[{"_id":"56d6e8bbf7404bd80a017edb","name":"document1","tags":["A area1","B area2","C area3"]},{"_id":"56d6e8bbf7404bd82d017ede","name":"document2","tags":["b_area3","b_area4","b_area5"]}]

db.collection.aggregate([{$project:{tags:{$filter:{input:"$tags",cond:{$regexMatch:{input:"$$this",regex:"area3"}}}}}}])