const digits  = (1..9 -> pick(4))
const grammar = Regex(
    '^ (?&exp) \z
      (?(DEFINE)
          (?<exp> ( (?&term) (?&op) (?&term) )+ )
          (?<term> \( (?&exp) \) | [' + digits.join + '])
          (?<op> [\-+*/] )
      )', 'x'
)

say "Here are your digits: #{digits.join(' ')}"

loop {
    var input = read("Expression: ", String)

    var expr = input
    expr -= /\s+/g     # remove all whitespace

    if (input == 'q') {
        say "Goodbye.  Sorry you couldn't win."
        break
    }

    var given_digits = digits.map{.to_s}.sort.join
    var entry_digits = input.scan(/\d/).sort.join

    if ((given_digits != entry_digits) || (expr !~ grammar)) {
        say "That's not valid"
        next
    }

    given(var n = eval(input)) {
        when (24) { say "You win!"; break }
        default   { say "Sorry, your expression is #{n}, not 24" }
    }
}