What is the world coming to, that you have to explain vi commands on HN?
:%s/_/*/g
Which is, replace all occurrences on all lines of '_' with ' *'EDIT: Talk about formatting fail...
(And you have less points than either of the ones above you? Seriously?)
Don't worry! I didn't feel like I needed to explain. But the formatting on HN is odd, and the rule on how to use asterisks is obscure and bizarre.
s|s/_/*| s/_/*|
And since I haven't debugged that regex even if it was legible, add two spaces to the beginning of a line to reproduce text verbatim. (Even if it's on one line.)