code: public String group(String groupName) {
          int idx = groupIndex(groupName);
          if (idx < 0) {
            throw new IndexOutOfBoundsException("No group \"" + groupName + "\"");
          }
          return group(idx);
      }
docs: Returns the input subsequence captured by the named group during
  the previous match operation.
  
  @param groupName name of the capture group
  @return the subsequence
  @throws IndexOutOfBoundsException if group name not found