$re = '/\.(\w+)$/m';
$str = '1231231223123131_FILE_NAME.EXTENSION.OTHEREXTENSION
1_This_is_an_otherExample.mpg.OTHEREXTENSIONadasdassdassds34
1231231223123131_myFile.tar.gz2';
preg_match_all($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