db={"collection":[{"key":0,name:"one"},{"key":12,"name":"two"}],"other":[{"key2":2,name:"onebigname"}]}

db.collection.aggregate([{$lookup:{from:"other",let:{name:"$name"},pipeline:[{$match:{$expr:{$regexMatch:{input:"$name",regex:"$$name",options:"i"}}}}],as:"newDocument"}},])