  -*- Mode : LISP ; Syntax : COMMON - LISP ; Package : CL - PPCRE ; Base : 10 -*-
  $ Header : /cvsroot / portableaserve / portableaserve / libs / cl - ppcre / parser.lisp , v 1.1 2004/02/16 19:37:17 rudi Exp $

;;; The parser will - with the help of the lexer - parse a regex
;;; string and convert it into a "parse tree" (see docs for details
;;; about the syntax of these trees). Note that the lexer might return
;;; illegal parse trees. It is assumed that the conversion process
;;; later on will track them down.

  Copyright ( c ) 2002 - 2003 , Dr.   . All rights reserved .

;;; Redistribution and use in source and binary forms, with or without
;;; modification, are permitted provided that the following conditions
;;; are met:

;;;   * Redistributions of source code must retain the above copyright
;;;     notice, this list of conditions and the following disclaimer.

;;;   * Redistributions in binary form must reproduce the above
;;;     copyright notice, this list of conditions and the following
;;;     disclaimer in the documentation and/or other materials
;;;     provided with the distribution.

;;; THIS SOFTWARE IS PROVIDED BY THE AUTHOR 'AS IS' AND ANY EXPRESSED
;;; OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
;;; WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
;;; ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
  DIRECT , INDIRECT , INCIDENTAL , SPECIAL , EXEMPLARY , OR CONSEQUENTIAL
;;; DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
;;; GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  INTERRUPTION ) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY ,
;;; WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
;;; NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
;;; SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

(in-package #:cl-ppcre)

(defun group (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <group>.
The productions are: <group> -> \"(\"<regex>\")\"
                                \"(?:\"<regex>\")\"
                                \"(?<\"<regex>\")\"
                                \"(?<flags>:\"<regex>\")\"
                                \"(?=\"<regex>\")\"
                                \"(?!\"<regex>\")\"
                                \"(?<=\"<regex>\")\"
                                \"(?<!\"<regex>\")\"
                                \"(?(\"<num>\")\"<regex>\")\"
                                \"(?(\"<regex>\")\"<regex>\")\"
                                <legal-token>
where <flags> is parsed by the lexer function MAYBE-PARSE-FLAGS.
Will return <parse-tree> or (<grouping-type> <parse-tree>) where
<grouping-type> is one of six keywords - see source for details."
  (multiple-value-bind (open-token flags)
      (get-token lexer)
    (cond ((eq open-token :open-paren-paren)
            ;; special case for conditional regular expressions; note
            ;; that at this point we accept a couple of illegal
            ;; combinations which'll be sorted out later by the
            ;; converter
            (let* ((open-paren-pos (car (lexer-last-pos lexer)))
                   ;; check if what follows "(?(" is a number
                   (number (try-number lexer :no-whitespace-p t))
                   ;; make changes to extended-mode-p local
                   (*extended-mode-p* *extended-mode-p*))
              (declare (type fixnum open-paren-pos))
              (cond (number
                      ;; condition is a number (i.e. refers to a
                      ;; back-reference)
                      (let* ((inner-close-token (get-token lexer))
                             (reg-expr (reg-expr lexer))
                             (close-token (get-token lexer)))
                        (unless (eq inner-close-token :close-paren)
                          (signal-ppcre-syntax-error*
                           (+ open-paren-pos 2)
                           "Opening paren has no matching closing paren"))
                        (unless (eq close-token :close-paren)
                          (signal-ppcre-syntax-error*
                           open-paren-pos
                           "Opening paren has no matching closing paren"))
                        (list :branch number reg-expr)))
                    (t
                      ;; condition must be a full regex (actually a
                      ;; look-behind or look-ahead); and here comes a
                      ;; terrible kludge: instead of being cleanly
                      ;; separated from the lexer, the parser pushes
  back the lexer by one position , thereby
                      ;; landing in the middle of the 'token' "(?(" -
                      ;; yuck!!
                      (decf (lexer-pos lexer))
                      (let* ((inner-reg-expr (group lexer))
                             (reg-expr (reg-expr lexer))
                             (close-token (get-token lexer)))
                        (unless (eq close-token :close-paren)
                          (signal-ppcre-syntax-error*
                           open-paren-pos
                           "Opening paren has no matching closing paren"))
                        (list :branch inner-reg-expr reg-expr))))))
          ((member open-token '(:open-paren
                                :open-paren-colon
                                :open-paren-greater
                                :open-paren-equal
                                :open-paren-exclamation
                                :open-paren-less-equal
                                :open-paren-less-exclamation)
                   :test #'eq)
            ;; make changes to extended-mode-p local
            (let ((*extended-mode-p* *extended-mode-p*))
  we saw one of the six token representing opening
              ;; parentheses
              (let* ((open-paren-pos (car (lexer-last-pos lexer)))
                     (reg-expr (reg-expr lexer))
                     (close-token (get-token lexer)))
                (when (eq open-token :open-paren)
                  ;; if this is the "("<regex>")" production we have to
                  ;; increment the register counter of the lexer
                  (incf (lexer-reg lexer)))
                (unless (eq close-token :close-paren)
                  ;; the token following <regex> must be the closing
                  ;; parenthesis or this is a syntax error
                  (signal-ppcre-syntax-error*
                   open-paren-pos
                   "Opening paren has no matching closing paren"))
                (if flags
                  ;; if the lexer has returned a list of flags this must
                  ;; have been the "(?:"<regex>")" production
                  (cons :group (nconc flags (list reg-expr)))
                  (list (case open-token
                          ((:open-paren)
                            :register)
                          ((:open-paren-colon)
                            :group)
                          ((:open-paren-greater)
                            :standalone)
                          ((:open-paren-equal)
                            :positive-lookahead)
                          ((:open-paren-exclamation)
                            :negative-lookahead)
                          ((:open-paren-less-equal)
                            :positive-lookbehind)
                          ((:open-paren-less-exclamation)
                            :negative-lookbehind))
                        reg-expr)))))
          (t
            ;; this is the <legal-token> production; <legal-token> is
            ;; any token which passes START-OF-SUBEXPR-P (otherwise
  parsing had already stopped in the SEQ method )
            open-token))))

(defun greedy-quant (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <greedy-quant>.
The productions are: <greedy-quant> -> <group> | <group><quantifier>
where <quantifier> is parsed by the lexer function GET-QUANTIFIER.
Will return <parse-tree> or (:GREEDY-REPETITION <min> <max> <parse-tree>)."
  (let* ((group (group lexer))
         (token (get-quantifier lexer)))
    (if token
      ;; if GET-QUANTIFIER returned a non-NIL value it's the
  two - element list ( < min > < max > )
      (list :greedy-repetition (first token) (second token) group)
      group)))

(defun quant (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <quant>.
The productions are: <quant> -> <greedy-quant> | <greedy-quant>\"?\".
Will return the <parse-tree> returned by GREEDY-QUANT and optionally
change :GREEDY-REPETITION to :NON-GREEDY-REPETITION."
  (let* ((greedy-quant (greedy-quant lexer))
         (pos (lexer-pos lexer))
         (next-char (next-char lexer)))
    (when next-char
      (if (char= next-char #\?)
        (setf (car greedy-quant) :non-greedy-repetition)
        (setf (lexer-pos lexer) pos)))
    greedy-quant))

(defun seq (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <seq>.
The productions are: <seq> -> <quant> | <quant><seq>.
Will return <parse-tree> or (:SEQUENCE <parse-tree> <parse-tree>)."
  (flet ((make-array-from-two-chars (char1 char2)
           (let ((string (make-array 2
                                     :element-type 'character
                                     :fill-pointer t
                                     :adjustable t)))
             (setf (aref string 0) char1)
             (setf (aref string 1) char2)
             string)))
  Note that we 're calling START - OF - SUBEXPR - P before we actually try
    ;; to parse a <seq> or <quant> in order to catch empty regular
    ;; expressions
    (if (start-of-subexpr-p lexer)
      (let ((quant (quant lexer)))
        (if (start-of-subexpr-p lexer)
          (let* ((seq (seq lexer))
                 (quant-is-char-p (characterp quant))
                 (seq-is-sequence-p (and (consp seq)
                                         (eq (first seq) :sequence))))
            (cond ((and quant-is-char-p
                        (characterp seq))
                    (make-array-from-two-chars seq quant))
                  ((and quant-is-char-p
                        (stringp seq))
                    (vector-push-extend quant seq)
                    seq)
                  ((and quant-is-char-p
                        seq-is-sequence-p
                        (characterp (second seq)))
                    (cond ((cddr seq)
                            (setf (cdr seq)
                                    (cons
                                     (make-array-from-two-chars (second seq)
                                                                quant)
                                     (cddr seq)))
                            seq)
                          (t (make-array-from-two-chars (second seq) quant))))
                  ((and quant-is-char-p
                        seq-is-sequence-p
                        (stringp (second seq)))
                    (cond ((cddr seq)
                            (setf (cdr seq)
                                    (cons
                                     (progn
                                       (vector-push-extend quant (second seq))
                                       (second seq))
                                     (cddr seq)))
                            seq)
                          (t 
                            (vector-push-extend quant (second seq))
                            (second seq))))
                  (seq-is-sequence-p
                    ;; if <seq> is also a :SEQUENCE parse tree we merge
                    ;; both lists into one to avoid unnecessary consing
                    (setf (cdr seq)
                            (cons quant (cdr seq)))
                    seq)
                  (t (list :sequence quant seq))))
          quant))
      :void)))
  
(defun reg-expr (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <regex>, a complete regular expression.
The productions are: <regex> -> <seq> | <seq>\"|\"<regex>.
Will return <parse-tree> or (:ALTERNATION <parse-tree> <parse-tree>)."
  (let ((pos (lexer-pos lexer)))
    (case (next-char lexer)
      ((nil)
        ;; if we didn't get any token we return :VOID which stands for
        ;; "empty regular expression"
        :void)
      ((#\|)
        ;; now check whether the expression started with a vertical
        ;; bar, i.e. <seq> - the left alternation - is empty
        (list :alternation :void (reg-expr lexer)))
      (otherwise
  otherwise un - read the character we just saw and parse a
        ;; <seq> plus the character following it
        (setf (lexer-pos lexer) pos)
        (let* ((seq (seq lexer))
               (pos (lexer-pos lexer)))
          (case (next-char lexer)
            ((nil)
              ;; no further character, just a <seq>
              seq)
            ((#\|)
              ;; if the character was a vertical bar, this is an
  alternation and we have the second production
              (let ((reg-expr (reg-expr lexer)))
                (cond ((and (consp reg-expr)
                            (eq (first reg-expr) :alternation))
  again we try to merge as above in SEQ
                        (setf (cdr reg-expr)
                                (cons seq (cdr reg-expr)))
                        reg-expr)
                      (t (list :alternation seq reg-expr)))))
            (otherwise
              ;; a character which is not a vertical bar - this is
              ;; either a syntax error or we're inside of a group and
              ;; the next character is a closing parenthesis; so we
  just un - read the character and let another function
              ;; take care of it
              (setf (lexer-pos lexer) pos)
              seq)))))))

(defun reverse-strings (parse-tree)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  (cond ((stringp parse-tree)
          (nreverse parse-tree))
        ((consp parse-tree)
          (loop for parse-tree-rest on parse-tree
                while parse-tree-rest
                do (setf (car parse-tree-rest)
                           (reverse-strings (car parse-tree-rest))))
          parse-tree)
        (t parse-tree)))

(defun parse-string (string)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Translate the regex string STRING into a parse tree."
  (let* ((lexer (make-lexer string))
         (parse-tree (reverse-strings (reg-expr lexer))))
    ;; check whether we've consumed the whole regex string
    (if (end-of-string-p lexer)
      parse-tree
      (signal-ppcre-syntax-error*
       (lexer-pos lexer)
       "Expected end of string"))))
 Syntax : COMMON - LISP ; Package : CL - PPCRE ; Base : 10 -*-
 The parser will - with the help of the lexer - parse a regex
 string and convert it into a "parse tree" (see docs for details
 about the syntax of these trees). Note that the lexer might return
 illegal parse trees. It is assumed that the conversion process
 later on will track them down.
 Redistribution and use in source and binary forms, with or without
 modification, are permitted provided that the following conditions
 are met:
   * Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
   * Redistributions in binary form must reproduce the above
     copyright notice, this list of conditions and the following
     disclaimer in the documentation and/or other materials
     provided with the distribution.
 THIS SOFTWARE IS PROVIDED BY THE AUTHOR 'AS IS' AND ANY EXPRESSED
 OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
 WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY
 DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE
 GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
 WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 special case for conditional regular expressions; note
 that at this point we accept a couple of illegal
 combinations which'll be sorted out later by the
 converter
 check if what follows "(?(" is a number
 make changes to extended-mode-p local
 condition is a number (i.e. refers to a
 back-reference)
 condition must be a full regex (actually a
 look-behind or look-ahead); and here comes a
 terrible kludge: instead of being cleanly
 separated from the lexer, the parser pushes
 landing in the middle of the 'token' "(?(" -
 yuck!!
 make changes to extended-mode-p local
 parentheses
 if this is the "("<regex>")" production we have to
 increment the register counter of the lexer
 the token following <regex> must be the closing
 parenthesis or this is a syntax error
 if the lexer has returned a list of flags this must
 have been the "(?:"<regex>")" production
 this is the <legal-token> production; <legal-token> is
 any token which passes START-OF-SUBEXPR-P (otherwise
 if GET-QUANTIFIER returned a non-NIL value it's the
 to parse a <seq> or <quant> in order to catch empty regular
 expressions
 if <seq> is also a :SEQUENCE parse tree we merge
 both lists into one to avoid unnecessary consing
 if we didn't get any token we return :VOID which stands for
 "empty regular expression"
 now check whether the expression started with a vertical
 bar, i.e. <seq> - the left alternation - is empty
 <seq> plus the character following it
 no further character, just a <seq>
 if the character was a vertical bar, this is an
 a character which is not a vertical bar - this is
 either a syntax error or we're inside of a group and
 the next character is a closing parenthesis; so we
 take care of it
 check whether we've consumed the whole regex string
  $ Header : /cvsroot / portableaserve / portableaserve / libs / cl - ppcre / parser.lisp , v 1.1 2004/02/16 19:37:17 rudi Exp $


  Copyright ( c ) 2002 - 2003 , Dr.   . All rights reserved .




  DIRECT , INDIRECT , INCIDENTAL , SPECIAL , EXEMPLARY , OR CONSEQUENTIAL
  INTERRUPTION ) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY ,

(in-package #:cl-ppcre)

(defun group (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <group>.
The productions are: <group> -> \"(\"<regex>\")\"
                                \"(?:\"<regex>\")\"
                                \"(?<\"<regex>\")\"
                                \"(?<flags>:\"<regex>\")\"
                                \"(?=\"<regex>\")\"
                                \"(?!\"<regex>\")\"
                                \"(?<=\"<regex>\")\"
                                \"(?<!\"<regex>\")\"
                                \"(?(\"<num>\")\"<regex>\")\"
                                \"(?(\"<regex>\")\"<regex>\")\"
                                <legal-token>
where <flags> is parsed by the lexer function MAYBE-PARSE-FLAGS.
Will return <parse-tree> or (<grouping-type> <parse-tree>) where
<grouping-type> is one of six keywords - see source for details."
  (multiple-value-bind (open-token flags)
      (get-token lexer)
    (cond ((eq open-token :open-paren-paren)
            (let* ((open-paren-pos (car (lexer-last-pos lexer)))
                   (number (try-number lexer :no-whitespace-p t))
                   (*extended-mode-p* *extended-mode-p*))
              (declare (type fixnum open-paren-pos))
              (cond (number
                      (let* ((inner-close-token (get-token lexer))
                             (reg-expr (reg-expr lexer))
                             (close-token (get-token lexer)))
                        (unless (eq inner-close-token :close-paren)
                          (signal-ppcre-syntax-error*
                           (+ open-paren-pos 2)
                           "Opening paren has no matching closing paren"))
                        (unless (eq close-token :close-paren)
                          (signal-ppcre-syntax-error*
                           open-paren-pos
                           "Opening paren has no matching closing paren"))
                        (list :branch number reg-expr)))
                    (t
  back the lexer by one position , thereby
                      (decf (lexer-pos lexer))
                      (let* ((inner-reg-expr (group lexer))
                             (reg-expr (reg-expr lexer))
                             (close-token (get-token lexer)))
                        (unless (eq close-token :close-paren)
                          (signal-ppcre-syntax-error*
                           open-paren-pos
                           "Opening paren has no matching closing paren"))
                        (list :branch inner-reg-expr reg-expr))))))
          ((member open-token '(:open-paren
                                :open-paren-colon
                                :open-paren-greater
                                :open-paren-equal
                                :open-paren-exclamation
                                :open-paren-less-equal
                                :open-paren-less-exclamation)
                   :test #'eq)
            (let ((*extended-mode-p* *extended-mode-p*))
  we saw one of the six token representing opening
              (let* ((open-paren-pos (car (lexer-last-pos lexer)))
                     (reg-expr (reg-expr lexer))
                     (close-token (get-token lexer)))
                (when (eq open-token :open-paren)
                  (incf (lexer-reg lexer)))
                (unless (eq close-token :close-paren)
                  (signal-ppcre-syntax-error*
                   open-paren-pos
                   "Opening paren has no matching closing paren"))
                (if flags
                  (cons :group (nconc flags (list reg-expr)))
                  (list (case open-token
                          ((:open-paren)
                            :register)
                          ((:open-paren-colon)
                            :group)
                          ((:open-paren-greater)
                            :standalone)
                          ((:open-paren-equal)
                            :positive-lookahead)
                          ((:open-paren-exclamation)
                            :negative-lookahead)
                          ((:open-paren-less-equal)
                            :positive-lookbehind)
                          ((:open-paren-less-exclamation)
                            :negative-lookbehind))
                        reg-expr)))))
          (t
  parsing had already stopped in the SEQ method )
            open-token))))

(defun greedy-quant (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <greedy-quant>.
The productions are: <greedy-quant> -> <group> | <group><quantifier>
where <quantifier> is parsed by the lexer function GET-QUANTIFIER.
Will return <parse-tree> or (:GREEDY-REPETITION <min> <max> <parse-tree>)."
  (let* ((group (group lexer))
         (token (get-quantifier lexer)))
    (if token
  two - element list ( < min > < max > )
      (list :greedy-repetition (first token) (second token) group)
      group)))

(defun quant (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <quant>.
The productions are: <quant> -> <greedy-quant> | <greedy-quant>\"?\".
Will return the <parse-tree> returned by GREEDY-QUANT and optionally
change :GREEDY-REPETITION to :NON-GREEDY-REPETITION."
  (let* ((greedy-quant (greedy-quant lexer))
         (pos (lexer-pos lexer))
         (next-char (next-char lexer)))
    (when next-char
      (if (char= next-char #\?)
        (setf (car greedy-quant) :non-greedy-repetition)
        (setf (lexer-pos lexer) pos)))
    greedy-quant))

(defun seq (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <seq>.
The productions are: <seq> -> <quant> | <quant><seq>.
Will return <parse-tree> or (:SEQUENCE <parse-tree> <parse-tree>)."
  (flet ((make-array-from-two-chars (char1 char2)
           (let ((string (make-array 2
                                     :element-type 'character
                                     :fill-pointer t
                                     :adjustable t)))
             (setf (aref string 0) char1)
             (setf (aref string 1) char2)
             string)))
  Note that we 're calling START - OF - SUBEXPR - P before we actually try
    (if (start-of-subexpr-p lexer)
      (let ((quant (quant lexer)))
        (if (start-of-subexpr-p lexer)
          (let* ((seq (seq lexer))
                 (quant-is-char-p (characterp quant))
                 (seq-is-sequence-p (and (consp seq)
                                         (eq (first seq) :sequence))))
            (cond ((and quant-is-char-p
                        (characterp seq))
                    (make-array-from-two-chars seq quant))
                  ((and quant-is-char-p
                        (stringp seq))
                    (vector-push-extend quant seq)
                    seq)
                  ((and quant-is-char-p
                        seq-is-sequence-p
                        (characterp (second seq)))
                    (cond ((cddr seq)
                            (setf (cdr seq)
                                    (cons
                                     (make-array-from-two-chars (second seq)
                                                                quant)
                                     (cddr seq)))
                            seq)
                          (t (make-array-from-two-chars (second seq) quant))))
                  ((and quant-is-char-p
                        seq-is-sequence-p
                        (stringp (second seq)))
                    (cond ((cddr seq)
                            (setf (cdr seq)
                                    (cons
                                     (progn
                                       (vector-push-extend quant (second seq))
                                       (second seq))
                                     (cddr seq)))
                            seq)
                          (t 
                            (vector-push-extend quant (second seq))
                            (second seq))))
                  (seq-is-sequence-p
                    (setf (cdr seq)
                            (cons quant (cdr seq)))
                    seq)
                  (t (list :sequence quant seq))))
          quant))
      :void)))
  
(defun reg-expr (lexer)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Parses and consumes a <regex>, a complete regular expression.
The productions are: <regex> -> <seq> | <seq>\"|\"<regex>.
Will return <parse-tree> or (:ALTERNATION <parse-tree> <parse-tree>)."
  (let ((pos (lexer-pos lexer)))
    (case (next-char lexer)
      ((nil)
        :void)
      ((#\|)
        (list :alternation :void (reg-expr lexer)))
      (otherwise
  otherwise un - read the character we just saw and parse a
        (setf (lexer-pos lexer) pos)
        (let* ((seq (seq lexer))
               (pos (lexer-pos lexer)))
          (case (next-char lexer)
            ((nil)
              seq)
            ((#\|)
  alternation and we have the second production
              (let ((reg-expr (reg-expr lexer)))
                (cond ((and (consp reg-expr)
                            (eq (first reg-expr) :alternation))
  again we try to merge as above in SEQ
                        (setf (cdr reg-expr)
                                (cons seq (cdr reg-expr)))
                        reg-expr)
                      (t (list :alternation seq reg-expr)))))
            (otherwise
  just un - read the character and let another function
              (setf (lexer-pos lexer) pos)
              seq)))))))

(defun reverse-strings (parse-tree)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  (cond ((stringp parse-tree)
          (nreverse parse-tree))
        ((consp parse-tree)
          (loop for parse-tree-rest on parse-tree
                while parse-tree-rest
                do (setf (car parse-tree-rest)
                           (reverse-strings (car parse-tree-rest))))
          parse-tree)
        (t parse-tree)))

(defun parse-string (string)
  (declare (optimize speed
                     (safety 0)
                     (space 0)
                     (debug 0)
                     (compilation-speed 0)
                     #+:lispworks (hcl:fixnum-safety 0)))
  "Translate the regex string STRING into a parse tree."
  (let* ((lexer (make-lexer string))
         (parse-tree (reverse-strings (reg-expr lexer))))
    (if (end-of-string-p lexer)
      parse-tree
      (signal-ppcre-syntax-error*
       (lexer-pos lexer)
       "Expected end of string"))))