sys-apps/grep-2.5.1-r8 does not support the option --perl-regexp / -P, but the manpage does mention these options. Grep should either support it, or the manpage should be updated.
build grep with USE=pcre
Does not work for me, my grep (2.5.1-r8) does not seem to have a "pcre" USE flag. If it is fixed in another version, it is not yet in x86 obviously.
it's fixed in the latest ebuild
The ebuild that re-enabled pcre by default does not remove the ewarn saying that pcre support was removed.
root@10.10.32.133 ~ # emerge -pv =sys-apps/grep-2.5.1a-r1 These are the packages that would be merged, in order: Calculating dependencies... done! [ebuild U ] sys-apps/grep-2.5.1a-r1 [2.5.1-r8] USE="nls pcre%* -static (-build%)" 515 kB Total size of downloads: 515 kB