Sanitize string of regex characters before RegExp build?

Question

I want to use a string to perform a global regex, but it might have regex characters in it. What's the best way to escape all regex characters in a string before building a regex with it?

Basically I might have something like this;

var test = 'test.';
var regex = new RegExp(test, 'ig');

I need 'test.' to become 'test\.' so it doesn't behave in unexpected ways.