You're right, in my haste to get this question out there I made the jump from my original problem to japhy's post to talking about his benchmarks. One of my coworkers pointed out the same thing to me as I was headed out the door. Obviously I should be benchmarking the exact problem I want to solve, not some generally similar example. After I get the kids to bed I'll write a better benchmark and try your \K suggestion below too. Thanks.
Also, you made the point that none of my input ends with spaces. I think that's generally true in real life usage too. It's frustrating that we have this pervasive idiom in our code of "strip whitespace just in case", but I think most of the time the input is already just fine. In fact, I think much of the time the input is short and has no spaces at all. I wonder if I should be checking it with index() first to quickly rule out that case.
update: added paragraph spacing