$re = '/^(([^\nf].{2}|.[^\no].|.{2}[^\no]).*|.{0,2})$/m';
$str = 'foo text
1oo text
f1o text
fo1 text
1
12
123
More text
12345 text';
preg_match_all($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