new Regexp doesn't work

I'm trying to convert following expression to "new Regexp()" style:

http://jsfiddle.net/HDWBZ/

var Wyrazenie = /\btest[a-z]*/g;

I'm really confused with it and have no idea how to fix it. Below is what I've done but obviously it doesn't work.

var Wyraznie = new RegExp("\btest[a-z]*","g");

Also have a question how would it look if instead of "test" I would use variable?