#!/usr/bin/env perl

use 5.010;
use strict;
use warnings;

my $start_nonmask_nums = 6;
my $end_nonmask_nums = 4;


while (my $input = <>) {
    chomp $input;

    my $start_mask_pos = get_mask_pos($input, $start_nonmask_nums);
    my $start_string = substr $input, 0, $start_mask_pos;

    my $end_mask_pos = get_mask_pos(scalar reverse($input), $end_nonmask_nums);
    my $end_string = substr $input, -$end_mask_pos;

    my $mid_string = substr $input, $start_mask_pos, -$end_mask_pos;
    $mid_string =~ y{0-9}{X};

    my $masked_number = $start_string . $mid_string . $end_string;

    say $masked_number; 
}

sub get_mask_pos {
    my ($string, $count) = @_;

    $string =~ m{\d}g for (1 .. $count);

    return pos $string;
}