In Vim, I would do this:
:%s/ /^M/g
:g/_at/j
Where the ^M
is typed by pressing control-V (control-Q on Windows) followed by the Enter/Return key.
This assumes single spaces between tokens; as @Floris suggests, you can use s/ \+/^M/g
to turn multiple consecutive spaces into a single newline. Or you could use s/\v\s+/^M/g
to do the same thing with any consecutive whitespace including tabs as well as literal space characters.