_tmpl = '';

        /**
         * Optional class prefix for fenced code block.
         * @var string
         */
        public $code_class_prefix = "";

        /**
         * Class attribute for code blocks goes on the `code` tag;
         * setting this to true will put attributes on the `pre` tag instead.
         * @var boolean
         */
        public $code_attr_on_pre = false;

        /**
         * Predefined abbreviations.
         * @var array
         */
        public $predef_abbr = array();

        /**
         * Only convert atx-style headers if there's a space between the header and #
         * @var boolean
         */
        public $hashtag_protection = false;

        /**
         * Parser implementation
         */

        /**
         * Constructor function. Initialize the parser object.
         * @return void
         */
        public function __construct() {
                // Add extra escapable characters before parent constructor
                // initialize the table.
                $this->escape_chars .= ':|';

                // Insert extra document, block, and span transformations.
                // Parent constructor will do the sorting.
                $this->document_gamut += array(
                        "doFencedCodeBlocks" => 5,
                        "stripFootnotes"     => 15,
                        "stripAbbreviations" => 25,
                        "appendFootnotes"    => 50,
                );
                $this->block_gamut += array(
                        "doFencedCodeBlocks" => 5,
                        "doTables"           => 15,
                        "doDefLists"         => 45,
                );
                $this->span_gamut += array(
                        "doFootnotes"        => 5,
                        "doAbbreviations"    => 70,
                );

                $this->enhanced_ordered_list = true;
                parent::__construct();
        }


        /**
         * Extra variables used during extra transformations.
         * @var array
         */
        protected $footnotes = array();
        protected $footnotes_ordered = array();
        protected $footnotes_ref_count = array();
        protected $footnotes_numbers = array();
        protected $abbr_desciptions = array();
        /** @var string */
        protected $abbr_word_re = '';

        /**
         * Give the current footnote number.
         * @var integer
         */
        protected $footnote_counter = 1;

        /**
         * Setting up Extra-specific variables.
         */
        protected function setup() {
                parent::setup();

                $this->footnotes = array();
                $this->footnotes_ordered = array();
                $this->footnotes_ref_count = array();
                $this->footnotes_numbers = array();
                $this->abbr_desciptions = array();
                $this->abbr_word_re = '';
                $this->footnote_counter = 1;

                foreach ($this->predef_abbr as $abbr_word => $abbr_desc) {
                        if ($this->abbr_word_re)
                                $this->abbr_word_re .= '|';
                        $this->abbr_word_re .= preg_quote($abbr_word);
                        $this->abbr_desciptions[$abbr_word] = trim($abbr_desc);
                }
        }

        /**
         * Clearing Extra-specific variables.
         */
        protected function teardown() {
                $this->footnotes = array();
                $this->footnotes_ordered = array();
                $this->footnotes_ref_count = array();
                $this->footnotes_numbers = array();
                $this->abbr_desciptions = array();
                $this->abbr_word_re = '';

                parent::teardown();
        }


        /**
         * Extra attribute parser
         */

        /**
         * Expression to use to catch attributes (includes the braces)
         * @var string
         */
        protected $id_class_attr_catch_re = '\{((?>[ ]*[#.a-z][-_:a-zA-Z0-9=]+){1,})[ ]*\}';

        /**
         * Expression to use when parsing in a context when no capture is desired
         * @var string
         */
        protected $id_class_attr_nocatch_re = '\{(?>[ ]*[#.a-z][-_:a-zA-Z0-9=]+){1,}[ ]*\}';

        /**
         * Parse attributes caught by the $this->id_class_attr_catch_re expression
         * and return the HTML-formatted list of attributes.
         *
         * Currently supported attributes are .class and #id.
         *
         * In addition, this method also supports supplying a default Id value,
         * which will be used to populate the id attribute in case it was not
         * overridden.
         * @param  string $tag_name
         * @param  string $attr
         * @param  mixed  $defaultIdValue
         * @param  array  $classes
         * @return string
         */
        protected function doExtraAttributes($tag_name, $attr, $defaultIdValue = null, $classes = array()) {
                if (empty($attr) && !$defaultIdValue && empty($classes)) return "";

                // Split on components
                preg_match_all('/[#.a-z][-_:a-zA-Z0-9=]+/', $attr, $matches);
                $elements = $matches[0];

                // Handle classes and IDs (only first ID taken into account)
                $attributes = array();
                $id = false;
                foreach ($elements as $element) {
                        if ($element[0] == '.') {
                                $classes[] = substr($element, 1);
                        } else if ($element[0] == '#') {
                                if ($id === false) $id = substr($element, 1);
                        } else if (strpos($element, '=') > 0) {
                                $parts = explode('=', $element, 2);
                                $attributes[] = $parts[0] . '="' . $parts[1] . '"';
                        }
                }

                if (!$id) $id = $defaultIdValue;

                // Compose attributes as string
                $attr_str = "";
                if (!empty($id)) {
                        $attr_str .= ' id="'.$this->encodeAttribute($id) .'"';
                }
                if (!empty($classes)) {
                        $attr_str .= ' class="'. implode(" ", $classes) . '"';
                }
                if (!$this->no_markup && !empty($attributes)) {
                        $attr_str .= ' '.implode(" ", $attributes);
                }
                return $attr_str;
        }

        /**
         * 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 = $4
                                                                [")]
                                                                [ ]*
                                                        )?      # title is optional
                                        (?:[ ]* '.$this->id_class_attr_catch_re.' )?  # $5 = extra id & class attr
                                                        (?:\n+|\Z)
                        }xm',
                        array($this, '_stripLinkDefinitions_callback'),
                        $text);
                return $text;
        }

        /**
         * Strip link definition callback
         * @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];
                $this->ref_attr[$link_id] = $this->doExtraAttributes("", $dummy =& $matches[5]);
                return ''; // String that will replace the block
        }


        /**
         * HTML block parser
         */

        /**
         * Tags that are always treated as block tags
         * @var string
         */
        protected $block_tags_re = 'p|div|h[1-6]|blockquote|pre|table|dl|ol|ul|address|form|fieldset|iframe|hr|legend|article|section|nav|aside|hgroup|header|footer|figcaption|figure';

        /**
         * Tags treated as block tags only if the opening tag is alone on its line
         * @var string
         */
        protected $context_block_tags_re = 'script|noscript|style|ins|del|iframe|object|source|track|param|math|svg|canvas|audio|video';

        /**
         * Tags where markdown="1" default to span mode:
         * @var string
         */
        protected $contain_span_tags_re = 'p|h[1-6]|li|dd|dt|td|th|legend|address';

        /**
         * Tags which must not have their contents modified, no matter where
         * they appear
         * @var string
         */
        protected $clean_tags_re = 'script|style|math|svg';

        /**
         * Tags that do not need to be closed.
         * @var string
         */
        protected $auto_close_tags_re = 'hr|img|param|source|track';

        /**
         * Hashify HTML Blocks and "clean tags".
         *
         * 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.
         *
         * This works by calling _HashHTMLBlocks_InMarkdown, which then calls
         * _HashHTMLBlocks_InHTML when it encounter block tags. When the markdown="1"
         * attribute is found within a tag, _HashHTMLBlocks_InHTML calls back
         *  _HashHTMLBlocks_InMarkdown to handle the Markdown syntax within the tag.
         * These two functions are calling each other. It's recursive!
         * @param  string $text
         * @return string
         */
        protected function hashHTMLBlocks($text) {
                if ($this->no_markup) {
                        return $text;
                }

                // Call the HTML-in-Markdown hasher.
                list($text, ) = $this->_hashHTMLBlocks_inMarkdown($text);

                return $text;
        }

        /**
         * Parse markdown text, calling _HashHTMLBlocks_InHTML for block tags.
         *
         * *   $indent is the number of space to be ignored when checking for code
         *     blocks. This is important because if we don't take the indent into
         *     account, something like this (which looks right) won't work as expected:
         *
         *     <div>
         *         <div markdown="1">
         *         Hello World.  <-- Is this a Markdown code block or text?
         *         </div>  <-- Is this a Markdown code block or a real tag?
         *     <div>
         *
         *     If you don't like this, just don't indent the tag on which
         *     you apply the markdown="1" attribute.
         *
         * *   If $enclosing_tag_re is not empty, stops at the first unmatched closing
         *     tag with that name. Nested tags supported.
         *
         * *   If $span is true, text inside must treated as span. So any double
         *     newline will be replaced by a single newline so that it does not create
         *     paragraphs.
         *
         * Returns an array of that form: ( processed text , remaining text )
         *
         * @param  string  $text
         * @param  integer $indent
         * @param  string  $enclosing_tag_re
         * @param  boolean $span
         * @return array
         */
        protected function _hashHTMLBlocks_inMarkdown($text, $indent = 0,
                                                                                $enclosing_tag_re = '', $span = false)
        {

                if ($text === '') return array('', '');

                // Regex to check for the presense of newlines around a block tag.
                $newline_before_re = '/(?:^\n?|\n\n)*$/';
                $newline_after_re =
                        '{
                                ^                                               # Start of text following the tag.
                                (?>[ ]*<!--.*?-->)?             # Optional comment.
                                [ ]*\n                                  # Must be followed by newline.
                        }xs';

                // Regex to match any tag.
                $block_tag_re =
                        '{
                                (                                       # $2: Capture whole tag.
                                        </?                                     # Any opening or closing tag.
                                                (?>                             # Tag name.
                                                        ' . $this->block_tags_re . '                    |
                                                        ' . $this->context_block_tags_re . '    |
                                                        ' . $this->clean_tags_re . '            |
                                                        (?!\s)'.$enclosing_tag_re . '
                                                )
                                                (?:
                                                        (?=[\s"\'/a-zA-Z0-9])   # Allowed characters after tag name.
                                                        (?>
                                                                ".*?"           |       # Double quotes (can contain `>`)
                                                                \'.*?\'         |       # Single quotes (can contain `>`)
                                                                .+?                             # Anything but quotes and `>`.
                                                        )*?
                                                )?
                                        >                                       # End of tag.
                                |
                                        <!--    .*?     -->     # HTML Comment
                                |
                                        <\?.*?\?> | <%.*?%>     # Processing instruction
                                |
                                        <!\[CDATA\[.*?\]\]>     # CData Block
                                ' . ( !$span ? ' # If not in span.
                                |
                                        # Indented code block
                                        (?: ^[ ]*\n | ^ | \n[ ]*\n )
                                        [ ]{' . ($indent + 4) . '}[^\n]* \n
                                        (?>
                                                (?: [ ]{' . ($indent + 4) . '}[^\n]* | [ ]* ) \n
                                        )*
                                |
                                        # Fenced code block marker
                                        (?<= ^ | \n )
                                        [ ]{0,' . ($indent + 3) . '}(?:~{3,}|`{3,})
                                        [ ]*
                                        (?: \.?[-_:a-zA-Z0-9]+ )? # standalone class name
                                        [ ]*
                                        (?: ' . $this->id_class_attr_nocatch_re . ' )? # extra attributes
                                        [ ]*
                                        (?= \n )
                                ' : '' ) . ' # End (if not is span).
                                |
                                        # Code span marker
                                        # Note, this regex needs to go after backtick fenced
                                        # code blocks but it should also be kept outside of the
                                        # "if not in span" condition adding backticks to the parser
                                        `+
                                )
                        }xs';


                $depth = 0;             // Current depth inside the tag tree.
                $parsed = "";   // Parsed text that will be returned.

                // Loop through every tag until we find the closing tag of the parent
                // or loop until reaching the end of text if no parent tag specified.
                do {
                        // Split the text using the first $tag_match pattern found.
                        // Text before  pattern will be first in the array, text after
                        // pattern will be at the end, and between will be any catches made
                        // by the pattern.
                        $parts = preg_split($block_tag_re, $text, 2,
                                                                PREG_SPLIT_DELIM_CAPTURE);

                        // If in Markdown span mode, add a empty-string span-level hash
                        // after each newline to prevent triggering any block element.
                        if ($span) {
                                $void = $this->hashPart("", ':');
                                $newline = "\n$void";
                                $parts[0] = $void . str_replace("\n", $newline, $parts[0]) . $void;
                        }

                        $parsed .= $parts[0]; // Text before current tag.

                        // If end of $text has been reached. Stop loop.
                        if (count($parts) < 3) {
                                $text = "";
                                break;
                        }

                        $tag  = $parts[1]; // Tag to handle.
                        $text = $parts[2]; // Remaining text after current tag.
                        $tag_re = preg_quote($tag); // For use in a regular expression.

                        // Check for: Fenced code block marker.
                        // Note: need to recheck the whole tag to disambiguate backtick
                        // fences from code spans
                        if (preg_match('{^\n?([ ]{0,' . ($indent + 3) . '})(~{3,}|`{3,})[ ]*(?:\.?[-_:a-zA-Z0-9]+)?[ ]*(?:' . $this->id_class_attr_nocatch_re . ')?[ ]*\n?$}', $tag, $capture)) {
                                // Fenced code block marker: find matching end marker.
                                $fence_indent = strlen($capture[1]); // use captured indent in re
                                $fence_re = $capture[2]; // use captured fence in re
                                if (preg_match('{^(?>.*\n)*?[ ]{' . ($fence_indent) . '}' . $fence_re . '[ ]*(?:\n|$)}', $text,
                                        $matches))
                                {
                                        // End marker found: pass text unchanged until marker.
                                        $parsed .= $tag . $matches[0];
                                        $text = substr($text, strlen($matches[0]));
                                }
                                else {
                                        // No end marker: just skip it.
                                        $parsed .= $tag;
                                }
                        }
                        // Check for: Indented code block.
                        else if ($tag[0] == "\n" || $tag[0] == " ") {
                                // Indented code block: pass it unchanged, will be handled
                                // later.
                                $parsed .= $tag;
                        }
                        // Check for: Code span marker
                        // Note: need to check this after backtick fenced code blocks
                        else if ($tag[0] == "`") {
                                // Find corresponding end marker.
                                $tag_re = preg_quote($tag);
                                if (preg_match('{^(?>.+?|\n(?!\n))*?(?<!`)' . $tag_re . '(?!`)}',
                                        $text, $matches))
                                {
                                        // End marker found: pass text unchanged until marker.
                                        $parsed .= $tag . $matches[0];
                                        $text = substr($text, strlen($matches[0]));
                                }
                                else {
                                        // Unmatched marker: just skip it.
                                        $parsed .= $tag;
                                }
                        }
                        // Check for: Opening Block level tag or
                        //            Opening Context Block tag (like ins and del)
                        //               used as a block tag (tag is alone on it's line).
                        else if (preg_match('{^<(?:' . $this->block_tags_re . ')\b}', $tag) ||
                                (       preg_match('{^<(?:' . $this->context_block_tags_re . ')\b}', $tag) &&
                                        preg_match($newline_before_re, $parsed) &&
                                        preg_match($newline_after_re, $text)    )
                                )
                        {
                                // Need to parse tag and following text using the HTML parser.
                                list($block_text, $text) =
                                        $this->_hashHTMLBlocks_inHTML($tag . $text, "hashBlock", true);

                                // Make sure it stays outside of any paragraph by adding newlines.
                                $parsed .= "\n\n$block_text\n\n";
                        }
                        // Check for: Clean tag (like script, math)
                        //            HTML Comments, processing instructions.
                        else if (preg_match('{^<(?:' . $this->clean_tags_re . ')\b}', $tag) ||
                                $tag[1] == '!' || $tag[1] == '?')
                        {
                                // Need to parse tag and following text using the HTML parser.
                                // (don't check for markdown attribute)
                                list($block_text, $text) =
                                        $this->_hashHTMLBlocks_inHTML($tag . $text, "hashClean", false);

                                $parsed .= $block_text;
                        }
                        // Check for: Tag with same name as enclosing tag.
                        else if ($enclosing_tag_re !== '' &&
                                // Same name as enclosing tag.
                                preg_match('{^</?(?:' . $enclosing_tag_re . ')\b}', $tag))
                        {
                                // Increase/decrease nested tag count.
                                if ($tag[1] == '/')                                             $depth--;
                                else if ($tag[strlen($tag)-2] != '/')   $depth++;

                                if ($depth < 0) {
                                        // Going out of parent element. Clean up and break so we
                                        // return to the calling function.
                                        $text = $tag . $text;
                                        break;
                                }

                                $parsed .= $tag;
                        }
                        else {
                                $parsed .= $tag;
                        }
                } while ($depth >= 0);

                return array($parsed, $text);
        }

        /**
         * Parse HTML, calling _HashHTMLBlocks_InMarkdown for block tags.
         *
         * *   Calls $hash_method to convert any blocks.
         * *   Stops when the first opening tag closes.
         * *   $md_attr indicate if the use of the `markdown="1"` attribute is allowed.
         *     (it is not inside clean tags)
         *
         * Returns an array of that form: ( processed text , remaining text )
         * @param  string $text
         * @param  string $hash_method
         * @param  string $md_attr
         * @return array
         */
        protected function _hashHTMLBlocks_inHTML($text, $hash_method, $md_attr) {
                if ($text === '') return array('', '');

                // Regex to match `markdown` attribute inside of a tag.
                $markdown_attr_re = '
                        {
                                \s*                     # Eat whitespace before the `markdown` attribute
                                markdown
                                \s*=\s*
                                (?>
                                        (["\'])         # $1: quote delimiter
                                        (.*?)           # $2: attribute value
                                        \1                      # matching delimiter
                                |
                                        ([^\s>]*)       # $3: unquoted attribute value
                                )
                                ()                              # $4: make $3 always defined (avoid warnings)
                        }xs';

                // Regex to match any tag.
                $tag_re = '{
                                (                                       # $2: Capture whole tag.
                                        </?                                     # Any opening or closing tag.
                                                [\w:$]+                 # Tag name.
                                                (?:
                                                        (?=[\s"\'/a-zA-Z0-9])   # Allowed characters after tag name.
                                                        (?>
                                                                ".*?"           |       # Double quotes (can contain `>`)
                                                                \'.*?\'         |       # Single quotes (can contain `>`)
                                                                .+?                             # Anything but quotes and `>`.
                                                        )*?
                                                )?
                                        >                                       # End of tag.
                                |
                                        <!--    .*?     -->     # HTML Comment
                                |
                                        <\?.*?\?> | <%.*?%>     # Processing instruction
                                |
                                        <!\[CDATA\[.*?\]\]>     # CData Block
                                )
                        }xs';

                $original_text = $text;         // Save original text in case of faliure.

                $depth          = 0;    // Current depth inside the tag tree.
                $block_text     = "";   // Temporary text holder for current text.
                $parsed         = "";   // Parsed text that will be returned.

                // Get the name of the starting tag.
                // (This pattern makes $base_tag_name_re safe without quoting.)
                if (preg_match('/^<([\w:$]*)\b/', $text, $matches))
                        $base_tag_name_re = $matches[1];

                // Loop through every tag until we find the corresponding closing tag.
                do {
                        // Split the text using the first $tag_match pattern found.
                        // Text before  pattern will be first in the array, text after
                        // pattern will be at the end, and between will be any catches made
                        // by the pattern.
                        $parts = preg_split($tag_re, $text, 2, PREG_SPLIT_DELIM_CAPTURE);

                        if (count($parts) < 3) {
                                // End of $text reached with unbalenced tag(s).
                                // In that case, we return original text unchanged and pass the
                                // first character as filtered to prevent an infinite loop in the
                                // parent function.
                                return array($original_text[0], substr($original_text, 1));
                        }

                        $block_text .= $parts[0]; // Text before current tag.
                        $tag         = $parts[1]; // Tag to handle.
                        $text        = $parts[2]; // Remaining text after current tag.

                        // Check for: Auto-close tag (like <hr/>)
                        //                       Comments and Processing Instructions.
                        if (preg_match('{^</?(?:' . $this->auto_close_tags_re . ')\b}', $tag) ||
                                $tag[1] == '!' || $tag[1] == '?')
                        {
                                // Just add the tag to the block as if it was text.
                                $block_text .= $tag;
                        }
                        else {
                                // Increase/decrease nested tag count. Only do so if
                                // the tag's name match base tag's.
                                if (preg_match('{^</?' . $base_tag_name_re . '\b}', $tag)) {
                                        if ($tag[1] == '/')                                             $depth--;
                                        else if ($tag[strlen($tag)-2] != '/')   $depth++;
                                }

                                // Check for `markdown="1"` attribute and handle it.
                                if ($md_attr &&
                                        preg_match($markdown_attr_re, $tag, $attr_m) &&
                                        preg_match('/^1|block|span$/', $attr_m[2] . $attr_m[3]))
                                {
                                        // Remove `markdown` attribute from opening tag.
                                        $tag = preg_replace($markdown_attr_re, '', $tag);

                                        // Check if text inside this tag must be parsed in span mode.
                                        $this->mode = $attr_m[2] . $attr_m[3];
                                        $span_mode = $this->mode == 'span' || $this->mode != 'block' &&
                                                preg_match('{^<(?:' . $this->contain_span_tags_re . ')\b}', $tag);

                                        // Calculate indent before tag.
                                        if (preg_match('/(?:^|\n)( *?)(?! ).*?$/', $block_text, $matches)) {
                                                $strlen = $this->utf8_strlen;
                                                $indent = $strlen($matches[1], 'UTF-8');
                                        } else {
                                                $indent = 0;
                                        }

                                        // End preceding block with this tag.
                                        $block_text .= $tag;
                                        $parsed .= $this->$hash_method($block_text);

                                        // Get enclosing tag name for the ParseMarkdown function.
                                        // (This pattern makes $tag_name_re safe without quoting.)
                                        preg_match('/^<([\w:$]*)\b/', $tag, $matches);
                                        $tag_name_re = $matches[1];

                                        // Parse the content using the HTML-in-Markdown parser.
                                        list ($block_text, $text)
                                                = $this->_hashHTMLBlocks_inMarkdown($text, $indent,
                                                        $tag_name_re, $span_mode);

                                        // Outdent markdown text.
                                        if ($indent > 0) {
                                                $block_text = preg_replace("/^[ ]{1,$indent}/m", "",
                                                                                                        $block_text);
                                        }

                                        // Append tag content to parsed text.
                                        if (!$span_mode)        $parsed .= "\n\n$block_text\n\n";
                                        else                            $parsed .= "$block_text";

                                        // Start over with a new block.
                                        $block_text = "";
                                }
                                else $block_text .= $tag;
                        }

                } while ($depth > 0);

                // Hash last block text that wasn't processed inside the loop.
                $parsed .= $this->$hash_method($block_text);

                return array($parsed, $text);
        }

        /**
         * Called whenever a tag must be hashed when a function inserts a "clean" tag
         * in $text, it passes through this function and is automaticaly escaped,
         * blocking invalid nested overlap.
         * @param  string $text
         * @return string
         */
        protected function hashClean($text) {
                return $this->hashPart($text, 'C');
        }

        /**
         * 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
                          \[
                                (' . $this->nested_brackets_re . ')     # link text = $2
                          \]
                          \(                    # literal paren
                                [ \n]*
                                (?:
                                        <(.+?)> # href = $3
                                |
                                        (' . $this->nested_url_parenthesis_re . ')      # href = $4
                                )
                                [ \n]*
                                (                       # $5
                                  ([\'"])       # quote char = $6
                                  (.*?)         # Title = $7
                                  \6            # matching quote
                                  [ \n]*        # ignore any spaces/tabs between closing quote and )
                                )?                      # title is optional
                          \)
                          (?:[ ]? ' . $this->id_class_attr_catch_re . ' )?       # $8 = id/class attributes
                        )
                        }xs',
                        array($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 for reference anchors
         * @param  array $matches
         * @return string
         */
        protected function _doAnchors_reference_callback($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\"";
                        }
                        if (isset($this->ref_attr[$link_id]))
                                $result .= $this->ref_attr[$link_id];

                        $link_text = $this->runSpanGamut($link_text);
                        $result .= ">$link_text</a>";
                        $result = $this->hashPart($result);
                }
                else {
                        $result = $whole_match;
                }
                return $result;
        }

        /**
         * Callback for inline anchors
         * @param  array $matches
         * @return string
         */
        protected function _doAnchors_inline_callback($matches) {
                $whole_match    =  $matches[1];
                $link_text              =  $this->runSpanGamut($matches[2]);
                $url                    =  $matches[3] == '' ? $matches[4] : $matches[3];
                $title                  =& $matches[7];
                $attr  = $this->doExtraAttributes("a", $dummy =& $matches[8]);

                // 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\"";
                }
                $result .= $attr;

                $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
                          \(                    # literal paren
                                [ \n]*
                                (?:
                                        <(\S*)> # src url = $3
                                |
                                        (' . $this->nested_url_parenthesis_re . ')      # src url = $4
                                )
                                [ \n]*
                                (                       # $5
                                  ([\'"])       # quote char = $6
                                  (.*?)         # title = $7
                                  \6            # matching quote
                                  [ \n]*
                                )?                      # title is optional
                          \)
                          (?:[ ]? ' . $this->id_class_attr_catch_re . ' )?       # $8 = id/class attributes
                        )
                        }xs',
                        array($this, '_doImages_inline_callback'), $text);

                return $text;
        }

        /**
         * Callback for referenced images
         * @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\"";
                        }
                        if (isset($this->ref_attr[$link_id]))
                                $result .= $this->ref_attr[$link_id];
                        $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 for inline images
         * @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];
                $attr  = $this->doExtraAttributes("img", $dummy =& $matches[8]);

                $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 .= $attr;
                $result .= $this->empty_element_suffix;

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

        /**
         * Process markdown headers. Redefined to add ID and class attribute support.
         * @param  string $text
         * @return string
         */
        protected function doHeaders($text) {
                // Setext-style headers:
                //  Header 1  {#header1}
                //        ========
                //
                //        Header 2  {#header2 .class1 .class2}
                //        --------
                //
                $text = preg_replace_callback(
                        '{
                                (^.+?)                                                          # $1: Header text
                                (?:[ ]+ ' . $this->id_class_attr_catch_re . ' )?         # $3 = id/class attributes
                                [ ]*\n(=+|-+)[ ]*\n+                            # $3: Header footer
                        }mx',
                        array($this, '_doHeaders_callback_setext'), $text);

                // atx-style headers:
                //      # Header 1        {#header1}
                //      ## Header 2       {#header2}
                //      ## Header 2 with closing hashes ##  {#header3.class1.class2}
                //      ...
                //      ###### Header 6   {.class2}
                //
                $text = preg_replace_callback('{
                                ^(\#{1,6})      # $1 = string of #\'s
                                [ ]'.($this->hashtag_protection ? '+' : '*').'
                                (.+?)           # $2 = Header text
                                [ ]*
                                \#*                     # optional closing #\'s (not counted)
                                (?:[ ]+ ' . $this->id_class_attr_catch_re . ' )?         # $3 = id/class attributes
                                [ ]*
                                \n+
                        }xm',
                        array($this, '_doHeaders_callback_atx'), $text);

                return $text;
        }

        /**
         * Callback for setext headers
         * @param  array $matches
         * @return string
         */
        protected function _doHeaders_callback_setext($matches) {
                if ($matches[3] == '-' && preg_match('{^- }', $matches[1])) {
                        return $matches[0];
                }

                $level = $matches[3][0] == '=' ? 1 : 2;

                $defaultId = is_callable($this->header_id_func) ? call_user_func($this->header_id_func, $matches[1]) : null;

                $attr  = $this->doExtraAttributes("h$level", $dummy =& $matches[2], $defaultId);
                $block = "<h$level$attr>" . $this->runSpanGamut($matches[1]) . "</h$level>";
                return "\n" . $this->hashBlock($block) . "\n\n";
        }

        /**
         * Callback for atx headers
         * @param  array $matches
         * @return string
         */
        protected function _doHeaders_callback_atx($matches) {
                $level = strlen($matches[1]);

                $defaultId = is_callable($this->header_id_func) ? call_user_func($this->header_id_func, $matches[2]) : null;
                $attr  = $this->doExtraAttributes("h$level", $dummy =& $matches[3], $defaultId);
                $block = "<h$level$attr>" . $this->runSpanGamut($matches[2]) . "</h$level>";
                return "\n" . $this->hashBlock($block) . "\n\n";
        }

        /**
         * Form HTML tables.
         * @param  string $text
         * @return string
         */
        protected function doTables($text) {
                $less_than_tab = $this->tab_width - 1;
                // Find tables with leading pipe.
                //
                //      | Header 1 | Header 2
                //      | -------- | --------
                //      | Cell 1   | Cell 2
                //      | Cell 3   | Cell 4
                $text = preg_replace_callback('
                        {
                                ^                                                       # Start of a line
                                [ ]{0,' . $less_than_tab . '}   # Allowed whitespace.
                                [|]                                                     # Optional leading pipe (present)
                                (.+) \n                                         # $1: Header row (at least one pipe)

                                [ ]{0,' . $less_than_tab . '}   # Allowed whitespace.
                                [|] ([ ]*[-:]+[-| :]*) \n       # $2: Header underline

                                (                                                       # $3: Cells
                                        (?>
                                                [ ]*                            # Allowed whitespace.
                                                [|] .* \n                       # Row content.
                                        )*
                                )
                                (?=\n|\Z)                                       # Stop at final double newline.
                        }xm',
                        array($this, '_doTable_leadingPipe_callback'), $text);

                // Find tables without leading pipe.
                //
                //      Header 1 | Header 2
                //      -------- | --------
                //      Cell 1   | Cell 2
                //      Cell 3   | Cell 4
                $text = preg_replace_callback('
                        {
                                ^                                                       # Start of a line
                                [ ]{0,' . $less_than_tab . '}   # Allowed whitespace.
                                (\S.*[|].*) \n                          # $1: Header row (at least one pipe)

                                [ ]{0,' . $less_than_tab . '}   # Allowed whitespace.
                                ([-:]+[ ]*[|][-| :]*) \n        # $2: Header underline

                                (                                                       # $3: Cells
                                        (?>
                                                .* [|] .* \n            # Row content
                                        )*
                                )
                                (?=\n|\Z)                                       # Stop at final double newline.
                        }xm',
                        array($this, '_DoTable_callback'), $text);

                return $text;
        }

        /**
         * Callback for removing the leading pipe for each row
         * @param  array $matches
         * @return string
         */
        protected function _doTable_leadingPipe_callback($matches) {
                $head           = $matches[1];
                $underline      = $matches[2];
                $content        = $matches[3];

                $content        = preg_replace('/^ *[|]/m', '', $content);

                return $this->_doTable_callback(array($matches[0], $head, $underline, $content));
        }

        /**
         * Make the align attribute in a table
         * @param  string $alignname
         * @return string
         */
        protected function _doTable_makeAlignAttr($alignname)
        {
                if (empty($this->table_align_class_tmpl)) {
                        return " align=\"$alignname\"";
                }

                $classname = str_replace('%%', $alignname, $this->table_align_class_tmpl);
                return " class=\"$classname\"";
        }

        /**
         * Calback for processing tables
         * @param  array $matches
         * @return string
         */
        protected function _doTable_callback($matches) {
                $head           = $matches[1];
                $underline      = $matches[2];
                $content        = $matches[3];

                // Remove any tailing pipes for each line.
                $head           = preg_replace('/[|] *$/m', '', $head);
                $underline      = preg_replace('/[|] *$/m', '', $underline);
                $content        = preg_replace('/[|] *$/m', '', $content);

                // Reading alignement from header underline.
                $separators     = preg_split('/ *[|] */', $underline);
                foreach ($separators as $n => $s) {
                        if (preg_match('/^ *-+: *$/', $s))
                                $attr[$n] = $this->_doTable_makeAlignAttr('right');
                        else if (preg_match('/^ *:-+: *$/', $s))
                                $attr[$n] = $this->_doTable_makeAlignAttr('center');
                        else if (preg_match('/^ *:-+ *$/', $s))
                                $attr[$n] = $this->_doTable_makeAlignAttr('left');
                        else
                                $attr[$n] = '';
                }

                // Parsing span elements, including code spans, character escapes,
                // and inline HTML tags, so that pipes inside those gets ignored.
                $head           = $this->parseSpan($head);
                $headers        = preg_split('/ *[|] */', $head);
                $col_count      = count($headers);
                $attr       = array_pad($attr, $col_count, '');

                // Write column headers.
                $text = "<table>\n";
                $text .= "<thead>\n";
                $text .= "<tr>\n";
                foreach ($headers as $n => $header)
                        $text .= "  <th$attr[$n]>" . $this->runSpanGamut(trim($header)) . "</th>\n";
                $text .= "</tr>\n";
                $text .= "</thead>\n";

                // Split content by row.
                $rows = explode("\n", trim($content, "\n"));

                $text .= "<tbody>\n";
                foreach ($rows as $row) {
                        // Parsing span elements, including code spans, character escapes,
                        // and inline HTML tags, so that pipes inside those gets ignored.
                        $row = $this->parseSpan($row);

                        // Split row by cell.
                        $row_cells = preg_split('/ *[|] */', $row, $col_count);
                        $row_cells = array_pad($row_cells, $col_count, '');

                        $text .= "<tr>\n";
                        foreach ($row_cells as $n => $cell)
                                $text .= "  <td$attr[$n]>" . $this->runSpanGamut(trim($cell)) . "</td>\n";
                        $text .= "</tr>\n";
                }
                $text .= "</tbody>\n";
                $text .= "</table>";

                return $this->hashBlock($text) . "\n";
        }

        /**
         * Form HTML definition lists.
         * @param  string $text
         * @return string
         */
        protected function doDefLists($text) {
                $less_than_tab = $this->tab_width - 1;

                // Re-usable pattern to match any entire dl list:
                $whole_list_re = '(?>
                        (                                                               # $1 = whole list
                          (                                                             # $2
                                [ ]{0,' . $less_than_tab . '}
                                ((?>.*\S.*\n)+)                         # $3 = defined term
                                \n?
                                [ ]{0,' . $less_than_tab . '}:[ ]+ # colon starting definition
                          )
                          (?s:.+?)
                          (                                                             # $4
                                  \z
                                |
                                  \n{2,}
                                  (?=\S)
                                  (?!                                           # Negative lookahead for another term
                                        [ ]{0,' . $less_than_tab . '}
                                        (?: \S.*\n )+?                  # defined term
                                        \n?
                                        [ ]{0,' . $less_than_tab . '}:[ ]+ # colon starting definition
                                  )
                                  (?!                                           # Negative lookahead for another definition
                                        [ ]{0,' . $less_than_tab . '}:[ ]+ # colon starting definition
                                  )
                          )
                        )
                )'; // mx

                $text = preg_replace_callback('{
                                (?>\A\n?|(?<=\n\n))
                                ' . $whole_list_re . '
                        }mx',
                        array($this, '_doDefLists_callback'), $text);

                return $text;
        }

        /**
         * Callback for processing definition lists
         * @param  array $matches
         * @return string
         */
        protected function _doDefLists_callback($matches) {
                // Re-usable patterns to match list item bullets and number markers:
                $list = $matches[1];

                // Turn double returns into triple returns, so that we can make a
                // paragraph for the last item in a list, if necessary:
                $result = trim($this->processDefListItems($list));
                $result = "<dl>\n" . $result . "\n</dl>";
                return $this->hashBlock($result) . "\n\n";
        }

        /**
         * Process the contents of a single definition list, splitting it
         * into individual term and definition list items.
         * @param  string $list_str
         * @return string
         */
        protected function processDefListItems($list_str) {

                $less_than_tab = $this->tab_width - 1;

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

                // Process definition terms.
                $list_str = preg_replace_callback('{
                        (?>\A\n?|\n\n+)                                         # leading line
                        (                                                                       # definition terms = $1
                                [ ]{0,' . $less_than_tab . '}   # leading whitespace
                                (?!\:[ ]|[ ])                                   # negative lookahead for a definition
                                                                                                #   mark (colon) or more whitespace.
                                (?> \S.* \n)+?                                  # actual term (not whitespace).
                        )
                        (?=\n?[ ]{0,3}:[ ])                                     # lookahead for following line feed
                                                                                                #   with a definition mark.
                        }xm',
                        array($this, '_processDefListItems_callback_dt'), $list_str);

                // Process actual definitions.
                $list_str = preg_replace_callback('{
                        \n(\n+)?                                                        # leading line = $1
                        (                                                                       # marker space = $2
                                [ ]{0,' . $less_than_tab . '}   # whitespace before colon
                                \:[ ]+                                                  # definition mark (colon)
                        )
                        ((?s:.+?))                                                      # definition text = $3
                        (?= \n+                                                         # stop at next definition mark,
                                (?:                                                             # next term or end of text
                                        [ ]{0,' . $less_than_tab . '} \:[ ]     |
                                        <dt> | \z
                                )
                        )
                        }xm',
                        array($this, '_processDefListItems_callback_dd'), $list_str);

                return $list_str;
        }

        /**
         * Callback for <dt> elements in definition lists
         * @param  array $matches
         * @return string
         */
        protected function _processDefListItems_callback_dt($matches) {
                $terms = explode("\n", trim($matches[1]));
                $text = '';
                foreach ($terms as $term) {
                        $term = $this->runSpanGamut(trim($term));
                        $text .= "\n<dt>" . $term . "</dt>";
                }
                return $text . "\n";
        }

        /**
         * Callback for <dd> elements in definition lists
         * @param  array $matches
         * @return string
         */
        protected function _processDefListItems_callback_dd($matches) {
                $leading_line   = $matches[1];
                $marker_space   = $matches[2];
                $def                    = $matches[3];

                if ($leading_line || preg_match('/\n{2,}/', $def)) {
                        // Replace marker with the appropriate whitespace indentation
                        $def = str_repeat(' ', strlen($marker_space)) . $def;
                        $def = $this->runBlockGamut($this->outdent($def . "\n\n"));
                        $def = "\n". $def ."\n";
                }
                else {
                        $def = rtrim($def);
                        $def = $this->runSpanGamut($this->outdent($def));
                }

                return "\n<dd>" . $def . "</dd>\n";
        }

        /**
         * Adding the fenced code block syntax to regular Markdown:
         *
         * ~~~
         * Code block
         * ~~~
         *
         * @param  string $text
         * @return string
         */
        protected function doFencedCodeBlocks($text) {

                $less_than_tab = $this->tab_width;

                $text = preg_replace_callback('{
                                (?:\n|\A)
                                # 1: Opening marker
                                (
                                        (?:~{3,}|`{3,}) # 3 or more tildes/backticks.
                                )
                                [ ]*
                                (?:
                                        \.?([-_:a-zA-Z0-9]+) # 2: standalone class name
                                )?
                                [ ]*
                                (?:
                                        ' . $this->id_class_attr_catch_re . ' # 3: Extra attributes
                                )?
                                [ ]* \n # Whitespace and newline following marker.

                                # 4: Content
                                (
                                        (?>
                                                (?!\1 [ ]* \n)  # Not a closing marker.
                                                .*\n+
                                        )+
                                )

                                # Closing marker.
                                \1 [ ]* (?= \n )
                        }xm',
                        array($this, '_doFencedCodeBlocks_callback'), $text);

                return $text;
        }

        /**
         * Callback to process fenced code blocks
         * @param  array $matches
         * @return string
         */
        protected function _doFencedCodeBlocks_callback($matches) {
                $classname =& $matches[2];
                $attrs     =& $matches[3];
                $codeblock = $matches[4];

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

                $codeblock = preg_replace_callback('/^\n+/',
                        array($this, '_doFencedCodeBlocks_newlines'), $codeblock);

                $classes = array();
                if ($classname != "") {
                        if ($classname[0] == '.')
                                $classname = substr($classname, 1);
                        $classes[] = $this->code_class_prefix . $classname;
                }
                $attr_str = $this->doExtraAttributes($this->code_attr_on_pre ? "pre" : "code", $attrs, null, $classes);
                $pre_attr_str  = $this->code_attr_on_pre ? $attr_str : '';
                $code_attr_str = $this->code_attr_on_pre ? '' : $attr_str;
                $codeblock  = "<pre$pre_attr_str><code$code_attr_str>$codeblock</code></pre>";

                return "\n\n".$this->hashBlock($codeblock)."\n\n";
        }

        /**
         * Replace new lines in fenced code blocks
         * @param  array $matches
         * @return string
         */
        protected function _doFencedCodeBlocks_newlines($matches) {
                return str_repeat("<br$this->empty_element_suffix",
                        strlen($matches[0]));
        }

        /**
         * Redefining emphasis markers so that emphasis by underscore does not
         * work in the middle of a word.
         * @var array
         */
        protected $em_relist = array(
                ''  => '(?:(?<!\*)\*(?!\*)|(?<![a-zA-Z0-9_])_(?!_))(?![\.,:;]?\s)',
                '*' => '(?<![\s*])\*(?!\*)',
                '_' => '(?<![\s_])_(?![a-zA-Z0-9_])',
        );
        protected $strong_relist = array(
                ''   => '(?:(?<!\*)\*\*(?!\*)|(?<![a-zA-Z0-9_])__(?!_))(?![\.,:;]?\s)',
                '**' => '(?<![\s*])\*\*(?!\*)',
                '__' => '(?<![\s_])__(?![a-zA-Z0-9_])',
        );
        protected $em_strong_relist = array(
                ''    => '(?:(?<!\*)\*\*\*(?!\*)|(?<![a-zA-Z0-9_])___(?!_))(?![\.,:;]?\s)',
                '***' => '(?<![\s*])\*\*\*(?!\*)',
                '___' => '(?<![\s_])___(?![a-zA-Z0-9_])',
        );

        /**
         * Parse text into paragraphs
         * @param  string $text String to process in paragraphs
         * @param  boolean $wrap_in_p Whether paragraphs should be wrapped in <p> tags
         * @return string       HTML output
         */
        protected function formParagraphs($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) {
                        $value = trim($this->runSpanGamut($value));

                        // Check if this should be enclosed in a paragraph.
                        // Clean tag hashes & block tag hashes are left alone.
                        $is_p = $wrap_in_p && !preg_match('/^B\x1A[0-9]+B|^C\x1A[0-9]+C$/', $value);

                        if ($is_p) {
                                $value = "<p>$value</p>";
                        }
                        $grafs[$key] = $value;
                }

                // Join grafs in one text, then unhash HTML tags.
                $text = implode("\n\n", $grafs);

                // Finish by removing any tag hashes still present in $text.
                $text = $this->unhash($text);

                return $text;
        }


        /**
         * Footnotes - Strips link definitions from text, stores the URLs and
         * titles in hash references.
         * @param  string $text
         * @return string
         */
        protected function stripFootnotes($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 . '}\[\^(.+?)\][ ]?:  # note_id = $1
                          [ ]*
                          \n?                                   # maybe *one* newline
                        (                                               # text = $2 (no blank lines allowed)
                                (?:
                                        .+                              # actual text
                                |
                                        \n                              # newlines but
                                        (?!\[.+?\][ ]?:\s)# negative lookahead for footnote or link definition marker.
                                        (?!\n+[ ]{0,3}\S)# ensure line is not blank and followed
                                                                        # by non-indented content
                                )*
                        )
                        }xm',
                        array($this, '_stripFootnotes_callback'),
                        $text);
                return $text;
        }

        /**
         * Callback for stripping footnotes
         * @param  array $matches
         * @return string
         */
        protected function _stripFootnotes_callback($matches) {
                $note_id = $this->fn_id_prefix . $matches[1];
                $this->footnotes[$note_id] = $this->outdent($matches[2]);
                return ''; // String that will replace the block
        }

        /**
         * Replace footnote references in $text [^id] with a special text-token
         * which will be replaced by the actual footnote marker in appendFootnotes.
         * @param  string $text
         * @return string
         */
        protected function doFootnotes($text) {
                if (!$this->in_anchor) {
                        $text = preg_replace('{\[\^(.+?)\]}', "F\x1Afn:\\1\x1A:", $text);
                }
                return $text;
        }

        /**
         * Append footnote list to text
         * @param  string $text
         * @return string
         */
        protected function appendFootnotes($text) {
                $text = preg_replace_callback('{F\x1Afn:(.*?)\x1A:}',
                        array($this, '_appendFootnotes_callback'), $text);

                if (!empty($this->footnotes_ordered)) {
                        $text .= "\n\n";
                        $text .= "<div class=\"footnotes\" role=\"doc-endnotes\">\n";
                        $text .= "<hr" . $this->empty_element_suffix . "\n";
                        $text .= "<ol>\n\n";

                        $attr = "";
                        if ($this->fn_backlink_class != "") {
                                $class = $this->fn_backlink_class;
                                $class = $this->encodeAttribute($class);
                                $attr .= " class=\"$class\"";
                        }
                        if ($this->fn_backlink_title != "") {
                                $title = $this->fn_backlink_title;
                                $title = $this->encodeAttribute($title);
                                $attr .= " title=\"$title\"";
                                $attr .= " aria-label=\"$title\"";
                        }
                        $attr .= " role=\"doc-backlink\"";
                        $backlink_text = $this->fn_backlink_html;
                        $num = 0;

                        while (!empty($this->footnotes_ordered)) {
                                $footnote = reset($this->footnotes_ordered);
                                $note_id = key($this->footnotes_ordered);
                                unset($this->footnotes_ordered[$note_id]);
                                $ref_count = $this->footnotes_ref_count[$note_id];
                                unset($this->footnotes_ref_count[$note_id]);
                                unset($this->footnotes[$note_id]);

                                $footnote .= "\n"; // Need to append newline before parsing.
                                $footnote = $this->runBlockGamut("$footnote\n");
                                $footnote = preg_replace_callback('{F\x1Afn:(.*?)\x1A:}',
                                        array($this, '_appendFootnotes_callback'), $footnote);

                                $attr = str_replace("%%", ++$num, $attr);
                                $note_id = $this->encodeAttribute($note_id);

                                // Prepare backlink, multiple backlinks if multiple references
                                $backlink = "<a href=\"#fnref:$note_id\"$attr>$backlink_text</a>";
                                for ($ref_num = 2; $ref_num <= $ref_count; ++$ref_num) {
                                        $backlink .= " <a href=\"#fnref$ref_num:$note_id\"$attr>$backlink_text</a>";
                                }
                                // Add backlink to last paragraph; create new paragraph if needed.
                                if (preg_match('{</p>$}', $footnote)) {
                                        $footnote = substr($footnote, 0, -4) . "&#160;$backlink</p>";
                                } else {
                                        $footnote .= "\n\n<p>$backlink</p>";
                                }

                                $text .= "<li id=\"fn:$note_id\" role=\"doc-endnote\">\n";
                                $text .= $footnote . "\n";
                                $text .= "</li>\n\n";
                        }

                        $text .= "</ol>\n";
                        $text .= "</div>";
                }
                return $text;
        }

        /**
         * Callback for appending footnotes
         * @param  array $matches
         * @return string
         */
        protected function _appendFootnotes_callback($matches) {
                $node_id = $this->fn_id_prefix . $matches[1];

                // Create footnote marker only if it has a corresponding footnote *and*
                // the footnote hasn't been used by another marker.
                if (isset($this->footnotes[$node_id])) {
                        $num =& $this->footnotes_numbers[$node_id];
                        if (!isset($num)) {
                                // Transfer footnote content to the ordered list and give it its
                                // number
                                $this->footnotes_ordered[$node_id] = $this->footnotes[$node_id];
                                $this->footnotes_ref_count[$node_id] = 1;
                                $num = $this->footnote_counter++;
                                $ref_count_mark = '';
                        } else {
                                $ref_count_mark = $this->footnotes_ref_count[$node_id] += 1;
                        }

                        $attr = "";
                        if ($this->fn_link_class != "") {
                                $class = $this->fn_link_class;
                                $class = $this->encodeAttribute($class);
                                $attr .= " class=\"$class\"";
                        }
                        if ($this->fn_link_title != "") {
                                $title = $this->fn_link_title;
                                $title = $this->encodeAttribute($title);
                                $attr .= " title=\"$title\"";
                        }
                        $attr .= " role=\"doc-noteref\"";

                        $attr = str_replace("%%", $num, $attr);
                        $node_id = $this->encodeAttribute($node_id);

                        return
                                "<sup id=\"fnref$ref_count_mark:$node_id\">".
                                "<a href=\"#fn:$node_id\"$attr>$num</a>".
                                "</sup>";
                }

                return "[^" . $matches[1] . "]";
        }


        /**
         * Abbreviations - strips abbreviations from text, stores titles in hash
         * references.
         * @param  string $text
         * @return string
         */
        protected function stripAbbreviations($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 . '}\*\[(.+?)\][ ]?:  # abbr_id = $1
                        (.*)                                    # text = $2 (no blank lines allowed)
                        }xm',
                        array($this, '_stripAbbreviations_callback'),
                        $text);
                return $text;
        }

        /**
         * Callback for stripping abbreviations
         * @param  array $matches
         * @return string
         */
        protected function _stripAbbreviations_callback($matches) {
                $abbr_word = $matches[1];
                $abbr_desc = $matches[2];
                if ($this->abbr_word_re) {
                        $this->abbr_word_re .= '|';
                }
                $this->abbr_word_re .= preg_quote($abbr_word);
                $this->abbr_desciptions[$abbr_word] = trim($abbr_desc);
                return ''; // String that will replace the block
        }

        /**
         * Find defined abbreviations in text and wrap them in <abbr> elements.
         * @param  string $text
         * @return string
         */
        protected function doAbbreviations($text) {
                if ($this->abbr_word_re) {
                        // cannot use the /x modifier because abbr_word_re may
                        // contain significant spaces:
                        $text = preg_replace_callback('{' .
                                '(?<![\w\x1A])' .
                                '(?:' . $this->abbr_word_re . ')' .
                                '(?![\w\x1A])' .
                                '}',
                                array($this, '_doAbbreviations_callback'), $text);
                }
                return $text;
        }

        /**
         * Callback for processing abbreviations
         * @param  array $matches
         * @return string
         */
        protected function _doAbbreviations_callback($matches) {
                $abbr = $matches[0];
                if (isset($this->abbr_desciptions[$abbr])) {
                        $desc = $this->abbr_desciptions[$abbr];
                        if (empty($desc)) {
                                return $this->hashPart("<abbr>$abbr</abbr>");
                        } else {
                                $desc = $this->encodeAttribute($desc);
                                return $this->hashPart("<abbr title=\"$desc\">$abbr</abbr>");
                        }
                } else {
                        return $matches[0];
                }
        }
}