Everyone is always quick with the regex, but in this case, I think you are painting yourself into a corner when you go with that approach. Just because you can, doesn't mean you should and all that.

So, here's a solution which is really straight-up, and it converts both ways.

Sample input:

3h4m            11040      3h4m      
1h2m3s          3723       1h2m3s    
1h5s            3605       1h5s      
3m4s            184        3m4s      
1d2h3m4s        93784      1d2h3m4s  
3x              -1                   
2m2m            -1                   
23h59m59s       86399      23h59m59s
[download]

The conversion routines:

my @time_values = qw [ d h m s ];
my %time_value  = ( d => 86400, h => 3600, m => 60, s => 1 );

sub string_to_time
{
        my ($time) = @_;
        my $value  = 0;

        foreach my $letter (@time_values)
        {
                if ($time =~ s/(\d+)$letter//)
                {
                        $value += $1 * $time_value{$letter};
                }
        }

        return -1 if length $time;

        return $value;
}

sub time_to_string
{
        my ($time) = @_;
        my $value  = '';

        foreach my $letter (@time_values)
        {
                my $time_value = $time_value{$letter};

                if ($time > $time_value)
                {
                        $value .= int($time/$time_value).$letter;
                        $time %= $time_value;
                }
        }

        return $value;
}
[download]

And a quick test-harness for comparisons with other routines:

#!/usr/bin/perl -w

use strict;
my @test = qw [ 3h4m 1h2m3s 1h5s 3m4s 1d2h3m4s 3x 2m2m 23h59m59s ];

foreach (@test)
{
        my $s2t = string_to_time($_);
        my $t2s = time_to_string($s2t);
        printf ("%-15s %-10s %-10s\n", $_, $s2t, $t2s);
}
[download]

A couple of notes:

• I'm not sure if you want to use -1 as an "invalid" return. You might want 0, or some other trigger, if you are working with values from 1969, for example. Maybe you have some really, really old DNS data that predates ARPANET.
• You can add new letters to the spec, and it will convert accordingly. I'm not sure how your tool displays really, really long values, such as months, or years, especially since these are subjective things. There is no ISO standard month length.
• These could be compacted, of course, even to the point of golf.

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