I don't use homebrew, but your comment made me realize what I needed to do to successfully compile git with pcre. As a result, this now works:

  git grep -P "outer\b"

Yay! Thank you and moonboots.

This is an important feature for me, because I organize my code lexically (primarily by being disciplined about using unique names for things) and rely on grep to quickly find what I want. Occasionally one unique name overlaps with another, and then I really want to match the word boundary.