To specify a nonprintable character, you can use a hexadecimal string constant with the following format:

The introductory X identifies the string as a hexadecimal string constant. You must specify the nonprintable character as two hexadecimal digits (nn) in the range 0‑9, a‑f, or A‑F, and the string must contain an even number of characters. There must be no intervening white space between the X and the single‑quoted string of hexadecimal digits. The X and the hexadecimal digits are case insensitive. RBF translates the hexadecimal constant into its corresponding character value.