The notation can be confusing (it's alien, really). But it's worth learning. I thought I knew it pretty well, but some of the stuff I see here is really awesome.

To answer your questions, I don't know what :g//d does. g generally means "global" and d "delete", but in the VI I'm using it doesn't work.

You already know what \(...\) means, you said.

s/.*//g means replace everything with nothing (where /.*/ means everything and // means nothing).

To replace

With