WordPress: Add rel tag to links for Lightbox

functions.php

add_filter( 'the_content', 'we_addlightboxrel_replace', 12 );
add_filter( 'get_comment_text', 'we_addlightboxrel_replace' );
/**
 * Add rel tag to links for Lightbox
 */
function we_addlightboxrel_replace($content){
	global $post;
	$pattern = "/<a(.*?)href=('|\")([^>]*).(bmp|gif|jpeg|jpg|png)('|\")(.*?)>(.*?)<\/a>/i";
	$replacement = '<a$1href=$2$3.$4$5 rel="prettyPhoto"$6>$7';
	$content = preg_replace($pattern, $replacement, $content);
	return $content;
}