public static Iterable<String> expandAtNFilepattern(String filepattern) {
    ImmutableList.Builder<String> builder = ImmutableList.builder();
    Matcher match = AT_N_SPEC.matcher(filepattern);
    if (!match.find()) {
      builder.add(filepattern);
    } else {
      int numShards = Integer.parseInt(match.group("N"));
      String formatString = "-%0" + getShardWidth(numShards, filepattern) + "d-of-%05d";
      for (int i = 0; i < numShards; ++i) {
        builder.add(
            AT_N_SPEC.matcher(filepattern).replaceAll(String.format(formatString, i, numShards)));
      }
      if (match.find()) {
        throw new IllegalArgumentException(
            "More than one @N wildcard found in filepattern: " + filepattern);
      }
    }
    return builder.build();
  }
@Test
  public void testExpandAtNFilepatternNoPattern() throws Exception {
    assertThat(
        Filepatterns.expandAtNFilepattern("gs://bucket/object@google.ism"),
        contains("gs://bucket/object@google.ism"));
  }