[{"_id":1,"name":"docName","fieldA":".*"},{"_id":2,"name":"docName","fieldA":"AB.*"},{"_id":3,"name":"docName","fieldA":"AB.+"},{"_id":4,"name":"docName","fieldA":"BAC"}]

db.collection.aggregate([{$match:{name:"docName",$expr:{$regexMatch:{input:"ABC",regex:"$fieldA"}}}}])