[{"a":"its me hello","b":"it does not have value","c":"It has hello"}]

db.collection.aggregate([{"$project":{"data":{"$objectToArray":"$$ROOT"}}},{$unwind:"$data"},{"$match":{"data.v":{$regex:"hello"}}},{$group:{"_id":"$_id",data:{"$addToSet":{k:"$data.k",v:"$data.v"}}}},{"$project":{"data":{"$arrayToObject":"$data"}}}])