Database
[{_id:1,"name":"alice","ssn":"123-45-6789"},{_id:2,name:"bob",ssn:"234-23-2345"},{_id:3,name:"clancy",ssn:"123-123-1234"}]
Query
db.collection.aggregate([{$addFields:{maskedssn:{$concat:["XXX-XX-",{$arrayElemAt:[{$getField:{field:"captures",input:{$regexFind:{input:"$ssn",regex:"\\d{3}-\\d{2}-(\\d{4})"}}}},0]}]}}}])