[{"_id":ObjectId("5e158e30e6facf7181cc3bdb"),"word":"adrenocorticotrophic"},{"_id":ObjectId("5e158e2ee6facf7181cc38cf"),"word":"attributive genitive"},{"_id":ObjectId("5e6f99eac16b5679b44becde"),"word":"attributive genitive new"}]

db.collection.aggregate([{$match:{$expr:{$eq:[{$strLenCP:"$word"},20]}}}])