static String substr(final String filename, final int group) {
        final Matcher m = FILENAME_FORMAT.matcher(filename);
        if (!m.matches()) {
            throw new IllegalArgumentException(format("Not a topic partition directory file: %s", filename));
        }
        return m.group(group);
    }
Creates a new {@link RemoteTopicPartitionDirectory} instance for the directory with the given
name under the root directory of the local tiered storage. This method throws an
{@link IllegalArgumentException} if the directory does not exist.
storage/src/test/java/org/apache/kafka/server/log/remote/storage/RemoteTopicPartitionDirectory.java