• patterns should be enclosed in //:
  • incorrect: |pattern|
  • correct: /pattern/
• all special characters must be escaped with \:
  • incorrrect:/([[)(.*?)|(.*?)(]])/
  • corrrect:/(\[\[)(.*?)\|(.*?)(\]\])/
• backreferences are referenced with $n, where n is a number:
  • incorrrect:<strong><em>2</em></strong>
  • corrrect:<strong><em>$2</em></strong>

Here's the updated working version:

<?php
/*
Plugin Name: MediaWiki Markup for bbPress
Plugin URI: http://bbpress.org/forums/topic/713
Description: Add a subset of MediaWiki markups to bbPress
Version: 0.1
Author: Jason Godesky
Author URI: http://anthropik.com/
*/

/*  Copyright 2006  Jason Godesky  (email : jason@anthropik.com)

    This program is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.

    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.

    You should have received a copy of the GNU General Public License
    along with this program; if not, write to the Free Software
    Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA  02111-1307  USA
*/

/*
These parameters specify the functioning of this plugin.
Edit accordingly for your specific situation.
*/

# Wiki root; "internal" links will point to the concatenation
# of this root and the link specified.
$mediawiki_filter_params["wiki"] = "http://en.wikipedia.org/wiki";

/*
Stop editing; actual plugin functionality follows.
*/

function bb_mediawikitext($text) {
    global $mediawiki_filter_params;
    // BASIC FORMATTING
    // Bold and italic
    $text = preg_replace("/(\\\'\\\'\\\'\\\'\\\')(.*?)(\\\'\\\'\\\'\\\'\\\')/",
        "<strong><em>$2</em></strong>", $text);

    // Bold
    $text = preg_replace("/(\\\'\\\'\\\')(.*?)(\\\'\\\'\\\')/",
        "<strong>$2</strong>", $text);
    // Italic
    $text = preg_replace("/(\\\'\\\')(.*?)(\\\'\\\')/",
        "<em>$2</em>", $text);

    // LINKS
    // Internal links with aliases
    $text = preg_replace("/(\\[\\[)(.*?)\\|(.*?)(\\]\\])/",
        "<a href=".'"'.$mediawiki_filter_params["wiki"]."$2".'"'.">$3</a>",
        $text);
    // Internal links without aliases
    $text = preg_replace("/(\\[\\[)(.*?)(\\]\\])/",
        "<a href=".'"'.$mediawiki_filter_params["wiki"]."$2".'"'.">$2</a>",
        $text);
    // External links with descriptions
    $text = preg_replace("/(\\[)(.*?) (.*?)(\\])/",
        "<a href=".'"'."$2".'"'.">$3</a>", $text);
    // External links with no description
    $count   = 1;
    $replace = TRUE;
    while ($replace) {
        $before = $text;
        $text = preg_replace("/(\\[)(.*?)(\\])/",
            "<a href=".'"'."$2".'"'.">[".$count."]</a>",
            $text, 1);
        if ($before==$text) { $replace = FALSE; }
        $count++;
    }

    // HEADINGS
    $text = preg_replace("/(======)(.*?)(======)/",
        "<h6>$2</h6>", $text);
    $text = preg_replace("/(=====)(.*?)(=====)/",
        "<h5>$2</h5>", $text);
    $text = preg_replace("/(====)(.*?)(====)/",
        "<h4>$2</h4>", $text);
    $text = preg_replace("/(===)(.*?)(===)/",
        "<h3>$2</h3>", $text);
    $text = preg_replace("/(==)(.*?)(==)/",
        "<h2>$2</h2>", $text);

    // RETURN
    return $text;
}

add_filter('pre_post', 'bb_mediawikitext', 1);

?>

<!--plugin-data-installation-->
<p>Add mediawiki-wikitext-filter.php to your /my-plugins/ directory.</p>

<!--plugin-data-faq-->
<h4>What markup is available?</h4>

<p>The color of the stars is set in the bb-ratings.css stylesheet file. The yellow color comes from "background-color: #fc0;",
and the red color comes from "background-color: #d00;". You can adjust these values to your taste.</p>

1. Basic formatting: Bolding and Italicizing
2. Basic internal links. Aliases work, but this does not include template transclusion, image embedding, or anything fancy like that.
3. External links. External links without descriptions are numbered, and descriptions are displayed.

<h4>Does it support lists? Tables of contents?</h4>

<p>No, but it would really be great if it did, so by all means give it a go. That's the glory of open source, right?</p>

<!--plugin-data-other_notes-->
<h3>Configuration</h3>
<p>At present, there is only one configuration option to change, on line 35: $mediawikifilterparams["wiki"]. This establishes the wiki you wish to use for "internal" links. "Internal" links work by concatenating the link onto the end of this parameter. The default value, "http://en.wikipedia.org/wiki/", will point to the English Wikipedia.</p>