docs: public String format(String format)
      {
          Matcher m = REF.matcher(format);
          StringBuffer sb = new StringBuffer();
          while (m.find())
          {
              Object ob = get(m.group(1));
              if (ob != null)
              {
                  m.appendReplacement(sb, ob.toString());
              }
              else
              {
                  m.appendReplacement(sb, m.group());
              }
          }
          m.appendTail(sb);
          return sb.toString();
      }