18. A hexadecimal digit is '0'..'9', or 'a'..'f', or 'A'..'F'. A hexadecimal constant is a sequence of hexadecimal digits. Examples include 3,

a, 0d, and FF4e. Use the direct-code technique for implementing an FSM as in the program of Figure 7.29 to parse a hexadecimal constant and convert it to a nonnegative integer. The input/output should be similar to that in the figure, with invalid input producing an error message and a valid hexadecimal input string producing the nonnegative integer value.