[{_id:1,Lookups:[{REG:"ABC"},{REG:"123"}]},{_id:2,Lookups:[{REG:"XYZ"},{REG:"123"}]}]

db.collection.find({"Lookups.REG":"ABC"},{Lookups:{$filter:{input:"$Lookups",as:"lookup",cond:{$eq:["$$lookup.REG","ABC"]}}}})