[{"_id":{"$oid":"6396c58284bfad036f960288"},"title":"This is a nice title.","time":"3266 sec"}]

db.collection.aggregate([{$set:{time:{$regexFind:{input:"$time",regex:"(\\d+) sec"}}}},{$set:{time:{$concat:["PT",{$arrayElemAt:["$time.captures",0]},"S"]}}}])