filter.php

<?php
// This file is part of Moodle - http://moodle.org/
//
// Moodle 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 3 of the License, or
// (at your option) any later version.
//
// Moodle 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 Moodle.  If not, see <http://www.gnu.org/licenses/>.

/**
 * Multi-Embed filter logic
 *
 * @package    filter_multiembed
 * @copyright  2016-2017 Frederic Nevers, www.iteachwithmoodle.com
 * @author     Frederic Nevers
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */

defined('MOODLE_INTERNAL') || die();


/**
 * Multi-Embed filter definition
 *
 * Class filter_multiembed
 *
 * @copyright  2016-2017 Frederic Nevers, www.iteachwithmoodle.com
 * @license    http://www.gnu.org/copyleft/gpl.html GNU GPL v3 or later
 */
class filter_multiembed extends moodle_text_filter {

    /**
     * Define the filter and run regex for each service
     * TODO: can the function be split up into smaller chunks?
     *
     * @param  string $text contained in page
     * @param  array $options
     * @return string $newtext filtered text
     */
    public function filter($text, array $options = array()) {
        global $PAGE;

        if (!is_string($text) or empty($text)) {
            // Non-string data can not be filtered anyway.
            return $text;
        }

        if (stripos($text, '</a>') === false) {
            // Performance shortcut - if there is no </a> tag, nothing can match.
            return $text;
        }

        $blacklistpages = explode(',', get_config('filter_multiembed', 'blacklistedpages'));

        foreach ($blacklistpages as $blacklistpage) {
            // Do not filter content in page types specified by admin.
            if ($PAGE->bodyid == $blacklistpage) {
                return $text;
            }
        }

        // Return the original text if the regex fails.
        $newtext = $text;

        // Some parts of the regexes are nearly always the same.
        $nofilter = get_config('filter_multiembed', 'nofilter');
        $regexstart = '/<a\s[^>]*href="(?![^ "]*\?'.$nofilter.')(https?:\/\/';
        $regexend = '"([^>]*)>(.*?)<\/a>/is';

        // Search for Book Creator books.
        if (get_config('filter_multiembed', 'bookcreator')) {
            $search = $regexstart.'(app|read\.)?)(bookcreator\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_bookcreatorcallback', $newtext);
        }

        // Search for Canva art.
        if (get_config('filter_multiembed', 'canva')) {
            $search = $regexstart.'(www\.)?)(canva\.com)\/(design)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_canvacallback', $newtext);
        }

        // Search for ClassTools tools.
        if (get_config('filter_multiembed', 'classtools')) {
            $search = $regexstart.'(www\.)?)(classtools\.net)\/(movietext|SMS|';
            $search .= 'brainybox|qwikslides|telescopic|arcade|hexagon|random-name-picker)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_classtoolscallback', $newtext);
        }

        // Search for CodePen snippets.
        if (get_config('filter_multiembed', 'codepen')) {
            $search = $regexstart.'(www\.)?)(codepen\.io)\/([^"]*?)\/(pen)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_codepencallback', $newtext);
        }

        // Search for Desmos calculators.
        if (get_config('filter_multiembed', 'desmos')) {
            $search = $regexstart.'(www\.)?)(desmos\.com)\/(calculator)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_desmoscallback', $newtext);
        }

        // Search for DiagnosticQuestions Questions and Quizzes.
        if (get_config('filter_multiembed', 'diagnosticq')) {
            $search = $regexstart.'(www\.)?)(diagnosticquestions\.com)\/(Questions|Quizzes)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_diagnosticqcallback', $newtext);
        }

        // Search for eMaze.
        if (get_config('filter_multiembed', 'emaze')) {
            $search = $regexstart.'(www\.)?)(emaze\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_emazecallback', $newtext);
        }

        // Search for Etherpad.
        if (get_config('filter_multiembed', 'etherpad')) {
            $search = $regexstart.'(etherpad\.)openstack\.org)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_etherpadcallback', $newtext);
        }

        // Search for Google Docs, Drawings, Forms, Presentations and Sheets.
        if (get_config('filter_multiembed', 'gdocs')) {
            $search = $regexstart.'(docs\.))(google\.com)\/(document|drawings|forms|';
            $search .= 'presentation|spreadsheets)\/([^"]*)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_gdocscallback', $newtext);
        }

        // Search for Google Drive documents (images, videos, PDFs, etc.).
        if (get_config('filter_multiembed', 'gdrive')) {
            $search = $regexstart.'(drive\.))(google\.com)\/(file)\/([^"]*)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_gdrivecallback', $newtext);
        }

        // Search for GSuite shared documents.
        if (get_config('filter_multiembed', 'gsuite')) {
            $search = $regexstart.'(docs\.))(google\.com)\/([^"]*)\/([^"]*)';
            $search .= '\/([^"]*)\/([^"]*)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_gsuitecallback', $newtext);
        }

        // Search for Haiku decks.
        if (get_config('filter_multiembed', 'haiku')) {
            $search = $regexstart.'(www\.)?)(haikudeck\.com)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_haikucallback', $newtext);
        }

        // Search for Imgur images.
        if (get_config('filter_multiembed', 'imgur')) {
            $search = $regexstart.'(www\.)?)(imgur\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_imgurcallback', $newtext);
        }

        // Search for Infogr.am visualisations.
        if (get_config('filter_multiembed', 'infogram')) {
            $search = $regexstart.'(www\.)?)(infogr\.am)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_infogramcallback', $newtext);
        }

        // Search for Learningapps.
        if (get_config('filter_multiembed', 'learningapps')) {
            $search = $regexstart.'(www\.)?)(learningapps\.org)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_learningappscallback', $newtext);
        }

        // Search for Padlet boards.
        if (get_config('filter_multiembed', 'padlet')) {
            $search = $regexstart.'(www\.)?)(padlet\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_padletcallback', $newtext);
        }

        // Search for PBS videos.
        if (get_config('filter_multiembed', 'pbs')) {
            $search = $regexstart.'(www\.)?)(pbs\.org)\/(video)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_pbscallback', $newtext);
        }

        // Search for Piktochart visualisations.
        if (get_config('filter_multiembed', 'piktochart')) {
            $search = $regexstart.'(magic|create\.)?)(piktochart\.com)\/(output)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_piktochartcallback', $newtext);
        }

        // Search for PollEverywhere polls.
        if (get_config('filter_multiembed', 'pollev')) {
            $search = $regexstart.'(www\.)?)(polleverywhere\.com|PollEv.com)\/(discourses|';
            $search .= 'polls|multiple_choice_polls|free_text_polls|ranking_polls)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_pollevcallback', $newtext);
        }

        // Search for Prezi presentations.
        if (get_config('filter_multiembed', 'prezi')) {
            $search = $regexstart.'(www\.)?)(prezi\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_prezicallback', $newtext);
        }

        // Search for Quizlet actvities.
        if (get_config('filter_multiembed', 'quizlet')) {
            $search = $regexstart.'(www\.)?)(quizlet\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_quizletcallback', $newtext);
        }

        // Search for Riddle quizzes.
        if (get_config('filter_multiembed', 'riddle')) {
            $search = $regexstart.'(www\.)?)(riddle\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_riddlecallback', $newtext);
        }

        // Search for Slid.es presentations.
        if (get_config('filter_multiembed', 'slides')) {
            $search = $regexstart.'(www\.)?)(slides\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_slidescallback', $newtext);
        }

        // Search for Smore creations.
        if (get_config('filter_multiembed', 'smore')) {
            $search = $regexstart.'(www\.)?)(smore\.com)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_smorecallback', $newtext);
        }

        // Search for Soundcloud tracks.
        if (get_config('filter_multiembed', 'soundcloud')) {
            $search = $regexstart.'(www\.)?)(soundcloud\.com)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_soundcloudcallback', $newtext);
        }

        // Search for Studystack items.
        if (get_config('filter_multiembed', 'studystack')) {
            $search = $regexstart.'(www\.)?)(studystack\.com)\/([^"]*)\-([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_studystackcallback', $newtext);
        }

        // Search for Sutori timelines.
        if (get_config('filter_multiembed', 'sutori')) {
            $search = $regexstart.'(www\.)?)(sutori\.com)\/timeline\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_sutoricallback', $newtext);
        }

        // Search for TED Videos.
        if (get_config('filter_multiembed', 'ted')) {
            $search = $regexstart.'(www\.)?)(ted\.com)\/talks\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_tedcallback', $newtext);
        }

        // Search for Thinglink images.
        if (get_config('filter_multiembed', 'thinglink')) {
            $search = $regexstart.'(www\.)?)(thinglink\.com)\/scene\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_thinglinkcallback', $newtext);
        }

        // Search for Trello boards or cards.
        if (get_config('filter_multiembed', 'trello')) {
            $search = $regexstart.'(www\.)?)(trello\.com)\/([^"]*)\/([^"]*)';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_trellocallback', $newtext);
        }

        // Search for Vimeo Videos.
        if (get_config('filter_multiembed', 'vimeo')) {
            $search = $regexstart;
            $search .= '(vimeo\.com\/)';
            $search .= '([0-9]+))';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_vimeocallback', $newtext);
        }

        // Search for YouTube videos.
        if (get_config('filter_multiembed', 'youtube')) {
            $search = '/<a\s[^>]*href="((?![^ "]*\?'.$nofilter.')';
            $search .= '(https?:\/\/(www\.)?)(youtube\.com|youtu\.be|youtube\.googleapis.com)';
            $search .= '\/(?:embed\/|v\/|watch\?v=|watch\?.+?&amp;v=|watch\?.+?&v=)?((\w|-){11})(.*?))';
            $search .= $regexend;
            $newtext = preg_replace_callback($search, 'filter_multiembed_youtubecallback', $newtext);
        }

        // Exception checks.
        if (empty($newtext) or $newtext === $text) {
            // Error or not filtered.
            unset($newtext);
            return $text;
        }

        return $newtext;
    }

}

/**
 * Turns a Book Creator link into an embedded book reader
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_bookcreatorcallback($link) {
    $embedcode = '<div style="display:inline-block;vertical-align:top;width:300px;margin:20px auto;color:#333;';
    $embedcode .= 'background:#fff;border:1px solid #ddd;line-height:1.2;text-decoration:none;padding:0;">';
    $embedcode .= '<a href="https://read.bookcreator.com/';
    $embedcode .= $link[4]; // Book creator first ID is in 4th capturing group of the regex.
    $embedcode .= '/';
    $embedcode .= $link[5]; // Book creator second ID ID is in 6th capturing group of the regex.
    $embedcode .= '" style="display:block;color:#333;line-height:1.2;text-decoration:none;text-align:left;padding:0;';
    $embedcode .= 'font-weight:normal;" target="_blank"><img class="lazyload" data-src="https://read.bookcreator.com/assets/';
    $embedcode .= $link[4]; // Book creator first ID is in 4th capturing group of the regex.
    $embedcode .= '/';
    $embedcode .= $link[5]; // Book creator second ID ID is in 6th capturing group of the regex.
    $embedcode .= '/cover" style="max-height:300px;max-width:100%;display:block;margin:0 auto;padding:0;border:none;"';
    $embedcode .= ' alt=""/></a><div style="display:block;padding:20px;overflow:hidden;overflow-x:hidden;';
    $embedcode .= 'border-top:1px solid #ddd;"><div style="display:block;color:#333;line-height:1.2;';
    $embedcode .= 'text-decoration:none;text-align:left;padding:0;font-weight:normal;font-size:16px;margin:0 0 0.5em;">';
    $embedcode .= '<a href="https://read.bookcreator.com/';
    $embedcode .= $link[4]; // Book creator first ID is in 4th capturing group of the regex.
    $embedcode .= '/';
    $embedcode .= $link[5]; // Book creator second ID ID is in 6th capturing group of the regex.
    $embedcode .= '" style="display:block;color:#333;line-height:1.2;text-decoration:none;text-align:left;padding:0;';
    $embedcode .= 'font-weight:normal;" target="_blank">Click to read this book, made with Book Creator</a></div>';
    $embedcode .= '<div style="display:block;color:#455a64;line-height:1.2;text-decoration:none;text-align:left;';
    $embedcode .= 'padding:0;font-weight:bold;overflow:hidden;text-overflow:ellipsis;white-space:nowrap;font-size:14px;">';
    $embedcode .= '<a href="https://read.bookcreator.com/';
    $embedcode .= $link[4]; // Book creator first ID is in 4th capturing group of the regex.
    $embedcode .= '/';
    $embedcode .= $link[5]; // Book creator second ID ID is in 6th capturing group of the regex.
    $embedcode .= '" style="display:block;color:#333;line-height:1.2;text-decoration:none;text-align:left;padding:0;';
    $embedcode .= 'font-weight:normal;" target="_blank">https://read.bookcreator.com</a></div></div></div>';
    return $embedcode;
}

/**
 * Turns a Canva link into an embedded artwork piece
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_canvacallback($link) {
    $embedcode = '<div class="canva-embed" data-height-ratio="0.7095" data-design-id="';
    $embedcode .= strtok($link[5], '/'); // Canva artwork ID is in 5th capturing group of the regex.
    $embedcode .= '" style="padding:70.95% 5px 5px 5px;background:rgba(0,0,0,0.03);border-radius:8px;"></div>';
    $embedcode .= '<script async src="https://sdk.canva.com/v1/embed.js"></script><a href="https://www.canva.com/design/';
    $embedcode .= strtok($link[5], '/'); // Canva artwork ID is in 5th capturing group of the regex.
    $embedcode .= '/view?utm_content=';
    $embedcode .= strtok($link[5], '/'); // Canva artwork ID is in 5th capturing group of the regex.
    $embedcode .= '&utm_campaign=designshare&utm_medium=embeds&utm_source=link" target="_blank">Created using Canva</a>';

    return $embedcode;
}

/**
 * Turns a ClassTools link into an embedded item
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_classtoolscallback($link) {

    $itemtype = $link[4]; // ClassTools item type is in 4th group of regex.

    switch ($itemtype) {
        case 'arcade':
            $embedcode = '<p align="center"><iframe class="lazyload" scrolling="no" data-src="//www.classtools.net/widg/';
            $embedcode .= $link[5];
            $embedcode .= '" width="570" height="250" frameborder=0></iframe></p>';
            break;
        case 'brainybox':
            $embedcode = '<p align="center"><iframe class="lazyload" scrolling="no" data-src="//www.classtools.net/brainybox/';
            $embedcode .= $link[5];
            $embedcode .= '&widget=1" width="650" height="650" frameborder=0></iframe></p>';
            break;
        case 'hexagon':
            $embedcode = '<iframe class="lazyload" data-src="//www.classtools.net/hexagon/';
            $embedcode .= $link[5];
            $embedcode .= '" style="background-color:white;position:relative;top:0;left:0;width:750px';
            $embedcode .= ';height:300px;overflow:none;border:none"></iframe>';
            break;
        case 'movietext':
            $embedcode = '<iframe class="lazyload" data-src="//www.classtools.net/movietext/';
            $embedcode .= $link[5];
            $embedcode .= '" style="position:relative;top:0;left:0;width:900px;height:650px';
            $embedcode .= ';overflow:none;border:none"></iframe>';
            break;
        case 'qwikslides':
            $embedcode = '<iframe class="lazyload" data-src="//www.classtools.net/qwikslides/';
            $embedcode .= $link[5];
            $embedcode .= '" style="position:relative;top:0;left:0;width:675px;height:600px';
            $embedcode .= ';overflow:none;border:none"></iframe>';
            break;
        case 'random-name-picker':
            $embedcode = '<iframe class="lazyload" data-src="//www.classtools.net/random-name-picker/';
            $embedcode .= $link[5];
            $embedcode .= '" style="position:relative;top:0;left:0;width:675px';
            $embedcode .= ';height:600px;overflow:none;border:none"></iframe>';
            break;
        case 'SMS':
            $embedcode = '<iframe class="lazyload" data-src="//www.classtools.net/SMS/';
            $embedcode .= $link[5];
            $embedcode .= '" style="position:relative;top:0;left:0;width:267px';
            $embedcode .= ';height:460px;overflow:none;border:none"></iframe>';
            break;
        case 'telescopic':
            $embedcode = '<div id="telescopic" class="lazyload" style="position:relative;padding-bottom:45%;height:0;">';
            $embedcode .= '<iframe id="fr" data-src="//www.classtools.net/telescopic/';
            $embedcode .= $link[5];
            $embedcode .= '?embedded=1" style="position:absolute;top:0;left:0;width:100%';
            $embedcode .= ';height:90%;overflow:hidden;border:none"></iframe></div>';
            break;
        default:
            $embedcode = 'issue';
    }

    return $embedcode;
}

/**
 * Turns a CodePen link into an embedded snippet
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_codepencallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-16by9">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" scrolling="no" data-src="//codepen.io/';
    $embedcode .= $link[4]; // CodePen user ID is in 4th capturing group of the regex.
    $embedcode .= '/embed/';
    $embedcode .= $link[6]; // CodePen snippet ID is in 6th capturing group of the regex.
    $embedcode .= '/?theme-id=0&amp;default-tab=css,result&embed-version=2" frameborder="no"';
    $embedcode .= ' allowtransparency="true" allowfullscreen="true"></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Desmos link into an embedded graphic calculator
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_desmoscallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<a title="View with the Desmos Graphing Calculator" href="https://www.desmos.com/calculator/';
    $embedcode .= $link[5]; // Desmos graphing calculators are in 5th capturing group of the regex.
    $embedcode .= '">  <img class="lazyload embed-responsive-item" data-src="https://s3.amazonaws.com/calc_thumbs/production/';
    $embedcode .= $link[5].'.png'; // Desmos graphing calculators are in 5th capturing group of the regex.
    $embedcode .= '" style="border:1px solid #ccc; border-radius:5px"/></a></div>';

    return $embedcode;
}

/**
 * Turns a Diagnostic Questions link into an embedded question or quizz
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_diagnosticqcallback($link) {
    $embedcode = '<iframe class="lazyload" width="664" height="568" data-src="https://diagnosticquestions.com/';
    $embedcode .= $link[4]; // Diagnostic Questions item types are in 4th capturing group of the regex.
    $embedcode .= '/Embed#/';
    $embedcode .= $link[6]; // Diagnostic Questions IDs are in 4th capturing group of the regex.
    $embedcode .= '" frameborder="0"></iframe>';

    return $embedcode;
}

/**
 * Turns an emaze link into an embedded presentation
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_emazecallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//app.emaze.com/';
    $embedcode .= $link[4].'/'.$link[5]; // Emaze presentation IDs are in the 4th capturing group of the regex.
    $embedcode .= '" seamless webkitallowfullscreen';
    $embedcode .= ' mozallowfullscreen allowfullscreen></iframe></div>';

    return $embedcode;
}

/**
 * Turns an Etherpad link into an embedded document
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_etherpadcallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" name="embed_readwrite" data-src="//etherpad.openstack.org/p/';
    $embedcode .= $link[4]; // Etherpad document IDs are in the 4th capturing group of the regex.
    $embedcode .= '?showControls=true&showChat=true&showLineNumbers=true&useMonospaceFont=false"></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Google Doc, Drawing, Form, Sheet or Slides link into an embedded (editable) document
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_gdocscallback($link) {
    if ($link[4] == 'drawings') {
        $embedcode = '<img src="//docs.google.com/';
    } else {
        $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
        $embedcode .= '<iframe class="lazyload embed-responsive-item" border="0" data-src="//docs.google.com/';
    }
    $embedcode .= $link[4].'/'; // Service type is in 4th capturing group of regex.
    $embedcode .= $link[5].'/'; // Unsure letter is always the same.
    $embedcode .= $link[6].'/'; // Google Doc IDs are in the 6th capturing group of the regex.

    // Check if documents are published to Web.
    if (strpos($link[7], 'pub') !== false) {

        // Deal with different use cases.
        $itemtype = $link[4]; // Service type is in 4th capturing group of regex.

        switch ($itemtype) {
            case 'document':
                $embedcode .= $link[7];
                $embedcode .= '?embedded=true';
                $embedcode .= '"></iframe></div>';
                break;
            case 'presentation':
                $embedcode .= str_replace("pub", "embed", $link[7]);
                $embedcode .= '" allowfullscreen="true" mozallowfullscreen="true" webkitallowfullscreen="true';
                $embedcode .= '"></iframe></div>';
                break;
            case 'spreadsheets':
                $embedcode .= $link[7];
                $embedcode .= '&widget=true&headers=false';
                $embedcode .= '"></iframe></div>';
                break;
            case 'drawings':
                $embedcode .= $link[7];
                $embedcode .= '">';
                break;
            case 'forms':
                $embedcode .= '"></iframe></div>';
                break;
            default:
                $embedcode .= 'edit?usp=sharing"></iframe></div>';
        }

    } else {
        $embedcode .= 'edit?usp=sharing"></iframe></div>';
    }

    return $embedcode;
}

/**
 * Turns a Google Drive document link into an embedded document. This works with PDF, images, videos, etc.
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_gdrivecallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//drive.google.com/file/';
    $embedcode .= $link[5].'/'; // Unsure letter is always the same.
    $embedcode .= $link[6].'/preview'; // Google Drive IDs are in the 6th capturing group of the regex.
    $embedcode .= '"></iframe></div>';

    return $embedcode;
}

/**
 * Turns a GSuite owned and shared Google Doc, Drawing, Form, Sheet or Slides link into an embedded (editable) document
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_gsuitecallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" border="0" data-src="//docs.google.com/';
    $embedcode .= $link[6].'/'; // Service type is in 6th capturing group of regex.
    $embedcode .= $link[7].'/'; // Unsure letter is always the same (should be a 'd').

    if ($link[8] == 'e') {     // If the 8th capturing group is the letter 'e';
        $embedcode .= $link[8].'/'.strtok($link[9], '/').'/'; // Then the Google Doc IDs are in the 9th group of the regex.
    } else {
        $embedcode .= strtok($link[8], '/').'/'; // Otherwise the IDs are in the 8th capturing group of the regex.
    }

    // Google forms follow a slightly different logic.
    if ($link[6] != 'forms') {
        $embedcode .= 'edit?usp=sharing"></iframe></div>';
    } else {
        $embedcode .= 'viewform"></iframe></div>';
    }

    return $embedcode;
}

/**
 * Turns a Haiku link into an embedded presentation
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_haikucallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//www.haikudeck.com/e/';
    // Only keep the last 10 characters of any URL, those are the deck IDs.
    $embedcode .= substr($link[4], -10); // Haiku deck IDs are in the 4th capturing group of the regex.
    $embedcode .= '/?isUrlHashEnabled=false&isPreviewEnabled=false&isHeaderVisible=false "';
    $embedcode .= 'frameborder="0" marginheight="0" marginwidth="0"></iframe></div>';

    return $embedcode;
}

/**
 * Turns an Imgur link into an embedded image
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_imgurcallback($link) {
    $embedcode = '<blockquote class="imgur-embed-pub" lang="en" data-id="a/';
    $embedcode .= $link[5]; // Imgur images are in the 5th capturing group of the regex.
    $embedcode .= '"><a href="//imgur.com/';
    $embedcode .= $link[5];
    $embedcode .= '">';
    $embedcode .= $link[5];
    $embedcode .= '</a></blockquote><script async src="//s.imgur.com/min/embed.js" charset="utf-8"></script>';

    return $embedcode;
}

/**
 * Turns an Infogr.am link into an embedded visualisation
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_infogramcallback($link) {
    $embedcode = '<div class="infogram-embed" data-id="';
    $embedcode .= $link[4]; // Infogr.am visualisation IDs are in the 5th capturing group of the regex.
    $embedcode .= '" data-type="interactive" data-title="Copy: ">';
    $embedcode .= '</div><script>!function(e,i,n,s){var t="InfogramEmbeds",d=e.getElementsByTagName("script")[0]';
    $embedcode .= ';if(window[t]&&window[t].initialized)window[t].process&&window[t].process();else if';
    $embedcode .= '(!e.getElementById(n)){var o=e.createElement("script");o.async=1,o.id=n,o.src=';
    $embedcode .= '"https://e.infogram.com/js/dist/embed-loader-min.js",d.parentNode.insertBefore(o,d)}}';
    $embedcode .= '(document,0,"infogram-async");</script>';

    return $embedcode;
}


/**
 * Turns a Learningapps link into an embedded app
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 *
 */
function filter_multiembed_learningappscallback($link) {
    $embedcode = '<iframe src="https://learningapps.org/watch?app=';
    $embedcode .= $link[4]; // Learningapps IDs are in the 4th capturing group of the regex.
    $embedcode .= '" style="border:0px;width:100%;height:500px" webkitallowfullscreen="true"';
    $embedcode .= ' mozallowfullscreen="true" allowfullscreen="true"></iframe>';

    return $embedcode;
}


/**
 * Turns a Padlet link into an embedded video
 * iframe code from www.padlet.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_padletcallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//padlet.com/embed/';
    $embedcode .= $link[5]; // Padlet IDs are in the 4th capturing group of the regex.
    $embedcode .= '" frameborder="0" style="padding:0;margin:0;border:none"></iframe></div>';

    return $embedcode;
}

/**
 * Turns a PBS link into an embedded video
 * iframe code from www.pbs.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_pbscallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-16by9">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//player.pbs.org/viralplayer/';
    $embedcode .= $link[5]; // PBS IDs are in the 4th capturing group of the regex.
    $embedcode .= '" frameborder="0" marginwidth="0" marginheight="0" scrolling="no" seamless allowfullscreen></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Piktochart link into an embedded poll
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_piktochartcallback($link) {
    $embedcode = '<div class="piktowrapper-embed" style="height: 300px; position: relative;" data-uid="';
    $embedcode .= $link[5]; // Piktochart ID is in 5th capturing group of regular expression.
    $embedcode .= '"><div class="pikto-canvas-wrap"><div class="pikto-canvas">';
    $embedcode .= '<div class="embed-loading-overlay" style="width: 100%; height: 100%; position: absolute;';
    $embedcode .= ' text-align: center;"><img width="60px" alt="Loading..." style="margin-top: 100px"';
    $embedcode .= ' src="https://create.piktochart.com/loading.gif"/><p style="margin: 0; padding: 0;';
    $embedcode .= ' font-family: Lato, Helvetica, Arial, sans-serif; font-weight: 600; font-size: 16px">';
    $embedcode .= 'Loading...</p></div></div></div></div><script>(function(d){var js, id="pikto-embed-js"';
    $embedcode .= ', ref=d.getElementsByTagName("script")[0];if (d.getElementById(id))';
    $embedcode .= ' { return;}js=d.createElement("script"); js.id=id; js.async=true;js.src=';
    $embedcode .= '"https://create.piktochart.com/assets/embedding/embed.js";ref.parentNode.';
    $embedcode .= 'insertBefore(js, ref);}(document));</script>';

    return $embedcode;
}


/**
 * Turns a PollEverywhere link into an embedded poll
 * iframe code from www.polleverywhere.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_pollevcallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="https://embed.polleverywhere.com/';
    $embedcode .= $link[4].'/'; // Type of activity.
    // Strip any unwanted parts of ID.
    $embedcode .= strtok($link[5], "/"); // PollEv IDs are in the 5th capturing group of the regex.
    $embedcode .= '?controls=none&short_poll=true" frameBorder="0"></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Prezi link into an embedded presentation
 * iframe code from www.prezi.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_prezicallback($link) {
    $embedcode = '<iframe id="iframe_container" class="lazyload" frameborder="0" webkitallowfullscreen="" mozallowfullscreen=""';
    $embedcode .= 'allowfullscreen="" width="550" height="400" data-src="//prezi.com/embed/';
    $embedcode .= $link[4]; // Prezi IDs are in the 4th capturing group of the regex.
    $embedcode .= '/?bgcolor=ffffff&amp;lock_to_path=0&amp;autoplay=0&amp;autohide_ctrls=0"></iframe>';

    return $embedcode;
}

/**
 * Turns a Quizlet link into an embedded activity
 * iframe code from www.quizlet.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_quizletcallback($link) {
    $embedcode = '<iframe class="lazyload" data-src="//quizlet.com/';

    // If URL is a study set, only keep the Quizlet ID from the 4th capturing group of the Regex.
    if (strpos($link[4], '/') !== false) {
        $embedcode .= strtok($link[4], '/');
    } else {
        $embedcode .= $link[4]; // If not a study set, no need to modify the 4th group of the Regex.
    }

    // Match the activity type found in original URL (if available, and supported).
    if (preg_match('~\b(learn|flashcards|spell|test|match)\b~i', $link[5])) {
        $embedcode .= '/'.$link[5];
        $embedcode .= '/embed" height="500" width="100%" style="border:0"></iframe>';
    } else {
        // If activity type is not supported (or if study set), then use the default activity type.
        $embedcode .= '/flashcards/embed" height="500" width="100%" style="border:0"></iframe>';
    }

    return $embedcode;
}

/**
 * Turns a Riddle link into an embedded quizz
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_riddlecallback($link) {
    $embedcode = '<div class="riddle_target embed-responsive embed-responsive-4by3" data-rid-id="';
    $embedcode .= basename($link[5]); // Riddle IDs are in the 5th capturing group of the regex.
    $embedcode .= '" data-fg="#1486cd" data-bg="#FFFFFF" style="margin:0 auto;">';
    $embedcode .= '<script src="https://www.riddle.com/files/js/embed.js"></script>';
    $embedcode .= '<iframe style="width:100%;height:300px;border:1px solid #cfcfcf" src="//riddle.com/a/';
    $embedcode .= basename($link[5]);
    $embedcode .= '?"></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Slid.es link into an embedded presentation
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_slidescallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//slides.com/';
    $embedcode .= $link[4].'/'; // Slid.es user IDs are in the 5th capturing group of the regex.
    $embedcode .= strtok($link[5], "/"); // Slid.es slide IDs are in the 6th capturing group of the regex.
    $embedcode .= 'embed" scrolling="no" frameborder="0"';
    $embedcode .= 'webkitallowfullscreen mozallowfullscreen allowfullscreen></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Smore link into an embedded creation
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_smorecallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-4by3">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//www.smore.com/';
    $embedcode .= $link[4]; // Smore IDs are in the 4th capturing group of the regex.
    $embedcode .= '?embed=1" scrolling="auto" frameborder="0" allowtransparency="true"';
    $embedcode .= ' style="min-width: 320px;border: none;"></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Soundcloud link into an embedded song
 * Please note that this may not work forever,
 * As it is a workaround (soundcloud API normally asks for tracks IDs)
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_soundcloudcallback($link) {
    $embedcode = '<iframe class="lazyload" width="100%" height="166" scrolling="no" frameborder="no"';
    $embedcode .= ' data-src="//w.soundcloud.com/player/?url=https%3A//soundcloud.com/';
    $embedcode .= $link[4]; // Soundcloud tracks are in the 4th capturing group of the regex.
    $embedcode .= '&amp;color=ff5500&amp;auto_play=false&amp;hide_related=false&amp;';
    $embedcode .= 'show_comments=true&amp;show_user=true&amp;show_reposts=false"></iframe>';

    return $embedcode;
}

/**
 * Turns a Studystack link into an embedded item
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_studystackcallback($link) {

    $itemtype = $link[4]; // Studystack item type is in 4th group of regex.

    switch ($itemtype) {
        case 'bugmatch':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'ibugmatch';
            $scrolling = 'no';
            $width = '800';
            $height = '500';
            break;
        case 'choppedupwords':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'ichoppedupwords';
            $scrolling = 'no';
            $width = '900';
            $height = '540';
            break;
        case 'crossword':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'icrossword';
            $scrolling = 'yes';
            $width = '950';
            $height = '600';
            break;
        case 'fillin':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'ifillin';
            $scrolling = 'yes';
            $width = '747';
            $height = '1000';
            break;
        case 'flashcard':
            $iframeclass = 'studyStackFlashcardIframe';
            $urlstem = 'inewflashcard';
            $scrolling = 'no';
            $width = '850';
            $height = '440';
            break;
        case 'hangman':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'ihangman';
            $scrolling = 'no';
            $width = '520';
            $height = '540';
            break;
        case 'hungrybug':
            $iframeclass = 'studyStackFlashcardIframe';
            $urlstem = 'ihungrybug';
            $scrolling = 'no';
            $width = '890';
            $height = '600';
            break;
        case 'picmatch':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'ipicmatch';
            $scrolling = 'no';
            $width = '1038';
            $height = '660';
            break;
        case 'quiz':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'iquiz';
            $scrolling = 'yes';
            $width = '727';
            $height = '800';
            break;
        case 'studyslide':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'istudyslide';
            $scrolling = 'yes';
            $width = '1040';
            $height = '800';
            break;
        case 'studystack':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'istudystack';
            $scrolling = 'yes';
            $width = '603';
            $height = '380';
            break;
        case 'studytable':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'istudytable';
            $scrolling = 'yes';
            $width = '747';
            $height = '1000';
            break;
        case 'test':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'itest';
            $scrolling = 'yes';
            $width = '727';
            $height = '800';
            break;
        case 'wordscramble':
            $iframeclass = 'studyStackMatchingIframe';
            $urlstem = 'iwordscramble';
            $scrolling = 'yes';
            $width = '720';
            $height = '480';
            break;
        default:
            $iframeclass = 'issue';
            $urlstem = 'issue';
            $scrolling = 'no';
            $width = '850';
            $height = '440';
    }

    $embedcode = '<iframe class="';
    $embedcode .= $iframeclass.' lazyload" data-src="https://www.studystack.com/';
    $embedcode .= $urlstem.'-'.$link[5].'" '; // Studystack IDs are in the 5th capturing group of the regex.
    $embedcode .= 'width="'.$width.'" height="'.$height;
    $embedcode .= '" frameborder="0" scrolling="'.$scrolling;
    $embedcode .= '" style="overflow:hidden"></iframe><br /><a href="http://www.studystack.com">';
    $embedcode .= 'Flashcards and educational games by StudyStack</a><br />';

    return $embedcode;
}

/**
 * Turns a Sutori link into an embedded timeline
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_sutoricallback($link) {
    $embedcode = '<script src="https://d1ox703z8b11rg.cloudfront.net/assets/iframeResizer.js"></script>';
    $embedcode .= '<iframe src="//www.sutori.com/timeline/';
    $embedcode .= $link[4]; // Sutori timelines IDs are in the 4th capturing group of the regex.
    $embedcode .= '/embed" width="100%" scrolling="no" frameborder="0" allowfullscreen></iframe>';
    $embedcode .= '<script src="https://d1ox703z8b11rg.cloudfront.net/assets/iframeResizer.executer.js"></script>';

    return $embedcode;
}

/**
 * Turns a TED link into an embedded video
 * iframe code from www.ted.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_tedcallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-16by9">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//embed.ted.com/talks/';
    $embedcode .= $link[4]; // TED video IDs are in the 4th capturing group of the regex.
    $embedcode .= '" frameborder="0" scrolling="no" ';
    $embedcode .= 'webkitAllowFullScreen mozallowfullscreen allowFullScreen></iframe></div>';

    return $embedcode;
}

/**
 * Turns a Thinglink link into an embedded image
 * iframe code from www.thinglink.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_thinglinkcallback($link) {
    $embedcode = '<iframe class="lazyload" width="800" height="500" data-src="//www.thinglink.com/card/';
    $embedcode .= $link[4]; // Thinglink image IDs are in the 4th capturing group of the regex.
    $embedcode .= '" type="text/html" frameborder="0" webkitallowfullscreen mozallowfullscreen';
    $embedcode .= ' allowfullscreen scrolling="no"></iframe>';

    return $embedcode;
}

/**
 * Turns a Trello link into an embedded card or board
 * depending on code pasted.
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_trellocallback($link) {
    $itemtype = $link[4]; // Trello item type is in 4th group of regex.

    switch ($itemtype) {
        case 'b':
            $divclass = 'trello-board-compact';
            $title = 'Trello Board';
            break;
        case 'c':
            $divclass = 'trello-card';
            $title = 'Trello Card';
            break;
        default:
            $divclass = 'trello-board-compact';
            $title = 'Trello Board';
    }

    $embedcode = '<blockquote class="';
    $embedcode .= $divclass;
    $embedcode .= '"><a href="https://trello.com/';
    $embedcode .= $link[4]; // Trello itemtype determines URL structure.
    $embedcode .= '/';
    $embedcode .= strtok($link[5], '/'); // Trello board or card IDs are in the 5th capturing group of the regex.
    $embedcode .= '">';
    $embedcode .= $title;
    $embedcode .= '</a></blockquote><script src="https://p.trellocdn.com/embed.min.js"></script>';

    return $embedcode;
}

/**
 * Turns a Vimeo link into an embedded video
 * iframe code from vimeo.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_vimeocallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-16by9">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//player.vimeo.com/video/';
    $embedcode .= $link[3]; // Vimeo video IDs are in the 3d capturing group of the regex.
    $embedcode .= '" frameborder="0" allowfullscreen></iframe></div>';

    return $embedcode;
}


/**
 * Turns a YouTube link into an embedded video
 * iframe code from www.youtube.com website
 *
 * @param  string $link HTML tag containing a link
 * @return string HTML content after processing.
 */
function filter_multiembed_youtubecallback($link) {
    $embedcode = '<div class="embed-responsive embed-responsive-16by9">';
    $embedcode .= '<iframe class="lazyload embed-responsive-item" data-src="//www.youtube.com/embed/';
    $embedcode .= $link[5]; // YouTube video IDs are in the 5th capturing group of the regex.
    $embedcode .= '" frameborder="0" allowfullscreen></iframe></div>';

    return $embedcode;
}