Database

[{"_id":{"$oid":"573a1390f29313caabcd413b"},"title":"The Arrival of a Train","year":{"$numberInt":"1896"},"runtime":{"$numberInt":"1"},"released":{"$date":{"$numberLong":"-2335219200000"}},"poster":"http://ia.media-imdb.com/images/M/MV5BMjEyNDk5MDYzOV5BMl5BanBnXkFtZTgwNjIxMTEwMzE@._V1_SX300.jpg","plot":"A group of people are standing in a straight line along the platform of a railway station, waiting for a train, which is seen coming at some distance. When the train stops at the platform, ...","fullplot":"A group of people are standing in a straight line along the platform of a railway station, waiting for a train, which is seen coming at some distance. When the train stops at the platform, the line dissolves. The doors of the railway-cars open, and people on the platform help passengers to get off.","lastupdated":"2015-08-15 00:02:53.443000000","type":"movie","directors":["Auguste Lumière","Louis Lumière"],"imdb":{"rating":{"$numberDouble":"7.3"},"votes":{"$numberInt":"5043"},"id":{"$numberInt":"12"}},"cast":["Madeleine Koehler"],"countries":["France"],"genres":"[\"Documentary\", \"Short\"]",/** "genres": "",* "genres": "[]",*/"tomatoes":{"viewer":{"rating":{"$numberDouble":"3.7"},"numReviews":{"$numberInt":"59"}},"lastUpdated":{"$date":{"$numberLong":"1441993589000"}}},"num_mflix_comments":{"$numberInt":"1"}}]

Query

db.collection.update({"genres":{"$type":"string"}},[{"$set":{"genres":{"$map":{"input":{"$regexFindAll":{"input":"$genres","regex":"(?<=\")[^,]*?(?=\")"}},"as":"elem","in":"$$elem.match"}}}}],{"multi":true})

Result