> No, that's "ancient regex".

wouldn't the "ancient regex" be the ed "g/re/p" version?

  -E, --extended-regexp
    Interpret PATTERNS as extended regular expressions (EREs, see below).
  -G, --basic-regexp
    Interpret PATTERNS as basic regular expressions (BREs, see below).  This is the default.
  -P, --perl-regexp
    Interpret I<PATTERNS> as Perl-compatible regular expressions (PCREs).  This option is experimental when combined with the -z (--null-data) option, and grep  -P  may  warn of unimplemented features.

From the manpage it seems my grep make distinction between "Extended" "Basic" and "Perl" regexes.