code: private String performRemoveLeading(String source, Integer length, Matcher matcher) {
          if (length != null && length != 0) {
              matcher.region(0, length);
          }
          if (matcher.find()) {
              String matched = matcher.group();
              return source.substring(matched.length());
          }
          return source;
      }
docs: Perform the remove leading operation. Returns the original string if the matcher does not match with the string