Early versions of OS X had user lands which were more BSD-like than they are now

They still have a lot of userland BSD parts. Including the very broken grep from FreeBSD (which is fixed in FreeBSD now IIRC):

    $ echo "abcdef" | /usr/bin/grep -o "^..."
    abc
    def

(I have to use the full path to grep, because I replace macOS grep by GNU grep.)