I can use
:5,12s/foo/bar/g
to search for foo
and replace it by bar
between lines 5 and 12. How can I do that only in line 5 and 12 (and not in the lines in between)?
Vim has special regular expression atoms that match in certain lines, columns, etc.; you can use them (possibly in addition to the range) to limit the matches:
:5,12s/\(\%5l\|\%12l\)foo/bar/g
See :help /\%l