code: public static final String digitsAndPlusOnly(MatcherCompat matcher) {
          StringBuilder buffer = new StringBuilder();
          String matchingRegion = matcher.group();
          for (int i = 0, size = matchingRegion.length(); i < size; i++) {
              char character = matchingRegion.charAt(i);
              if (character == '+' || Character.isDigit(character)) {
                  buffer.append(character);
              }
          }
          return buffer.toString();
      }
docs: Convenience method to return only the digits and plus signs
  in the matching string.
  
  @param matcher      The Matcher object from which digits and plus will
  be extracted
  
  @return             A String comprising all of the digits and plus in
  the isMatch