Database
[{"data":"ABCS_auto"},{"data":"TRQWEY_car_2"},{"data":"ALPHAB1234RAVO_Garie_2__4_22__11"},{"data":""},]
Query
db.collection.aggregate([{$set:{data_regex:{$regexFind:{input:"$data",regex:"_.*"}}}},{$project:{data:{"$cond":{"if":{$not:{$eq:["$data_regex",null]}},"then":{"$trim":{"input":"$data_regex.match","chars":"_"}},"else":"No auto found"}}}}])