@Override
    public Optional<Integer> extractIndexNumber(final String indexName) {
        final int beginIndex = indexPrefixLength(indexName);
        if (indexName.length() < beginIndex) {
            return Optional.empty();
        }

        final String suffix = indexName.substring(beginIndex);
        try {
            return Optional.of(Integer.parseInt(suffix));
        } catch (NumberFormatException e) {
            return Optional.empty();
        }
    }
@Test
    public void testExtractIndexNumberWithMalformedFormatReturnsEmptyOptional() {
        assertThat(mongoIndexSet.extractIndexNumber("graylog2_hunderttausend")).isEmpty();
    }