[{"Thing_id":"100","Thing_name":"Thing100","Comments":["Good: [Thing100 is awesome]","Bad: [I will never buy Thing100 again.]"]},{"Thing_id":"101","Thing_name":"Thing101","Comments":["Comparative: [Thing101 is so much better than Thing100.]","Bad: [Who designed such piece of …]"]}]

db.collection.update({/** only update docs that have Comments*/"$expr":{"$gt":[{"$size":"$Comments"},0]}},[{"$set":{/** rewrite Comments*/"Comments":{"$map":{"input":"$Comments","as":"comment","in":{"$reduce":{"input":{/** get all matches without "[" or "]"* every comment needs to be a string*/"$regexFindAll":{"input":"$$comment","regex":"[^\\[\\]]*"}},"initialValue":"","in":{/** concat all matches*/"$concat":["$$value","$$this.match"]}}}}}}}],{"multi":true})