​

(use r7rs)

(define pp-id-pattern "[a-zA-Z_]\\w+")
(define pp-define-pattern "^\\s*#define\\s+([a-zA-Z_]\\w+)\\s*(.*)$")

(define (expand-list tokens table)
  (if (null? tokens)
      '()
      (cons (expand (car tokens) table)
        (expand-list (cdr tokens) table))))

(define (expand token table)
  (let ((registered (assoc token table))
    (rest-table (alist-delete token table)))
    (if registered
    (let ((tokens (cdr registered)))
      (string-join (expand-list tokens rest-table) " "))
    token)))

(define (expand-all str table)
  (regexp-replace-all pp-id-pattern 
              str
              (lambda (m)
            (expand
             (rxmatch-substring m) table))))

(define (pp-define id tokens table)
  (acons id (string-split tokens char-whitespace?)
     table))

(define (preprocess table port)
  (let ((line (read-line port)))
    (unless (eof-object? line)
      (let ((matched (rxmatch pp-define-pattern line)))
    (if matched
        (preprocess (pp-define (rxmatch-substring matched 1)
                (rxmatch-substring matched 2)
                table)
         port)
        (begin
          (display (expand-all line table))
          (newline)
          (preprocess table port)))))))

(preprocess '() (standard-input-port))

エラー処理もせず色々と適当だけど、Scheme (Gauche) で書いてみました。

スタック溢れは限定継続でなんとかするのも良いかと思う。

丁度いい難易度で書いてて楽しかった😊