db.collection.aggregate([
{$addFields: {
"year": {$regexFindAll: {
input: "$born_string",
regex: /\b\d{4}\b/
}},
}},
{$addFields: {
"year": {$arrayElemAt: ["$year",0]},
}},
{$addFields: {
"year": "$year.match",
}},
{$addFields: {
"year": {$toInt: "$year"},
}},
]);