In this lesson we'll capture groups of characters we wish to match, use quantifiers with those groups, and use references to those groups in String.prototype.replace.
As explained in the lesson, if you wanted to trim back on the memory allocated for storing the reference to the group you'd opt out. Perhaps you only wanted to utilize the alternation functionality, but did not need to reference the group later as expressed in something like:
var regex = /www\.google\.(?:com|net|org)/