(use prcc)

(define (regexp-parser)
  (letrec ((lw (act (one-of "abcdefghijklmnopqrstuvwxyz") (lambda (o)
                (str o))))
           (uw (act (one-of "ABCDEFGHIJKLMNOPQRSTUVWZYZ") (lambda (o)
                (str o))))
           (w  (sel lw uw (char #\_)))
           (d (act (one-of "0123456789") (lambda (o)
                (str o))))
           (dot (act (char #\.) (lambda (o)
                 (one-of "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWZYZ_0123456789"))))
           (aw (act (str "\\w") (lambda (o)
                 (one-of "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWZYZ_0123456789"))))
           (ad (act (str "\\d") (lambda (o)
                 (one-of "0123456789"))))
           (^ (act (char #\^) (lambda (o)
                (str o))))
           ($ (act (char #\$) (lambda (o)
                (eof))))
           (a  (sel (lazy es) (lazy ee) aw ad w d dot ^ $))
           (o? (act (seq a (char #\?)) (lambda (o)
                 (one? (car o)))))
           (r* (act (seq a (char #\*)) (lambda (o)
                 (rep (car o)))))
           (r+ (act (seq a (char #\+)) (lambda (o)
                 (rep+ (car o)))))
           (es (act (seq (char #\[) (rep+ a) (char #\])) (lambda (o)
                 (apply sel (cadr o)))))
           (s  (act (seq (lazy e) (char #\|) (lazy e)) (lambda (o)
                 (sel (car o) (caddr o)))))
           (ss (act (seq (char #\() s (char #\))) (lambda (o)
                 (cadr o))))
           (ee (act (seq (char #\() (lazy e) (char #\))) (lambda (o)
                 (cadr o))))
           (e  (act (rep (sel o? ss r* r+ a)) (lambda (o)
                 (apply seq o)))))
    (ind (seq (sel s e) (eof)) 0)))

(define (regexp str)
  (parse-string str (regexp-parser)))

(define (regexp-match r str)
  (parse-string str (regexp r)))

(display (regexp-match "a?[abc]\\w" "abc"))
(newline)