use warnings;
use strict;

my @test_values = qw/ 26 231 232 233 234 254 255 256 025 0000 /;

my $rx = qr /^ (?:
                    2                  # first digit is a 2
                      (?:
                           [6-9]       #    second and _last_ digit is 6-9
                         | 5           # or second digit 5
                                [0-5]? #    maybe third and _last_ digit 0-5
                         | [0-4]       # or second digit 0-4
                                \d?    #    maybe followed by a final third 0-9
                      )?
                   | [3-9]             # first digit 3-9
                           \d?         #    maybe followed by final 0-9

                   | 1                 # first digit 1
                       (?:
                           \d\d?       # maybe followed by 1 or 2 digits
                       )?
                   | 0                 # single-digit 0
                )
             $/x;

foreach my $number ( @test_values ) {
    print "$number:",
           ($number =~ /$rx/ ? 'true' : 'false'),
          "\n",
          ;
}