What is the world coming to, that you have to explain vi commands on HN?

That's technically just a regular expression (perl syntax). The vi equivalent would be

    :%s/_/*/g

Which is, replace all occurrences on all lines of '_' with ' *'

EDIT: Talk about formatting fail...

Isn't that actually ed/sed syntax?

What is the world coming to when a correction has to get 3 levels deep before it's actually correct on HN?

(And you have less points than either of the ones above you? Seriously?)

Indeed, quick memory recall failure on my part. It's ed syntax which bled into everything else via POSIX basic. However, given the limited number of symbols present, identification can be considered ambiguous :).

I actually couldn't get the asterisk to appear without adding text below it, and I didn't have anything else to add. So I just added an explanation.

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.)

I wasn't aware HN actually had formatting, I was just trying to use plaintext for emphasis... Or does it?

Text between asterisks will be in italics.