(import (chicken irregex))

(define (string->instr str)
  (let ((idx 0)
        (l (string-length str))
        (whitespace-irx (irregex "[ \t]*"))
        (newline-irx (irregex "\n"))
        (comment-irx (irregex ";[^\n]*")))
    (define (accept-token irx mandatory)
      (let ((wsmatch (irregex-match whitespace-irx (substr str idx))))
        (set! idx (+ idx (irregex-match-end-index wsmatch)))) ;Skip leading whitespace
      (let ((res (irregex-match irx (substring str idx))))
        (if res
            (begin
              (set! idx (+ idx (irregex-match-end-index res)))
              (irregex-match-substring res))
            (if mandatory
                (error "Unexpected token at input string index" idx)
                #f))))
    (define (accept-token-string token-str mandatory)
      (accept-token (irregex token-str) mandatory))
    (define (load-file)
      (let loop ()
        (if (line)
            (loop))))
    (define (line)
      (or (accept-token comment-irx #f)
          (accept-token newline-irx #f)
          (and(accept-token newline-irx #t))))
    (define (instruction)
      (and (opcode)
           (accept-token period-irx #t)
           (accept-modifier)))))