Do you like apis and nodejs as I do?, then this must be for you, have you noticed that all the famous apis have a common functionality? the ability to request partial responses with masks, so if you have a friend list you may want to return only a few fields you know just in case. You will end up requesting someting like /me/friends?fields=name,email,address. But also may want to return complex references like your friend's friends, so this regex let you match things like name,email,address,friends(name,email) as the 4 fields they are, then you can use a portion of the regex to extract the inner fields. Why the fancy introduction? we need some context :P enjoy