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$str = "aa_bb_cc.dd.txt";
preg_match('/.*_([\w]+)_([\w]+\.[\w]+).*/is', $str, $match);
// $match[1], $match[2]
explode('_', $string, $arr);
explode('.', $arr[2], $arr2);
echo $arr[1];
echo $arr2[0].".".$arr2[1];
$str = "a.01.abc";
echo substr($str, 0, strrpos($str, "."));