My favorite feature is:

       -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.

As everything (python, Go, javascript, etc, etc) uses perl regexps now-a-days and I can never remember which things I need to escape for old gods regexp.