WordPress: Adicionando rel=lightbox em todas as imgs do post

/*
Adicionando rel="lightbox" em todas as imagens do post automaticamente. Lembre-se de instalar o jquery lightbox (do Leandro Vieira - http://leandrovieira.com/projects/jquery/lightbox/).
*/

function add_lightbox_rel($content) {
	$pattern = '/<a(.*?)href="(.*?).(bmp|gif|jpeg|jpg|png)"(.*?)>/i';
  	$replacement = '<a$1href="$2.$3" rel=\'lightbox\'$4>';
	$content = preg_replace( $pattern, $replacement, $content );
	return $content;
}
add_filter( 'the_content', 'add_lightbox_rel' );