$re = '/\b(20[0-9][0-9])\b/';
$str = '1920
1930
2011
2
22
222
2323
23323
2011
2009
2012
2001
2020
2022
2040
';
preg_match_all($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