is-regex 
Is this value a JS regex? This module works cross-realm/iframe, and despite ES6 @@toStringTag.
Example
var isRegex = require('is-regex');
var assert = require('assert');
assert.notOk(isRegex(undefined));
assert.notOk(isRegex(null));
assert.notOk(isRegex(false));
assert.notOk(isRegex(true));
assert.notOk(isRegex(42));
assert.notOk(isRegex('foo'));
assert.notOk(isRegex(function () {}));
assert.notOk(isRegex([]));
assert.notOk(isRegex({}));
assert.ok(isRegex(/a/g));
assert.ok(isRegex(new RegExp('a', 'g')));
Tests
Simply clone the repo, npm install, and run npm test