SET TERM ^;
CREATE OR ALTER FUNCTION FQDN__CHECK
  (
    fqdName ASCII_STRING DEFAULT NULL
  )
  RETURNS BOOLEAN
AS
  DECLARE VARIABLE dot      INT64;
  DECLARE VARIABLE dotPart  ASCII_STRING;
  DECLARE VARIABLE dotTail  ASCII_STRING DEFAULT NULL;
  DECLARE VARIABLE regEx    ASCII_STRING
    DEFAULT '([a-zA-Z0-9][a-zA-Z0-9-]{0,61}[a-zA-Z0-9])|([a-zA-Z0-9]{1,63})'
BEGIN
  IF (:fqdName IS NULL) THEN RETURN NULL;
  IF (NOT :fqdName SIMILAR TO '[a-zA-Z0-9-.]{1,255}') THEN RETURN FALSE;

  dotTail = fqdName;
  WHILE (:dotTail IS NOT NULL) DO
    BEGIN
      dot = Position('.', dotTail);
      IF (:dot = 0) THEN
        BEGIN
          dotPart = dotTail;
          dotTail = NULL;
        END
      ELSE
        BEGIN
          dotPart = Left(dotTail, dot - 1);
          IF (Char_Length(:dotTail) = :dot) THEN
            dotTail = NULL;
          ELSE
            dotTail = Substring(dotTail FROM dot + 1);
        END
      IF (NOT :dotPart SIMILAR TO :regEx) THEN RETURN FALSE;
    END
  RETURN TRUE;
END^
SET TERM ;^

SET TERM ^;
CREATE OR ALTER FUNCTION FQDN__CHECK
  (
    fqdName ASCII_STRING DEFAULT NULL
  )
  RETURNS BOOLEAN
AS
  DECLARE VARIABLE dot    INT64;
  DECLARE VARIABLE part   ASCII_STRING;
  DECLARE VARIABLE tail   ASCII_STRING DEFAULT NULL;
  DECLARE VARIABLE regEx  ASCII_STRING
    DEFAULT '([a-zA-Z0-9][a-zA-Z0-9-]{0,61}[a-zA-Z0-9])|([a-zA-Z0-9]{1,63})';
  DECLARE VARIABLE permitted CHAR(38) CHARACTER SET ASCII
    DEFAULT 'ABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789-.';
BEGIN
  IF (:fqdName IS NULL) THEN RETURN NULL;
  tail = Upper(fqdName);
  dot = Char_Length(tail);
  IF (:dot = 0) THEN RETURN FALSE;
  WHILE (:dot != 0) DO
    IF (Position(Substring(:tail FROM :dot FOR 1), :permitted) = 0) THEN
      RETURN FALSE;
    ELSE
      dot = dot - 1;

  WHILE (:tail IS NOT NULL) DO
    BEGIN
      dot = Position('.', tail);
      IF (:dot = 0) THEN
        BEGIN
          part = tail;
          tail = NULL;
        END
      ELSE
        BEGIN
          part = Left(tail, dot - 1);
          IF (Char_Length(:tail) = :dot) THEN
            tail = NULL;
          ELSE
            tail = Substring(tail FROM dot + 1);
        END
      IF (NOT :part SIMILAR TO :regEx) THEN RETURN FALSE;
    END
  RETURN TRUE;
END^
SET TERM ;^