You've done very detailed work there sgifford, but I'm a bit nervous about you using the phrase "word boundary character". Thinking of \b and similar things as characters has got me into a lot of trouble in the past.

I don't know the perfect phrase to describe it, however, and "zero-width assertion" has never really appealed to me, so I'd rather just call it a "word boundary" and explain that as "place between a word-character and a non-word character".