mongodb如何实现更新一个字段的值为另外一个字段的值?

db.set_forbidden_log.find({'register_number':{$exists:false}}).forEach( function(u) { //语法同js语法var substr = u.forbidden_sys_log.match(/[:|：]{1}[A|D]?[\d]+/);if(substr[0]){    var register_number_str=substr[0];}else{    var register_number_str=':'} u.register_number = register_number_str.substring(1); db.set_forbidden_log.save(u);});

代码意思：操作表为set_forbidden_log，将表中forbidden_sys_log字段中的值按正则匹配出来赋值给新字段register_number