$re = '`^(?i).*(?P<fileExt>\.mka)$`';
$str = '[TxxZ&A.I.R.nesSub][Library_Wars][The_Last_Mission][BDRIP][1920x1080][HEVC_AC3x2].mka';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($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