<?php
$link = "cover_photos_s/yasinallana1984751717_post_notif_aurora_kuenzli_big.jpg";
$reg_for_key = "~^(cover_photos_s\/[a-z0-9A-Z]+)(_\w+?_\w+?_)(.*)$~";
$replacement = "$1_thumb_$3";
echo preg_replace($reg_for_key, $replacement, $link);
?>