db.getCollection('userset').find({type : "EntityBestItemsSet", subject : { $regex : "^http://data.europeana.eu/.*base.*"}}).count();

db.getCollection('userset').find({type : "EntityBestItemsSet", subject : { $regex : "^http://data.europeana.eu/.*base.*"}}).forEach(
    function (elem) {
     var subjectnewValue = elem.subject.map(a => a.replace('/base', ''));
     
     db.getCollection('userset').updateOne(
            {
                _id: elem._id
            },
            {
                $set: {
                    subject:subjectnewValue }
            }
        );
    }
);
