$str = '[[{"id":"","url":"","title":""xxxxxxxxxxxxxxxxxxxxxxxx"","thumb":"","description":"","color": "","size":"14px"}]]';
まず、","thumb" で始まり "xxxxxxxxxxxxxxxxxxxxxxxx" から "xxxxxxxxxxxxxxxxxxxxxxxxxxx" までの "title":" が必要です。 ご指導ください
望む結果かどうか確認してください
preg_replace("/title":""(.*)"","thumb/i","title:"\"\1\"" ,"親指",$str);