private String getIndexBaseName() {
        final Matcher indexNameMatcher = INDEX_BASE_NAME_PATTERN.matcher(indexName);
        if (indexNameMatcher.find()) {
            return indexNameMatcher.group(1);
        } else {
            throw new RuntimeException("Expected one or more characters in the index name before the first curly " +
                                       "brace.");
        }
    }
Get the index name stem, which is the text up to the first index name variable, as denoted by curly braces
stroom-search/stroom-search-elastic/src/main/java/stroom/search/elastic/indexing/ElasticIndexingFilter.java