#! perl -sw
use strict;

for ('6d', '3h', '4d 3h','6D', '3H', '4D 3H') {
	our ($d,$h) = (0,0);
	print "$d days $h hours\n" if
		m[^
			(?: (\d+)d (?{ $d = $+ || '0' }) )?
			\s*
			(?: (\d+)h (?{ $h = $+ || '0' }) )?
		$]xi;
}
__END__
C:\test>test
6 days 0 hours
0 days 3 hours
4 days 3 hours