Generated unions fail to enforce patterns

  typedef bgp-set-med-type {
    type union {
      type uint32;
      type string {
        pattern "^[+-][0-9]+";
      }
      type enumeration {
        enum IGP {
          description "set the MED value to the IGP cost toward the
          next hop for the route";
        }
      }
    }
    description
      "Type definition for specifying how the BGP MED can
      be set in BGP policy actions. The three choices are to set
      the MED directly, increment/decrement using +/- notation,
      and setting it to the IGP cost (predefined value).";
  }

    public static final List<String> PATTERN_CONSTANTS = ImmutableList.of("^(?:\\^[+-][0-9]+)$");
    private static final Pattern patterns = Pattern.compile(PATTERN_CONSTANTS.get(0));
    private static final String regexes = "^[+-][0-9]+";
    private final Long _uint32;
    private final String _string;
    private final Enumeration _enumeration;


    private static void checkUint32Range(final long value) {
        if (value >= 0L && value <= 4294967295L) {
            return;
        }
        CodeHelpers.throwInvalidRange("[[0..4294967295]]", value);
    }
    public BgpSetMedType(Long _uint32) {
        super();
        checkUint32Range(_uint32);
        
        this._uint32 = _uint32;
        this._string = null;
        this._enumeration = null;
    }
    
    private static void check_stringLength(final String value) {
    }
    public BgpSetMedType(String _string) {
        super();
        check_stringLength(_string);
        
        this._string = _string;
        this._uint32 = null;
        this._enumeration = null;
    }