Isn't that because GNU grep has more features than FreeBSD grep? Had to do a complex regex capturing to rewrite recently (automatic help based on Makefile command comments in a specific format), and having to have compatibility with FreeBSD grep was the main reason I couldn't do it with grep only and had to resort to Perl.

If anyone is curious, this is the resulting Perl oneliner:

    perl -WE 'printf ("COMMAND;DESCRIPTION;USAGE\n");while (my $line = <>) { $line =~ /^(.*):\s#\s(.*)\[Usage\s`(.\*)`]/ && printf("$1;$2;$3\n")};'  Makefile | column -c "3" -s ";" -t