<?php

$replacer = static function ($m) {
    return str_repeat('*', strlen($m[1])) . '.' . str_repeat('*', strlen($m[2]));
};


$ips = ['127.0.0.1', '127.0.0.100', '127.0.100.0'];

foreach ($ips as $ip) {
	$results[] = preg_replace_callback('/(\d{1})\.(\d{1,3})$/', $replacer, $ip);
}

print_r($results);