A hexadecimal character string literal begins with an X
    followed by a string of up to 16 characters enclosed in single
    quotation marks. This type of string literal lets you represent
    nonprintable ASCII characters by specifying the hexadecimal value
    of the characters within the quotation marks.

    Each ASCII character requires 2 hexadecimal digits to represent
    it, so you must provide an even number of characters within
    the quotation marks. The only valid characters for hexadecimal
    character string literals are 0 through 9 and A through F
    (uppercase or lowercase).

    In the following example, the hexadecimal character string
    literal represents two delete characters; the ASCII hexadecimal
    value for a delete character is FF:

    X'FFFF'