Ah-ah!

From https://wiki.freebsd.org/BSDgrep:

"BSD grep has been imported to 9.0-CURRENT on July 22, 2010. It has been obtained from the OpenBSD and the FreeGrep projects to make it appropriate for our needs to provide GNU compatibility and let ports build without problems. GNU compatibility is almost 100%, the only missing feature is -P (--perl-regex), which is not possible to support without having PCRE in base system and anyway, it was disabled in GNU grep for the same reason. It is still linked to GNU libregex because we needed compatibility for non-standard regexes, e.g. [foo|] is accepted by GNU, while it is considered an empty subexpression in stricter implementations."

FreeBSD links with GNU libregex. Traitors! :)