use warnings;
use strict;

my @examples = (
	[ 'Rack1-Unit2', 'Rack3-Unit1', 'Rack1-Unit1' ],
	[ 'R2U3', 'R1U4', 'R10U1', 'R1U1' ],
	[ 'R2-U3', 'R1-U4', 'R10-U1', 'R1-U1' ],
);

my $rackre = qr/R(?:ack)?(\d+)/i;
my $unitre = qr/U(?:nit)?(\d+)/i;
my %orderings = (
	rackfirst => sub { ($a =~ $rackre)[0] <=> ($b =~ $rackre)[0]
		or ($a =~ $unitre)[0] <=> ($b =~ $unitre)[0] },
	unitfirst => sub { ($a =~ $unitre)[0] <=> ($b =~ $unitre)[0]
		or ($a =~ $rackre)[0] <=> ($b =~ $rackre)[0] },
);

for my $ex (@examples) {
	print "Input: @$ex\n";
	for my $o (sort keys %orderings) {
		my @sorted = sort {&{$orderings{$o}}} @$ex;
		print "$o: @sorted\n";
	}
}

__END__

Input: Rack1-Unit2 Rack3-Unit1 Rack1-Unit1
rackfirst: Rack1-Unit1 Rack1-Unit2 Rack3-Unit1
unitfirst: Rack1-Unit1 Rack3-Unit1 Rack1-Unit2
Input: R2U3 R1U4 R10U1 R1U1
rackfirst: R1U1 R1U4 R2U3 R10U1
unitfirst: R1U1 R10U1 R2U3 R1U4
Input: R2-U3 R1-U4 R10-U1 R1-U1
rackfirst: R1-U1 R1-U4 R2-U3 R10-U1
unitfirst: R1-U1 R10-U1 R2-U3 R1-U4