define switch function isbn (value stream vs-s) as
  local integer li-total initial {0}
  local integer li-check
  repeat scan vs-s
    ; Any digits in an ISBN-13 may be separated by a hyphen or space
    match (digit => lpv-1 ('-' | space)? digit => lpv-3 ('-' | space)? ) => p
      ; Add the first digit to the total and add the second multiplied by 3
      increment li-total by (lpv-1 + (lpv-3 * 3))
    match any => lpv-c
      ; This is always the thirteenth digit, the 'check' digit
      set li-check to lpv-c
  again
  do when 10 - li-total modulo 10 = li-check
    return true
  else when li-total modulo 10 = 0 and li-check = 0
    return true
  else
    return false
  done

process
  ; These are the first four test ISBN-13s from the problem description, with
  ; the first and third being valid and the second and fourth invalid.
  submit "978-0596528126"
  submit "978-0596528120"
  submit "978-1788399081"
  submit "978-1788399083"

  ; This ISBN-13 is from https://isbn-information.com/the-13-digit-isbn.html
  submit "978-1-86197-876-9"
  submit "978 1 86197 876 9"

  ; These ISBNs are from https://en.wikipedia.org/wiki/ISBN
  submit "978-3-16-148410-0"
  submit "978 3 16 148410 9"
  submit "This is valid: 978-0-306-40615-7^"
  submit "978-03-0640-615 7."

  ; These strings show how non-ISBN-13 numbers must be (and are) ignored.
  submit "This is a 14-digit number - 555-30640610555 - so it is ignored.%n"
  submit "This is an ISBN-10, so it is ignored too: 0-306-40615-2.%n"

find ([any \ digit] | value-start) => lpv-s
     ((digit ('-' | space)?){12} digit) => lpv-n
     ([any \ digit] | value-end) => lpv-e
  output lpv-s || lpv-n || lpv-e
  do when isbn(lpv-n) = true
    output ' (good)%n'
  else
    output ' (bad)%n'
  done

find any => lpv-char
  output lpv-char