Also note that with newer Emacs version there is project.el, where all you have to do to replace all instances of "foo" with "bar" is: <Ctrl> + x + p + r "foo"

Also note that with newer Emacs version there is project.el, where all you have to do to replace all instances of "foo" with "bar" is: <Ctrl> + x + p + r "foo" <enter> "bar" <enter> which are 12 keystrokes, if I'm not mistaken, and does more.