use strict;
use warnings;
use Test::Simple qw(no_plan);
my %test_cases = (
    '   999' => 'valid',
    '  9999' => 'valid',
    '999999' => 'valid',
    '     9' => 'valid',
    ' 99 9'  => 'invalid',
);

foreach my $case (keys %test_cases) {
    my $does_match
        = $case =~ /
            ^\s*        # any number of leading spaces
            \d+         # followed by a number of digits
            (:?
            [^\s]*      # but cant have any spaces after a digit has been found
            \d+         # It must contain at least one digit at the end
            )?$
        /x ;
    ok( !($does_match xor $test_cases{$case} eq 'valid'),
        "'$case': $test_cases{$case}" );
}

##</code><code>##

ok 1 - '999999': valid
ok 2 - '   999': valid
ok 3 - '     9': valid
ok 4 - '  9999': valid
ok 5 - ' 99 9': invalid
1..5