Database
[{"originalValue":"DELETERANDOMVALUE_Keep_This_Words_DELETERANDOMVALUE"}]
Query
db.collection.aggregate([{"$addFields":{"first":{"$regexFind":{"input":"$originalValue","regex":"^([^_]*)_"}},"last":{"$regexFind":{"input":"$originalValue","regex":"_([^_]*)$"}}}},{"$project":{originalValue:{"$replaceAll":{"input":{"$replaceAll":{"input":"$originalValue","find":"$first.match","replacement":""}},"find":"$last.match","replacement":""}}}}])