$rx_4333 = /(?=\d{4}$)
   (?: 4     (?: 3     (?: 3     (?: 3
                                 |   [4-9]
                                 )
                       |   [4-9]
                       )
             |   [4-9]
             )
   |   [5-9]
   )
/x

##</code><code>##

$rx_4333 = /(?=\d{4}$)
   (?: 4     (?: 3     (?: 3     [3-9]
                       |   [4-9]
                       )
             |   [4-9]
             )
   |   [5-9]
   )
$/x

##</code><code>##

my @digits = $ProdBuild =~ /([0-9])([0-9])([0-9])([0-9])$/;

my $rx = '(?=\\d{4}$)';
for (@digits) {
   $rx .= "(?:$_";
}
for (reverse @digits) {
   local $_ = $_+1;
   $rx .= '|' . ($_ == 9 ? 9 : "[$_-9]") if $_ != 10;
   $rx .= ')';
}

# 4333 gives (?=\d{4}$)(?:4(?:3(?:3(?:3|[4-9])|[4-9])|[4-9])|[5-9])

foreach (@pattern) {
   print("Match: $_\n") if /$rx/;
}