To append a match efficiently in Vim :g/---/s/---/X/

Question

How can you refer to the match in the command g in Vim?

I would like to put X after the match without replacing the match. For instance, in

Answer 1

I'm not sure why you need the g portion of the command - the substitute will only act on matching lines. Here's what you're looking for:

:%s/create_title/&X/

The & represents the entire text which was matched.

Answer 2

:%s/\(create_title\)/\1X/g

works for me. (if I understand your question correctly).