Database
[{"_id":ObjectId("5a934e000102030405000000"),"key":1},{"_id":ObjectId("1a934e000102030405000001"),"key":2},{"_id":ObjectId("01a934e00010203040500002"),"key":2}]
Query
db.collection.aggregate([{$addFields:{convertedId:{$toString:"$_id"}}},{$match:{convertedId:{$regex:"^1a934e",$options:"i"}}},{$project:{convertedId:0}}])