Doh! I was a little mis-guided--my Mastering Regular Expressions book say on page 229 'Lookbehind, were it supported, would somehow look back to toward the left'. Turns out the book published in 1999, so shame on me for not checking further.

Your're correct in that it doesn't cover all possibilities, and that's something I still have to consider. I just got sidetracked with the problem at hand and wanted to address it for the learning experience. Thanks for pointing me in the right direction with lookbehind.