code: public int findIn(Source source) {
          if (regex != null) {
              Matcher matcher = regex.matcher(source.subToEnd());
              if (matcher.find()) {
                  return source.getOffset() + matcher.start();
              } else {
                  return -1;
              }
          } else {
              return -1;
          }
      }
docs: Find this element
  
  @param source text source
  @return start offset