Anonymize IPv4 and IPv6 addresses with PHP preg_replace?

I needed to anonymize IPv4 and IPv6 addresses so I coded this crude solution:

if (strlen($_SERVER['REMOTE_ADDR']) <= 15) {  // Sorry: 15 NOT 12
    echo htmlentities(substr_replace($_SERVER['REMOTE_ADDR'], 'XXX', -3), ENT_QUOTES);
} else {
    echo htmlentities(substr_replace($_SERVER['REMOTE_ADDR'], 'XXXX:XXXX', -9), ENT_QUOTES);
}

It works fine with full length IPv4 and IPv6 addresses like

207.142.131.005

2001:0db8:0000:08d3:0000:8a2e:0070:7344

but not with abbreviated addresses like

207.142.131.5

2001:0db8::8d3::8a2e:7:7344

I wonder if there is an elegant solution with preg_replace and some regular expression magic?

Answer

No conditional is necessary. You can write two patterns and two replacements for a single preg_replace() call to process.

Target the optional numbers after the last literal dot in the string for replacement. Then target the alphanumeric colon-delimited substrings at the end of the string.

Code: (Demo)

$tests = [
    "207.142.131.005",
    "2001:0db8:0000:08d3:0000:8a2e:0070:7344",
    "2001:0db8:0000:08d3:0000:8a2e:0070:734a",
    "207.142.131.5",
    "2001:0db8::8d3::8a2e:7:7344",
    "::1",
    "127.0.0.1"
];

$tests = preg_replace(
             ['/.d*$/', '/[da-f]*:[da-f]*$/'],
             ['.XXX', 'XXXX:XXXX'],
             $tests
         );

var_export($tests);

Output:

array (
  0 => '207.142.131.XXX',
  1 => '2001:0db8:0000:08d3:0000:8a2e:XXXX:XXXX',
  2 => '2001:0db8:0000:08d3:0000:8a2e:XXXX:XXXX',
  3 => '207.142.131.XXX',
  4 => '2001:0db8::8d3::8a2e:XXXX:XXXX',
  5 => ':XXXX:XXXX',
  6 => '127.0.0.XXX',
)

Pattern Explanations:

IPv4:

/         #Pattern delimiter
.        #Match dot literally
d*       #Match zero or more digits
$         #Match the end of the string
/         #Pattern delimiter

IPv6

/         #Pattern delimiter
[da-f]*  #Match zero or more (digits or a b c d e f)
:         #Match colon
[da-f]*  #Match zero or more (digits or a b c d e f)
$         #Match the end of the string
/         #Pattern delimiter