How to Decode JSON object in Laravel

I want to decode gallery array JSON objects in Laravel 5.1. my JSON is:

{
  "title": "aaaaaaaaaaaaaaaa",
  "category_id": "1",
  "user_id": "1",
  "gallery": "[{name: \"XCB808tvXNpqXKqekA2HlkJ8H.jpg\",size:5112},{name: \"s6kA6B0e5m1sdSAjPXqNwtiy4.jpg\", size: 13135}]"
}

When I use this code, return me null:

public function store(Request $request)
    {
         $json = json_decode($request['gallery'],true);
         return $json;
    }
}

via Chebli Mohamed