[{_id:"abc*hello*today*123","value":123},{_id:"abc*hello*today*999","value":999},{_id:"xyz*hello*tomorrow*123","value":123}]

db.collection.aggregate([{$group:{_id:{$slice:[{$split:["$_id","*"]},3]},results:{$push:"$$ROOT"}}},{$project:{_id:{$reduce:{input:"$_id",initialValue:"",in:{$concat:["$$value","*","$$this"]}}},results:1}}])