$re = '/(?:\b[\s]?|[^x])([\d]{1}[\d]{2})(?:[^pi]|[\s]?\b)/';
$str = 'This is (321) an example.
(321) also
including (321) //basically not possible, but can\'t hurt.
this (321) has another group with a p: 122p
this (321) has another group with a I: 123i
this x235 should be ignored cause (123) is what i want to match.
(123) is what i want, not x111 or 125p or 999i
in this 111 case there is no solution 555
101 202.something
';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($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