code: public static Map<String, List<String>> extractHTMLheaders(String html) {
          Map<String, List<String>> hxtagsMap = new HashMap<>();
          for(int i=1;i<=6;++i) {
              hxtagsMap.put("H"+i, new ArrayList<>());
          }
                  
          Matcher m = HX_PATTERN.matcher(html);
          while (m.find()) {
              if(m.groupCount()==2) {
                  String tagType = m.group(1).toUpperCase(Locale.ENGLISH);
                  String content = m.group(2);
                  hxtagsMap.get(tagType).add(clear(content));
              }
          }
          return hxtagsMap;
      }
docs: Extracts the HTML headers (h1-h6 tags) from an HTML page.
  
  @param html
  @return