I am trying to understand what's happening here.

(w*?@) seems to be a short way of writing w.join('@').

But what happens next escapes me still. Especially I do not understand the regex and how that plays together with gsub.

Apparently (I didn't know the syntax either) ? can be used to create literal strings of length one.

E.g.

irb(main):014:0> ?.
=> "."
irb(main):015:0> ?"
=> "\""
irb(main):016:0> ?-
=> "-"
irb(main):017:0> ?a
=> "a"

I guess this was just shorter than "." ^^