gistfile1.php

<?php
$eventName = $modx->event->name;
switch ($eventName) {
    case 'OnLoadWebDocument' :
        $output = $modx->codeContent = $modx->resource->getContent();
        preg_match_all('/<pre.*?>(.*?)<\/pre>/ims', $output, $matches);

        $i = 0;
        if (empty($matches[1])) return;
        if (empty($modx->codeMatches)) $modx->codeMatches = array();
        foreach ($matches[1] as $key => $match) {
            $output = str_replace($match, '~*code_' . $i . '*~', $output);
            $modx->codeMatches['~*code_' . $i . '*~'] = $match;
            $i++;
        }
        $modx->resource->setContent($output);
        break;
    case 'OnWebPagePrerender' :
        $html = & $modx->resource->_output;
        $output = $modx->resource->getContent();
        /*preg_match_all('/<pre.*?>(.*?)<\/pre>/ims', $html, $matches); */
            
        $i = 0;
        
        $replacements = array (
            '<' => '&lt;',
            '>' => '&gt;',
        );
        if (empty($modx->codeMatches)) return;
        foreach ($modx->codeMatches as $key => $match) {
            //$match = preg_replace('/&([^\s]*;)/i', '&amp;$1', $match);
            foreach ($replacements as $key => $value) {
                $match = str_replace($key, $value, $match);
            }

            $html = str_replace('~*code_' . $i . '*~', $match, $html);
            $output = str_replace('~*code_' . $i . '*~', $match, $output);
            $i++;
        }
        $modx->resource->setContent($output);
        $modx->codeContent = $output;
        break;
    case 'OnBeforeSaveWebPageCache' :
        $modx->resource->setContent($modx->codeContent);
        break;
    default :
        break;
}