instanceof regexp evalutes to false inside a sandbox

weird.js

var reg = /^[a-z]$/;

console.log('case 01: typeof /^[a-z]$/ returns %s', typeof reg);
console.log('case 02: /^[a-z]$/ instanceof RegExp returns %s', reg instanceof RegExp);
console.log('case 03: /^[a-z]$/.constructor.name === "RegExp" returns %s', reg.constructor.name === 'RegExp'); 

var util = require('util'),
    vm = require('vm'),
    sandbox = { u: reg };

vm.runInNewContext('out1 = /^[a-z]$/ instanceof RegExp; out2 = u instanceof RegExp; u_name = u.constructor.name', sandbox);
console.log(util.inspect(sandbox));

Is there a solution how to keep the regex object when passed to the vm?