[{"key_name":"1 ff 42"}]

db.collection.aggregate([{"$addFields":{"results":{"$regexFindAll":{"input":"$key_name","regex":/\d+/}}}},{"$project":{"number":{"$toInt":{"$reduce":{"input":"$results","initialValue":"","in":{"$concat":["$$value","$$this.match"]}}}}}},{"$match":{"number":142}}])