$re = '/[A-Z]{3}(?=\.)/';
$str = '/path/to/QAQC/Conclusions_Pakistan_2019-09-13_PAK.pdf
/path/to/QAQC/Conclusions_SouthAfrica_2019-09-13_ZAF.pdf
/path/to/QAQC/Conclusions_TheBahamas_2019-09-13_BHS.pdf
/path/to/QAQC/Conclusions_Venezuela_2019-09-13_VEN.pdf
/path/to/QAQC/Conclusions_Pakistan_2019-09-13_YEM.pdf
/path/to/QAQC/Conclusions_Iraq_2019-09-13_IRQ.pdf
/path/to/QAQC/Conclusions_Afghanistan_2019-09-13_AFG.pdf
';
preg_match_all($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