An answer of http://nabetani.sakura.ne.jp/hena/ord18notfork/

Regi.pm

package Regi;
use strict;
use warnings;
use Exporter qw(import);

our @EXPORT = qw(solve);

sub search_regi (@) { (sort { $a->length <=> $b->length } @_)[0] }

sub solve ($) {
    local $_ = shift;
    my @regis = map { RegiItem->new($_) } 2, 7, 3, 5, 2;

    while ($_) {
        if    (s/^\.//) { $_->flush for @regis }
        elsif (s/^.// ) { my $m = "append_$&"; (search_regi @regis)->$m }
    };

    join ',', map { $_->length } @regis;
}


package RegiItem;
use strict;
use warnings;
use List::Util qw(max);

sub new { bless [$_[1], 0, 0] => $_[0] }

sub flush {
    my $self = shift;
    $self->[1] = max($self->[1] - $self->[0], 0);
}

for my $n (1 .. 9) {
    no strict 'refs';
    *{"append_$n"} = sub { ($_[0][2] ? $_[0][2] : $_[0][1]) += $n };
}

sub append_x { $_[0][2]++ }

sub length { $_[0][1] + $_[0][2] }

1;

test.t

use strict;
use warnings;
use Regi;
use Test::More;

while (<DATA>) {
    tr/\r\n//d;
    my ($n, $i, $o) = split /\t/, $_, 3;
    is solve($i), $o, "# $n";
}

done_testing;

__END__
0	42873x.3.	0,4,2,0,0
1	1	1,0,0,0,0
2	.	0,0,0,0,0
3	x	1,0,0,0,0
4	31.	1,0,0,0,0
5	3x.	1,1,0,0,0
6	99569x	9,9,6,6,9
7	99569x33	9,9,9,9,9
8	99569x33.	7,2,6,4,7
9	99569x33..	5,0,4,0,5
10	12345x3333.	4,0,3,2,3
11	54321x3333.	3,0,3,0,4
12	51423x3333.	3,4,4,0,4
13	12x34x.	1,0,1,0,2
14	987x654x.32	7,6,4,10,5
15	99999999999x99999999.......9.	20,10,12,5,20
16	997	9,9,7,0,0
17	.3.9	1,9,0,0,0
18	832.6	6,6,0,0,0
19	.5.568	3,5,6,8,0
20	475..48	4,8,0,0,0
21	7.2..469	1,4,6,9,0
22	574x315.3	3,3,1,7,1
23	5.2893.x98	10,9,5,4,1
24	279.6xxx..4	2,1,4,1,1
25	1.1.39..93.x	7,1,0,0,0
26	7677749325927	16,12,17,18,12
27	x6235.87.56.9.	7,2,0,0,0
28	4.1168.6.197.6.	0,0,3,0,0
29	2.8.547.25..19.6	6,2,0,0,0
30	.5.3x82x32.1829..	5,0,5,0,7
31	x.1816..36.24.429.	1,0,0,0,7
32	79.2.6.81x..26x31.1	1,0,2,1,1
33	574296x6538984..5974	14,13,10,15,14
34	99.6244.4376636..72.6	5,6,0,0,3
35	1659.486x5637168278123	17,16,16,18,17
36	.5.17797.x626x5x9457.3.	14,0,3,5,8
37	..58624.85623..4.7..23.x	1,1,0,0,0
38	716.463.9.x.8..4.15.738x4	7,3,5,8,1
39	22xx.191.96469472.7232377.	10,11,18,12,9
40	24..4...343......4.41.6...2	2,0,0,0,0
41	32732.474x153.866..4x29.2573	7,5,7,8,5
42	786.1267x9937.17.15448.1x33.4	4,4,8,4,10
43	671714849.149.686852.178.895x3	13,16,13,10,12
44	86x.47.517..29621.61x937..xx935	7,11,8,8,10
45	.2233.78x.94.x59511.5.86x3.x714.	4,6,10,8,8
46	.793...218.687x415x13.1...x58576x	8,11,8,6,9
47	6.6x37.3x51x932.72x4x33.9363.x7761	15,13,15,12,15
48	6..4.x187..681.2x.2.713276.669x.252	6,7,8,6,5
49	.6.xx64..5146x897231.x.21265392x9775	19,17,19,20,17
50	334.85413.263314.x.6293921x3.6357647x	14,14,12,16,10
51	4.1..9..513.266..5999769852.2.38x79.x7	12,10,13,6,10