Not a direct answer to your question, but as a suggestion I wanted to point you in the direction of the Date::Manip module. Often in my scripts which dealt with time/date parsing, this module saved my day. It is able to do a lot of date 'magic' but may require some digging in order to understand :)

Update:
Here's a rendition of Ovid's code, but using the Date::Manip module. I agree that using the module for such a simple task may look like an overkill, but I have this feeling your program may have to deal with a lot more date/time manipulation going forward. Even if this is not the case, it's always good to have another way of doing a thing ;-) 
use strict;
use Date::Manip;

foreach (<DATA>) {
    chomp;
    next if /^$/;
    my @delta=split(/:/, ParseDateDelta($_));
    my ($day) = $delta[3];
    my ($hour) = $delta[4];
    print "$_\t=>\tDay: $day\tHour: $hour\n";
}

__DATA__
3d
3d 2h
6h

[download]
And the output the script generates is: 
3d      =>      Day: 3  Hour: 0
3d 2h   =>      Day: 3  Hour: 2
6h      =>      Day: 0  Hour: 6

[download]


_____________________ 
# Under Construction

[download]

In reply to Re: Help with a more precise regex by vladb
in thread Help with a more precise regex by LogicalChaos

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