use warnings;
use strict;
use Data::Dump;

my $re = qr{ \A \s*
	RepeatingGroup \s* = \s*
	(?<GroupID> \b[^\ \t]+\b ) \s*
	( , \s*
		(?: (?<Flatten> \bFlatten\b )
		  | (?<Direction> \b(?:in|out)\b ) )
	\s* ){1,2}
	\z }msx;

dd $_, /$re/ ? \%+ : "FAIL" while <DATA>;

__DATA__
RepeatingGroup = Waiver, Flatten, out
RepeatingGroup = Waiver, out
RepeatingGroup = Waiver, out, Flatten
RepeatingGroup = Waiver , in
RepeatingGroup = Waiver , Flatten