有个小问题

$pattern = '/(\d{4})\/(\d{2})\/(\d{2})/';

$subject='2017/10/10';

$res=preg_replace_callback($pattern,'nextyear',$subject);

function nextyear($var){

    return ($var[1]+1).'/'.($var[2]+1).'/'.($var[3]+1);

}

echo $res;

为什么当$pattern = '/(\d{4})(\d{2})(\d{2})/';时取不到内容