[{"data":"ABCS_auto"},{"data":"TRQWEY_car_2"},{"data":"ALPHAB1234RAVO_Garie_2__4_22__11"},{"data":""},]

db.collection.aggregate([{$set:{split_data:{$cond:{if:{$not:{$or:[{"$eq":["$data",null]},{$eq:["$data",""]}]}},then:{$split:["$data","_"]},else:[]}}}},{$project:{data:{"$cond":{"if":{$not:{$eq:["$split_data",[]]}},"then":{"$trim":{"input":{$reduce:{input:{"$slice":["$split_data",1,{$size:"$split_data"}]},initialValue:"",in:{$concat:["$$value","_","$$this"]}}},"chars":"_"}},"else":"No auto found"}}}}])