Add thickbox to wordpress images

<?php
// adds thickbox to all images embedded in a post from the media library including galleries
add_filter('the_content', 'brentini_addthickboxclass');
function brentini_addthickboxclass($content) {
add_thickbox();
    $pattern[0] ="/<a(.*?)href=('|\")(.*?).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>/i";
    $replacement[0] = '<a$1href=$2$3.$4$5 class="thickbox">';
    $pattern[1]     = "/(<a href=)('|\")([^\>]*?)(\.bmp|\.gif|\.jpg|\.jpeg|\.png)('|\")([^\>]*?)(>)(.*?) title=('|\")(.*?)('|\")(.*?)(<\/a>)/i";
    $replacement[1] = '$1$2$3$4$5$6 title=$9$10$11$7$8 title=$9$10$11$12$13';
    $pattern[2]     = "/(<a href=)('|\")([^\>]*?)(\.bmp|\.gif|\.jpg|\.jpeg|\.png)('|\")([^\>]*?) title=([^\>]*?) title=([^\>]*?)(>)(.*?)(<\/a>)/i";
    $replacement[2] = '$1$2$3$4$5$6 title=$7$9$10$11';
    $content = preg_replace($pattern, $replacement, $content);
    return $content;
}
function gallery_shortcode_tbi($attr) {
    global $post;
    if ( isset( $attr['orderby'] ) ) {
        $attr['orderby'] = sanitize_sql_orderby( $attr['orderby'] );
        if ( !$attr['orderby'] )
            unset( $attr['orderby'] );
    }
    extract(shortcode_atts(array(
        'orderby'    => 'menu_order ASC, ID ASC',
        'id'         => $post->ID,
        'itemtag'    => 'dl',
        'icontag'    => 'dt',
        'captiontag' => 'dd',
        'columns'    => 3,
        'size'       => 'thumbnail',
    ), $attr));
    $id = intval($id);
    $attachments = get_children("post_parent=$id&post_type=attachment&post_mime_type=image&orderby={$orderby}");
    if ( empty($attachments) )
        return '';
    if ( is_feed() ) {
        $output = "\n";
        foreach ( $attachments as $id => $attachment )
            $output .= wp_get_attachment_link($id, $size, true) . "\n";
        return $output;
    }
    $itemtag = tag_escape($itemtag);
    $captiontag = tag_escape($captiontag);
    $columns = intval($columns);
    $itemwidth = $columns > 0 ? floor(100/$columns) : 100;
    $output = apply_filters('gallery_style', "
        <style type='text/css'>
            .gallery {
                margin: auto;
            }
            .gallery-item {
                float: left;
                margin-top: 10px;
                text-align: center;
                width: {$itemwidth}%;           }
            .gallery img {
                border: 2px solid #cfcfcf;
            }
            .gallery-caption {
                margin-left: 0;
            }
        </style>
        <div class='gallery'>");
    foreach ( $attachments as $id => $attachment ) {
        $a_img = wp_get_attachment_url($id);
        $att_page = get_attachment_link($id);
        $img = wp_get_attachment_image_src($id, $size);
        $img = $img[0];
        $title = $attachment->post_excerpt;
        if($title == '') $title = $attachment->post_title;
        $output .= "<{$itemtag} class='gallery-item'>";
        $output .= "
            <{$icontag} class='gallery-icon'>
                    <a href=\"$a_img\" title=\"$title\" class=\"thickbox\" rel=\"gallery-$post->ID\">
                    <img src=\"$img\" alt=\"$title\" />
                    </a>
            </{$icontag}>";
        if ( $captiontag && trim($attachment->post_excerpt) ) {
            $output .= "
                <{$captiontag} class='gallery-caption'>
                {$attachment->post_excerpt}
                </{$captiontag}>";
        }
        $output .= "</{$itemtag}>";
        if ( $columns == 0 )
            $output .= '<br style="clear: both" />';
    }
    $output .= "
            <br style='clear: both;' />
        </div>\n";
    return $output;
}
remove_shortcode('gallery');
add_shortcode('gallery', 'gallery_shortcode_tbi');