code: public String matchOrDefault(String defaultString) {
          check();
          String match = context.getMatch();
          return match.length() == 0 ? defaultString : match;
      }
docs: <p>Returns the input text matched by the rule immediately preceding the action expression that is currently
  being evaluated. If the matched input text is empty the given default string is returned.
  This call can only be used in actions that are part of a Sequence rule and are not at first
  position in this Sequence.</p>
  
  @param defaultString the default string to return if the matched input text is empty
  @return the input text matched by the immediately preceding subrule or the default string