$re = '/(?(DEFINE)
(?<bit>[0-1][0-9]{1,2}|2[0-4][0-9]|25[0-5])
)
(
10\.
((?&bit)\.){2}((?&bit))
)|
(
172\.
(
(1[6-9])|
(2[0-9])|
(3[0-6])
)
\.
((?&bit)\.?+){2}
)|
(
192\.168\.
((?&bit)\.?+){2}
)/x';
$str = '172.31.253.126';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php