[{"data":{"someEntry":"someValue","xy_bs":"will be matched","xy_abc":"will be also matched"}},{"data":{"someOtherEntry":"123"}},{"data":{"xy_123":"will be matched"}},]

db.collection.aggregate([{$project:{data:{$objectToArray:"$data"}}},{"$unwind":"$data"},{$match:{"data.k":{$regex:"^xy"}}}])