code: public static StrMatcher charSetMatcher(final String chars) {
          if (StringUtils.isEmpty(chars)) {
              return NONE_MATCHER;
          }
          if (chars.length() == 1) {
              return new CharMatcher(chars.charAt(0));
          }
          return new CharSetMatcher(chars.toCharArray());
      }
docs: Constructor that creates a matcher from a string representing a set of characters.
  
  @param chars  the characters to match, null or empty matches nothing
  @return a new Matcher for the given characters