Re: peg.el --- Parsing Expression Grammars in Emacs Lisp

[weber]

This is nice, thanks. I'll try to start using it in my scripts :)
A quick note: the function characterp doesn't exist here (i'm using
22.3)
Aliasing it to numberp seems to work for now..
Cheers,
Hugo

Helmut Eller wrote:
> New in this version:
>
>  * fix a few bugs, most notably *-expressions didn't backtrack properly.
>
>  * a more convenient syntax for character sets, e.g. [A-Z "-]"] now
>    means all charecters from A to Z, minus, and right brackets.
>
>  * more realistic examples
>
>  * minimal test suite
>
> Helmut.
>
> ;;; peg.el --- Parsing Expression Grammars in Emacs Lisp
> ;;
> ;; Copyright 2008  Helmut Eller <address@hidden>.
> ;;
> ;; This file is licensed under the terms of the GNU General Public
> ;; License as distributed with Emacs (press C-h C-c for details).
> ;;
> ;;; Commentary:
> ;;
> ;; Parsing Expression Grammars (PEG) are a formalism in the spirit of
> ;; Context Free Grammars (CFG) with some simplifications which makes
> ;; the implementation of PEGs as top-down parser particularly simple
> ;; and easy to understand [**].
> ;;
> ;; This file implements a macro `peg-parse' which parses the current
> ;; buffer according to a PEG.  E.g. we can match integers with a PEG
> ;; like this:
> ;;
> ;;  (peg-parse (number   sign digit (* digit))
> ;;             (sign     (or "+" "-" ""))
> ;;             (digit    '"09")))
> ;;
> ;; In contrast to regexps, PEGs allow us to define recursive rules.  A
> ;; PEG is a list of rules.  A rule is written as (NAME . PE).
> ;; E.g. (sign (or "+" "-" "")) is a rule with the name "sign".  For
> ;; convenience, PE is implicitly wrapped in a and. The syntax for
> ;; Parsing Expression (PE) is a follows:
> ;;
> ;; Description        Lisp            Haskell as in [*]
> ;; Sequence           (and e1 e2)     e1 e2
> ;; Prioritized Choice   (or e1 e2)    e1 / e2
> ;; Not-predicate      (not e)         !e
> ;; And-predicate      (if e)          &e
> ;; Any character      (any)           .
> ;; Literal string     "abc"           "abc"
> ;; Character C                (char c)        'c'
> ;; Zero-or-more       (* e)           e*
> ;; One-or-more                (+ e)           e+
> ;; Optional           (opt e)         e?
> ;; Character range    (range a b)     [a-b]
> ;; Character set      [a-b "+*" ?x]   [a-b+*x]
> ;;
> ;; `peg-parse' also supports parsing actions, i.e. Lisp snippets which
> ;; are executed when a PE matches.  This can be used to construct
> ;; syntax trees or for similar tasks.  Actions are written as
> ;;
> ;;  (action FORM)          ; evaluate FORM
> ;;  `(VAR... -- FORM...)   ; stack action
> ;;
> ;; Actions don't consume input, but are executed at the point of
> ;; match.  A "stack action" takes VARs from the "value stack" and
> ;; pushes the result of evaluating FORMs to that stack.  See
> ;; `peg-ex-parse-int' for an example.
> ;;
> ;; References:
> ;;
> ;; [*] Bryan Ford. Parsing Expression Grammars: a Recognition-Based
> ;; Syntactic Foundation. In POPL'04: Proceedings of the 31st ACM
> ;; SIGPLAN-SIGACT symposium on Principles of Programming Languages,
> ;; pages 111-122, New York, NY, USA, 2004. ACM Press.
> ;;
> ;; [**] Baker, Henry G. "Pragmatic Parsing in Common Lisp".  ACM Lisp
> ;; Pointers 4(2), April--June 1991, pp. 3--15.
> ;;
>
> ;;; Code:
>
> (defmacro peg-parse (&rest rules)
>   "Match RULES at point.
> Return (T STACK) if the match succeed and nil on failure."
>   (peg-translate-rules rules))
>
> (defmacro peg-parse-exp (exp)
>   "Match the parsing expression EXP at point.
> Note: a PE can't \"call\" rules by name."
>   `(let ((peg-thunks nil))
>      (when ,(peg-translate-exp (peg-normalize exp))
>        (peg-postprocess peg-thunks))))
>
> ;; A table of the PEG rules.  Used during compilation to resolve
> ;; references to named rules.
> (defvar peg-rules)
>
> ;; used at runtime for backtracking.  It's a list ((POS . THUNK)...).
> ;; Each THUNK is executed at the corresponding POS.  Thunks are
> ;; executed in a postprocessing step, not during parsing.
> (defvar peg-thunks)
>
> ;; The basic idea is to translate each rule to a lisp function.
> ;; The result looks like
> ;;   (let ((rule1 (lambda () code-for-rule1))
> ;;         ...
> ;;         (ruleN (lambda () code-for-ruleN)))
> ;;     (funcall rule1))
> ;;
> ;; code-for-ruleX returns t if the rule matches and nil otherwise.
> ;;
> (defun peg-translate-rules (rules)
>   "Translate the PEG RULES, to a top-down parser."
>   (let ((peg-rules (make-hash-table :size 20)))
>     (dolist (rule rules)
>       (puthash (car rule) 'defer peg-rules))
>     (dolist (rule rules)
>       (puthash (car rule) (peg-normalize `(and . ,(cdr rule))) peg-rules))
>     (peg-check-cycles peg-rules)
>     `(let ((peg-thunks '()))
>        (let ,(mapcar (lambda (rule)
>                      (let ((name (car rule)))
>                        `(,name
>                          (lambda ()
>                            ,(peg-translate-exp (gethash name peg-rules))))))
>                    rules)
>        (when (funcall ,(car (car rules)))
>          (peg-postprocess peg-thunks))))))
>
> (defun peg-method-table-name (method-name)
>   (intern (format "peg-%s-methods" method-name)))
>
> (defmacro peg-define-method-table (name)
>   (let ((tab (peg-method-table-name name)))
>   `(progn
>      (defvar ,tab)
>      (setq ,tab (make-hash-table :size 20)))))
>
> (defmacro peg-add-method (method type args &rest body)
>   (declare (indent 3))
>   `(puthash ',type (lambda ,args . ,body) ,(peg-method-table-name method)))
>
> (peg-define-method-table normalize)
>
> ;; Internally we use a regularized syntax, e.g. we only have binary OR
> ;; nodes.  Regularized nodes are lists of the form (OP ARGS...).
> (defun peg-normalize (exp)
>   "Return a \"normalized\" form of EXP."
>   (cond ((and (consp exp)
>             (let ((fun (gethash (car exp) peg-normalize-methods)))
>               (and fun
>                    (apply fun (cdr exp))))))
>       ((stringp exp)
>        (let ((len (length exp)))
>          (cond ((zerop len) '(null))
>                ((= len 1) `(char ,(aref exp 0)))
>                (t `(str ,exp)))))
>       ((and (symbolp exp) exp)
>        (when (not (gethash exp peg-rules))
>          (error "Reference to undefined PEG rule: %S" exp))
>        `(call ,exp))
>       ((vectorp exp)
>        (peg-normalize `(set . ,(append exp '()))))
>       (t
>        (error "Invalid parsing expression: %S" exp))))
>
> (defvar peg-leaf-types '(null fail any call action char range str eob
>                             set))
>
> (dolist (type peg-leaf-types)
>   (puthash type `(lambda (&rest args) (cons ',type args))
>          peg-normalize-methods))
>
> (peg-add-method normalize or (&rest args)
>   (cond ((null args) '(fail))
>       ((null (cdr args)) (peg-normalize (car args)))
>       (t `(or ,(peg-normalize (car args))
>               ,(peg-normalize `(or . ,(cdr args)))))))
>
> (peg-add-method normalize and (&rest args)
>   (cond ((null args) '(null))
>       ((null (cdr args)) (peg-normalize (car args)))
>       (t `(and ,(peg-normalize (car args))
>                ,(peg-normalize `(and . ,(cdr args)))))))
>
> (peg-add-method normalize * (&rest args)
>   `(* ,(peg-normalize `(and . ,args))))
>
> (peg-add-method normalize + (&rest args)
>   (let ((e (peg-normalize `(and . ,args))))
>     `(and ,e (* ,e))))
>
> (peg-add-method normalize opt (&rest args)
>   (let ((e (peg-normalize `(and . ,args))))
>     `(or ,e (null))))
>
> (peg-add-method normalize if (&rest args)
>   `(if ,(peg-normalize `(and . ,args))))
>
> (peg-add-method normalize not (&rest args)
>   `(not ,(peg-normalize `(and . ,args))))
>
> (peg-add-method normalize \` (form)
>   (unless (member '-- form)
>     (error "Malformed stack action: %S" form))
>   (let ((args (cdr (member '-- (reverse form))))
>       (values (cdr (member '-- form))))
>     (let ((form `(let ,(mapcar (lambda (var) `(,var (pop peg-stack)))
>                              args)
>                  (setq peg-stack
>                        (append (list . ,values) peg-stack)))))
>       `(action ,form))))
>
> (peg-add-method normalize set (&rest specs)
>   (cond ((null specs) '(fail))
>       ((and (null (cdr specs))
>             (let ((range (peg-range-designator (car specs))))
>               (and range `(range . ,range)))))
>       (t
>        (let ((args '()))
>          (while specs
>            (let* ((spec (pop specs))
>                   (range (peg-range-designator spec)))
>              (cond (range
>                     (push (cons (car range) (cadr range)) args))
>                    ((or (and (symbolp spec) (eq spec nil))
>                         (characterp spec))
>                     (push spec args))
>                    ((stringp spec)
>                     (setq args (append spec args)))
>                    (t (error "Invalid set specifier: %S" spec)))))
>          `(set ,(rx-to-string `(any . ,args) t))))))
>
> (peg-add-method normalize *list (&rest args)
>   (peg-normalize
>    `(and `(-- ())
>        (* ,@args `(l e -- (cons e l)))
>        `(l -- (nreverse l)))))
>
> (peg-add-method normalize substring (&rest args)
>   (peg-normalize
>    `(and `(-- (point))
>        ,@args
>        `(start --
>                (buffer-substring-no-properties start (point))))))
>
> (defun peg-range-designator (x)
>   (and (symbolp x)
>        (let ((str (symbol-name x)))
>        (and (= (length str) 3)
>             (eq (aref str 1) ?-)
>             (list (aref str 0) (aref str 2))))))
>
> (peg-add-method normalize quote (form)
>   (error "quote is reverved for future use"))
>
> (peg-define-method-table translate)
>
> ;; This is the main translation function.
> (defun peg-translate-exp (exp)
>   "Return the ELisp code to match the PE EXP."
>   (let ((translator (or (gethash (car exp) peg-translate-methods)
>                       (error "No translator for: %S" (car exp)))))
>     (apply translator (cdr exp))))
>
> (peg-add-method translate and (e1 e2)
>   `(and ,(peg-translate-exp e1)
>       ,(peg-translate-exp e2)))
>
> (peg-add-method translate or (e1 e2)
>   (let ((cp (peg-make-choicepoint)))
>     `(,@(peg-save-choicepoint cp)
>       (or ,(peg-translate-exp e1)
>         (,@(peg-restore-choicepoint cp)
>          ,(peg-translate-exp e2))))))
>
> ;; Choicepoints are used for backtracking.  At a choicepoint we save
> ;; enough state, so that we can continue from there if needed.
> (defun peg-make-choicepoint ()
>   (cons (make-symbol "point") (make-symbol "thunks")))
>
> (defun peg-save-choicepoint (choicepoint)
>   `(let ((,(car choicepoint) (point))
>        (,(cdr choicepoint) peg-thunks))))
>
> (defun peg-restore-choicepoint (choicepoint)
>   `(progn
>      (goto-char ,(car choicepoint))
>      (setq peg-thunks ,(cdr choicepoint))))
>
> ;; match empty strings
> (peg-add-method translate null ()
>   `t)
>
> ;; match nothing
> (peg-add-method translate fail ()
>   `nil)
>
> (peg-add-method translate eob ()
>   '(eobp))
>
> (peg-add-method translate * (e)
>   (let ((cp (peg-make-choicepoint)))
>     `(progn (while (,@(peg-save-choicepoint cp)
>                   (cond (,(peg-translate-exp e))
>                         (t ,(peg-restore-choicepoint cp)
>                            nil))))
>           t)))
>
> (peg-add-method translate if (e)
>   (let ((cp (peg-make-choicepoint)))
>     `(,@(peg-save-choicepoint cp)
>       (when ,(peg-translate-exp e)
>       ,(peg-restore-choicepoint cp)
>       t))))
>
> (peg-add-method translate not (e)
>   (let ((cp (peg-make-choicepoint)))
>     `(,@(peg-save-choicepoint cp)
>       (when (not ,(peg-translate-exp e))
>       ,(peg-restore-choicepoint cp)
>       t))))
>
> (peg-add-method translate any ()
>   '(when (not (eobp))
>      (forward-char)
>      t))
>
> (peg-add-method translate char (c)
>   `(when (eq (char-after) ',c)
>      (forward-char)
>      t))
>
> (peg-add-method translate set (charset-regexp)
>   `(when (looking-at ',charset-regexp)
>      (forward-char)
>      t))
>
> (peg-add-method translate range (from to)
>   `(when (and (<= ',from (char-after))
>             (<= (char-after) ',to))
>      (forward-char)
>      t))
>
> (peg-add-method translate str (str)
>   `(when (looking-at ',(regexp-quote str))
>      (goto-char (match-end 0))
>      t))
>
> (peg-add-method translate call (name)
>   (or (gethash name peg-rules)
>       (error "Reference to unknown rule: %S" name))
>   `(funcall ,name))
>
> (peg-add-method translate action (form)
>   `(progn
>      (push (cons (point) (lambda () ,form)) peg-thunks)
>      t))
>
> (defvar peg-stack)
> (defun peg-postprocess (thunks)
>   "Execute \"actions\"."
>   (let  ((peg-stack '()))
>     (dolist (thunk (reverse thunks))
>       (goto-char (car thunk))
>       (funcall (cdr thunk)))
>     (list t peg-stack)))
>
> ;; Left recursion is presumably a common mistate when using PEGs.
> ;; Here we try to detect such mistakes.  Essentailly we traverse the
> ;; graph as long as we can without consuming input.  When we find a
> ;; recursive call we signal an error.
>
> (defun peg-check-cycles (peg-rules)
>   (maphash (lambda (name exp)
>            (peg-detect-cycles exp (list name))
>            (dolist (node (peg-find-star-nodes exp))
>              (peg-detect-cycles node '())))
>          peg-rules))
>
> (defun peg-find-star-nodes (exp)
>   (let ((type (car exp)))
>     (cond ((memq type peg-leaf-types) '())
>         (t (let ((kids (apply #'append
>                               (mapcar #'peg-find-star-nodes (cdr exp)))))
>              (if (eq type '*)
>                  (cons exp kids)
>                kids))))))
>
>
> (peg-define-method-table detect-cycles)
>
> (defun peg-detect-cycles (exp path)
>   "Signal an error on a cycle.
> Otherwise traverse EXP recursively and return T if EXP can match
> without consuming input.  Return nil if EXP definetly consumes
> input.  PATH is the list of rules that we have visited so far."
>   (apply (or (gethash (car exp) peg-detect-cycles-methods)
>            (error "No detect-cycle method for: %S" exp))
>        path (cdr exp)))
>
> (peg-add-method detect-cycles call (path name)
>   (cond ((member name path)
>        (error "Possible left recursion: %s"
>               (mapconcat (lambda (x) (format "%s" x))
>                          (reverse (cons name path)) " -> ")))
>       (t
>        (peg-detect-cycles (gethash name peg-rules) (cons name path)))))
>
> (peg-add-method detect-cycles and (path e1 e2)
>   (and (peg-detect-cycles e1 path)
>        (peg-detect-cycles e2 path)))
>
> (peg-add-method detect-cycles or (path e1 e2)
>   (or (peg-detect-cycles e1 path)
>       (peg-detect-cycles e2 path)))
>
> (peg-add-method detect-cycles * (path e)
>   (when (peg-detect-cycles e path)
>     (error "Infinite *-loop: %S matches empty string" e))
>   t)
>
> (peg-add-method detect-cycles if  (path e) (peg-unary-nullable e path))
> (peg-add-method detect-cycles not (path e) (peg-unary-nullable e path))
>
> (defun peg-unary-nullable (exp path)
>   (peg-detect-cycles exp path)
>   t)
>
> (peg-add-method detect-cycles any   (path)       nil)
> (peg-add-method detect-cycles char  (path c)     nil)
> (peg-add-method detect-cycles set   (path s)     nil)
> (peg-add-method detect-cycles range (path c1 c2) nil)
> (peg-add-method detect-cycles str   (path s)     (equal s ""))
> (peg-add-method detect-cycles null  (path)       t)
> (peg-add-method detect-cycles fail  (path)       nil)
> (peg-add-method detect-cycles eob   (path)       t)
> (peg-add-method detect-cycles action (path form) t)
>
> ;;; Tests:
>
> (defmacro peg-parse-string (rules string)
>   `(with-temp-buffer
>      (insert ,string)
>      (goto-char (point-min))
>      (peg-parse . ,rules)))
>
> (defun peg-test ()
>   (interactive)
>   (assert (peg-parse-string ((s "a")) "a"))
>   (assert (not (peg-parse-string ((s "a")) "b")))
>   (assert (peg-parse-string ((s (not "a"))) "b"))
>   (assert (not (peg-parse-string ((s (not "a"))) "a")))
>   (assert (peg-parse-string ((s (if "a"))) "a"))
>   (assert (not (peg-parse-string ((s (if "a"))) "b")))
>   (assert (peg-parse-string ((s "ab")) "ab"))
>   (assert (not (peg-parse-string ((s "ab")) "ba")))
>   (assert (not (peg-parse-string ((s "ab")) "a")))
>   (assert (peg-parse-string ((s (range ?0 ?9))) "0"))
>   (assert (not (peg-parse-string ((s (range ?0 ?9))) "a")))
>   (assert (peg-parse-string ((s [0-9])) "0"))
>   (assert (peg-parse-string ((s (any))) "0"))
>   (assert (not (peg-parse-string ((s (any))) "")))
>   (assert (peg-parse-string ((s (eob))) ""))
>   (assert (peg-parse-string ((s (not (eob)))) "a"))
>   (assert (peg-parse-string ((s (or "a" "b"))) "a"))
>   (assert (peg-parse-string ((s (or "a" "b"))) "b"))
>   (assert (not (peg-parse-string ((s (or "a" "b"))) "c")))
>   (assert (peg-parse-string ((s (and "a" "b"))) "ab"))
>   (assert (peg-parse-string ((s (and "a" "b"))) "abc"))
>   (assert (not (peg-parse-string ((s (and "a" "b"))) "ba")))
>   (assert (peg-parse-string ((s (and "a" "b" "c"))) "abc"))
>   (assert (peg-parse-string ((s (* "a") "b" (eob))) "ab"))
>   (assert (not (peg-parse-string ((s (* "a") "b" (eob))) "abc")))
>   (assert (peg-parse-string ((s "")) "abc"))
>   (assert (peg-parse-string ((s "" (eob))) ""))
>   (assert (peg-parse-string ((s (opt "a") "b")) "abc"))
>   (assert (peg-parse-string ((s (opt "a") "b")) "bc"))
>   (assert (not (peg-parse-string ((s (or))) "ab")))
>   (assert (peg-parse-string ((s (and))) "ab")))
>
> (when (featurep 'cl)
>   (peg-test))
>
> ;;; Examples:
>
> ;; peg-ex-recognize-int recognizes integers.  An integer begins with a
> ;; optional sign, then follows one or more digits.  Digits are all
> ;; characters from 0 to 9.
> ;;
> ;; Notes:
> ;; 1) "" matches the empty sequence, i.e. matches without
> ;; consuming input.
> ;; 2) [0-9] is the character range from 0 to 9.  This can also be
> ;; written as (range ?0 ?9). [Note that 0-9 is a symbol.]
> (defun peg-ex-recognize-int ()
>   (peg-parse (number   sign digit (* digit))
>            (sign     (or "+" "-" ""))
>            (digit    [0-9])))
>
> ;; peg-ex-parse-int recognizes integers and computes the corresponding
> ;; value.  The grammer is the same as for `peg-ex-recognize-int' added
> ;; with parsing actions.  Unfortunaletly, the actions add quite a bit
> ;; of clutter.
> ;;
> ;; The action for the sign rule pushes t on the stack for a minus sign
> ;; and nil for plus or no sign.
> ;;
> ;; The action for the digit rule pushes the value for a single digit.
> ;;
> ;; The action `(a b -- (+ (* a 10) b)), takes two items from the stack
> ;; and pushes the first digit times 10 added to second digit.
> ;;
> ;; The action `(minus val -- (if minus (- val) val)), negates the
> ;; value if the minus flag is true.
> (defun peg-ex-parse-int ()
>   (peg-parse (number sign
>                    digit
>                    (* digit `(a b -- (+ (* a 10) b)))
>                    `(minus val -- (if minus (- val) val)))
>            (sign (or (and "+" `(-- nil))
>                      (and "-" `(-- t))
>                      (and "" `(-- nil))))
>            (digit [0-9] `(-- (- (char-before) ?0)))))
>
> ;; Put point after the ) and press C-x C-e
> ;; (peg-ex-parse-int)-234234
>
> ;; Parse arithmetic expressions and compute the result as side effect.
> (defun peg-ex-arith ()
>   (peg-parse
>    (expr (or (and ws sum eol)
>            (and (* (not eol) (any)) eol error)))
>    (sum product (* (or (and plus product `(a b -- (+ a b)))
>                      (and minus product `(a b -- (- a b))))))
>    (product value (* (or (and times value `(a b -- (* a b)))
>                        (and divide value `(a b -- (/ a b))))))
>    (value (or (and (substring number) `(string -- (string-to-number string)))
>             (and open sum close)))
>    (number (+ [0-9]) ws)
>    (plus "+" ws)
>    (minus "-" ws)
>    (times "*" ws)
>    (divide "/" ws)
>    (open "(" ws)
>    (close ")" ws)
>    (ws (* (or " " "\t")))
>    (eol (or "\n" "\r\n" "\r"))
>    (error (action (error "Parse error at: %s" (point))))))
>
> ;; (peg-ex-arith)   1 + 2 * 3 * (4 + 5)
>
> ;; Parse URI according to RFC 2396.
> (defun peg-ex-uri ()
>   (peg-parse
>    (URI-reference (or absoluteURI relativeURI)
>                 (or (and "#" (substring fragment))
>                     `(-- nil)))
>    (absoluteURI (substring scheme) ":" (or hier-part opaque-part)
>               `(scheme user host port path query -- (list :scheme scheme
>                                                           :user user
>                                                           :host host
>                                                           :port port
>                                                           :path path
>                                                           :query query)))
>    (hier-part (or net-path abs-path)
>             (or (and "?" (substring query))
>                 `(-- nil)))
>    (net-path "//" authority (or abs-path `(-- nil)))
>    (abs-path "/" path-segments)
>    (path-segments segment (*list "/" segment) `(s l -- (cons s l)))
>    (segment (substring (* pchar) (* ";" param)))
>    (param (* pchar))
>    (pchar (or unreserved escaped [":@&=+$,"]))
>    (query (* uric))
>    (fragment (* uric))
>    (relativeURI (or net-path abs-path rel-path) (opt "?" query))
>    (rel-path rel-segment (opt abs-path))
>    (rel-segment (+ unreserved escaped [";@&=+$,"]))
>    (authority (or server reg-name))
>    (server (or (and (or (and (substring userinfo) "@")
>                       `(-- nil))
>                   hostport)
>              `(-- nil nil nil)))
>    (userinfo (* (or unreserved escaped [";:&=+$,"])))
>    (hostport (substring host) (or (and ":" (substring port))
>                                 `(-- nil)))
>    (host (or hostname ipv4address))
>    (hostname (* domainlabel ".") toplabel (opt "."))
>    (domainlabel alphanum
>               (opt (* (or alphanum "-") (if alphanum))
>                    alphanum))
>    (toplabel alpha
>            (* (or alphanum "-") (if alphanum))
>            alphanum)
>    (ipv4address (+ digit) "." (+ digit) "." (+ digit) "." (+ digit))
>    (port (* digit))
>    (scheme alpha (* (or alpha digit ["+-."])))
>    (reg-name (or unreserved escaped ["$,;:@&=+"]))
>    (opaque-part uric-no-slash (* uric))
>    (uric (or reserved unreserved escaped))
>    (uric-no-slash (or unreserved escaped [";?:@&=+$,"]))
>    (reserved (set ";/?:@&=+$,"))
>    (unreserved (or alphanum mark))
>    (escaped "%" hex hex)
>    (hex (or digit [A-F] [a-f]))
>    (mark (set "-_.!~*'()"))
>    (alphanum (or alpha digit))
>    (alpha (or lowalpha upalpha))
>    (lowalpha [a-z])
>    (upalpha [A-Z])
>    (digit [0-9])))
>
> ;; (peg-ex-uri)file:/bar/baz.html?foo=df
> ;; (peg-ex-uri)http://address@hidden:8080/bar/baz.html?x=1#foo
>
> ;; Parse a lisp style Sexp.
> ;; [To keep the example short, ' and . are handled as ordinary symbol.]
> (defun peg-ex-lisp ()
>   (peg-parse
>    (sexp (* (or blank comment)) (or string list number symbol))
>    (ws (* blank))
>    (blank [" \n\t"])
>    (comment ";" (* (not (or "\n" (eob))) (any)))
>    (string "\"" (substring  (* (not "\"") (any))) "\"")
>    (number (substring (opt (set "+-")) (+ digit))
>          (if terminating)
>          `(string -- (string-to-number string)))
>    (symbol (substring (and symchar (* (not terminating) symchar)))
>          `(s -- (intern s)))
>    (symchar [a-z A-Z 0-9 "-;!#%&'*+,./:;<=>address@hidden|}~"])
>    (list "("                  `(-- (cons nil nil)) `(hd -- hd hd)
>        (* sexp        `(tl e -- (setcdr tl (list e)))
>           ) ws ")"    `(hd tl -- (cdr hd)))
>    (digit [0-9])
>    (terminating (or (set " \n\t();\"'") (eob)))))
>
> ;; (peg-ex-lisp)
>
> ;; We try to detect left recursion and report it as error.
> (defun peg-ex-left-recursion ()
>   (eval '(peg-parse (exp (or term
>                            (and exp "+" exp)))
>                   (term (or digit
>                             (and term "*" term)))
>                   (digit [0-9]))))
>
> (defun peg-ex-infinite-loop ()
>   (eval '(peg-parse (exp (* (or "x"
>                               "y"
>                               (action (foo))))))))
>
> (provide 'peg)
>
> ;;; peg.el ends here