code: public static Set<String> extractPlaceHolders(String string) {
          Matcher matcher = Pattern.compile(PLACEHOLDER_PATTERN).matcher(string);
          Set<String> placeHolders = Sets.newHashSet();
          while (matcher.find()) {
              placeHolders.add(matcher.group(1));
          }
          return placeHolders;
      }
docs: Extract all the placeholder's names of the string
  
  @param string The whole string with placeholders
  @return A set with all the placeholder's names