I'm using Terminal on Snow Leopard.
At the command line, if I've typed foo.bar.baz.bang.quuz.quux, when i tap option-B, it moves the cursor backward word by word -- stopping at every period, because it considers a period to be a word boundary. Likewise, option-F moves forward word by word.
In irb (0.9.5, ruby 1.8.7), option-B and -F also have this behavior, but the period is no longer treated as a word boundary, which makes these keyboard shortcuts significantly less useful.
How can I change this?
EDIT: Curiouser and curiouser: On an EC2 instance which has the same irb and ruby versions, the period is treated as a word boundary.
Could this be of relevance here?
http://jorgebernal.info/2009/11/18/fixing-snow-leopard-ruby-readline/
In any case make sure option-B/F are actually bound to forward and backward-word in your inputrc files, like John pointed out.
Also word boundaries are determined by your locale (see the "locale" command), and more specifically by LC_CTYPE (character classification). I don't think that's the problem here, but you might want to check out and compare your locale settings just in case.