Program to translate HEX and ASCII, rewritten in Kotlin

HexLib.kt

// Program to encode/decode hexadecimal and ASCII/Unicode

/**
* Decodes a string of hexadecimal numbers into a string of characters.
*
* @param input A string with only valid HEX numbers separated by spaces and/or commas.
*/
fun decodeHex(input: String): String {
    return input
        .split(Regex("(\\s|,)+")) // Split into individual values
        .map { Integer.parseInt(it, 16) } // Parse as hex into numbers
        .map { it.toChar() } // Convert numbers to ASCII
        .joinToString(""); // Merge results into a string
}

/**
* Converts a regular string into the equivalent hexadecimal values.
*
* @param input Any string of valid ASCII/Unicode characters.
*/
fun encodeHex(input: String): String {
    return input
        .map{it.code.toString(16)} // Convert to number, then hex
        .joinToString(" "); // Merge with space separators
}

/** Main Program */
fun main() {
    print("(E)ncode, (D)ecode, or (Q)uit?\n> ");
    val opt = readLine()?.first(); // Prompt to select function

    when (opt) {
        'e','E' -> { // Encode to HEX
            print("Enter string to encode.\n> ");
            println("= " + encodeHex(readLine().orEmpty()) + "\n");
            main();
        }

        'f','F' -> { // Press F to pay respects.
            println("Respects have been paid.\n")
            main();
        }

        'd','D' -> {
            println("Enter HEX string to decode."); // Decode from HEX
            print("Separate values with commas and/or spaces.\n> ");
            try {
                println("= " + decodeHex(readLine().orEmpty()) + "\n");
            } catch (e: NumberFormatException) {
                // If user enters invalid string, give error and retry.
                println("Please enter valid hexadecimal, and use " +
                        "only commas and spaces " +
                        "to separate each value.\n");
            } catch (e: Exception) {
                System.err.println("An unknown error has occurred."); // If there's some other error, print
                System.exit(1); // message and quit w/ error code 1.
            } finally {
                main();
            }
        }

        'q','Q' -> { // Quit program
            System.exit(0);
        }

        else -> { // If other selection is made,
            println("Please enter a valid command.\n"); // give error message.
            main();
        }
    }
} // :]