Section 14,95: hexadecimal_constant

 hexadecimal constant

 A constant that is a string of hexadecimal digits (range 0 to 9, or
 an uppercase or lowercase letter in the range A to F) enclosed by
 apostrophes and followed by the letter X.  In DEC Fortran, the
 letter X can alternatively precede the string, and the letter Z can
 alternatively precede or follow the string.