code: public static PennTreebankPOS tag(String word) {
          for (int i = 0; i < REGEX.length; i++) {
              if (REGEX[i].matcher(word).matches()) {
                  return REGEX_POS[i];
              }
          }
  
          return null;
      }
docs: Returns the POS tag of a given word based on the regular expression over
  word string. Returns null if no match.