[{userid:"1",notes:["one","two","asdfffff","three","asdfasdas","as","dfaasdaasdfs","four"],},{userid:"2",notes:["one","two","asdfffff",],},{userid:"3",notes:["asdfasdas","as","dfaasdaasdfs","four"],}]

db.collection.aggregate([{$project:{userid:1,notes:{$filter:{input:"$notes",as:"note",cond:{"$regexMatch":{"input":"$$note","regex":"asdf","options":"i"}}}}}}])