Add thickbox to WordPress images and galleries -simple

<?php
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 $content;
}
function brentini_add_thickbox_class_to_head(){
{?>
<script type="text/javascript">jQuery(document).ready(function($){
  $('dt.gallery-icon a').addClass('thickbox');
});</script>
<?php }}
add_action('wp_head', 'brentini_add_thickbox_class_to_head');