@c -*-texinfo-*-
@c This is part of the GNU Emacs Lisp Reference Manual.
@c Copyright (C) 1990--1995, 1998--1999, 2001--2023 Free Software
@c Foundation, Inc.
@c See the file elisp.texi for copying conditions.
@node Parsing Expression Grammars
@chapter Parsing Expression Grammars
@cindex text parsing
@cindex parsing expression grammar

  Emacs Lisp provides several tools for parsing and matching text,
from regular expressions (@pxref{Regular Expressions}) to full
@acronym{LL} grammar parsers (@pxref{Top,, Bovine parser
development,bovine}).  @dfn{Parsing Expression Grammars}
(@acronym{PEG}) are another approach to text parsing that offer more
structure and composibility than regular expressions, but less
complexity than context-free grammars.

A @acronym{PEG} parser is defined as a list of named rules, each of
which matches text patterns, and/or contains references to other
rules.  Parsing is initiated with the function @code{peg-run} or the
macro @code{peg-parse} (see below), and parses text after point in the
current buffer, using a given set of rules.

@cindex parsing expression
The definition of each rule is referred to as a @dfn{parsing
expression} (@acronym{PEX}), and can consist of a literal string, a
regexp-like character range or set, a peg-specific construct
resembling an elisp function call, a reference to another rule, or a
combination of any of these.  A grammar is expressed as a tree of
rules in which one rule is typically treated as a ``root'' or
``entry-point'' rule.  For instance:

@example
@group
((number sign digit (* digit))
 (sign   (or "+" "-" ""))
 (digit  [0-9]))
@end group
@end example

Once defined, grammars can be used to parse text after point in the
current buffer, in the following ways:

@defmac peg-parse &rest pexs
Match @var{pexs} at point.  If @var{pexs} is a list of PEG rules, the
first rule is considered the ``entry-point'':
@end defmac

@example
@group
(peg-parse
  ((number sign digit (* digit))
   (sign   (or "+" "-" ""))
   (digit  [0-9])))
@end group
@end example

This macro represents the simplest use of the @acronym{PEG} library,
but also the least flexible, as the rules must be written directly
into the source code.  A more flexible approach involves use of three
macros in conjunction: @code{with-peg-rules}, a @code{let}-like
construct that makes a set of rules available within the macro body;
@code{peg-run}, which initiates parsing given a single rule; and
@code{peg}, which is used to wrap the entry-point rule name.  In fact,
a call to @code{peg-parse} expands to just this set of calls.  The
above example could be written as:

@example
@group
(with-peg-rules
    ((number sign digit (* digit))
     (sign   (or "+" "-" ""))
     (digit  [0-9]))
  (peg-run (peg number)))
@end group
@end example

This allows more explicit control over the ``entry-point'' of parsing,
and allows the combination of rules from different sources.

Individual rules can also be defined using a more @code{defun}-like
syntax, using the macro @code{define-peg-rule}:

@example
(define-peg-rule digit ()
  [0-9])
@end example

This also allows for rules that accept an argument (supplied by the
@code{funcall} PEG rule).

Another possibility is to define a named set of rules with
@code{define-peg-ruleset}:

@example
(define-peg-ruleset number-grammar
        '((number sign digit (* digit))
          digit  ;; A reference to the definition above.
          (sign (or "+" "-" ""))))
@end example

Rules and rulesets defined this way can be referred to by name in
later calls to @code{peg-run} or @code{with-peg-rules}:

@example
(with-peg-rules number-grammar
  (peg-run (peg number)))
@end example

By default, calls to @code{peg-run} or @code{peg-parse} produce no
output: parsing simply moves point.  In order to return or otherwise
act upon parsed strings, rules can include @dfn{actions}, see
@ref{Parsing Actions}.

@menu
* PEX Definitions::    The syntax of PEX rules.
* Parsing Actions::    Running actions upon successful parsing.
* Writing PEG Rules::  Tips for writing parsing rules.
@end menu

@node PEX Definitions
@section PEX Definitions

Parsing expressions can be defined using the following syntax:

@table @code
@item (and E1 E2 ...)
A sequence of @acronym{PEX}s that must all be matched.  The @code{and} form is
optional and implicit.

@item (or E1 E2 ...)
Prioritized choices, meaning that, as in Elisp, the choices are tried
in order, and the first successful match is used.  Note that this is
distinct from context-free grammars, in which selection between
multiple matches is indeterminate.

@item (any)
Matches any single character, as the regexp ``.''.

@item @var{string}
A literal string.

@item (char @var{C})
A single character @var{C}, as an Elisp character literal.

@item (* @var{E})
Zero or more instances of expression @var{E}, as the regexp @samp{*}.
Matching is always ``greedy''.

@item (+ @var{E})
One or more instances of expression @var{E}, as the regexp @samp{+}.
Matching is always ``greedy''.

@item (opt @var{E})
Zero or one instance of expression @var{E}, as the regexp @samp{?}.

@item SYMBOL
A symbol representing a previously-defined PEG rule.

@item (range CH1 CH2)
The character range between CH1 and CH2, as the regexp @samp{[CH1-CH2]}.

@item [CH1-CH2 "+*" ?x]
A character set, which can include ranges, character literals, or
strings of characters.

@item [ascii cntrl]
A list of named character classes.

@item (syntax-class @var{NAME})
A single syntax class.

@item (funcall E ARGS...)
Call @acronym{PEX} E (previously defined with @code{define-peg-rule})
with arguments @var{ARGS}.

@item (null)
The empty string.

@end table

The following expressions are used as anchors or tests -- they do not
move point, but return a boolean value which can be used to constrain
matches as a way of controlling the parsing process (@pxref{Writing
PEG Rules}).

@table @code
@item (bob)
Beginning of buffer.

@item (eob)
End of buffer.

@item (bol)
Beginning of line.

@item (eol)
End of line.

@item (bow)
Beginning of word.

@item (eow)
End of word.

@item (bos)
Beginning of symbol.

@item (eos)
End of symbol.

@item (if E)
Returns non-@code{nil} if parsing @acronym{PEX} E from point succeeds (point
is not moved).

@item (not E)
Returns non-@code{nil} if parsing @acronym{PEX} E from point fails (point
is not moved).

@item (guard EXP)
Treats the value of the Lisp expression EXP as a boolean.

@end table

@vindex peg-char-classes
Character class matching can use the same named character classes as
in regular expressions (@pxref{Top,, Character Classes,elisp})

@node Parsing Actions
@section Parsing Actions

@cindex parsing actions
@cindex parsing stack
By default the process of parsing simply moves point in the current
buffer, ultimately returning @code{t} if the parsing succeeds, and
@code{nil} if it doesn't.  It's also possible to define ``actions''
that can run arbitrary Elisp at certain points in the parsed text.
These actions can optionally affect something called the @dfn{parsing
stack}, which is a list of values returned by the parsing process.
These actions only run (and only return values) if the parsing process
ultimately succeeds; if it fails the action code is not run at all.

Actions can be added anywhere in the definition of a rule.  They are
distinguished from parsing expressions by an initial backquote
(@samp{`}), followed by a parenthetical form that must contain a pair
of hyphens (@samp{--}) somewhere within it.  Symbols to the left of
the hyphens are bound to values popped from the stack (they are
somewhat analogous to the argument list of a lambda form).  Values
produced by code to the right are pushed to the stack (analogous to
the return value of the lambda).  For instance, the previous grammar
can be augmented with actions to return the parsed number as an actual
integer:

@example
(with-peg-rules ((number sign digit (* digit
                                       `(a b -- (+ (* a 10) b)))
                         `(sign val -- (* sign val)))
                 (sign (or (and "+" `(-- 1))
                           (and "-" `(-- -1))
                           (and ""  `(-- 1))))
                 (digit [0-9] `(-- (- (char-before) ?0))))
  (peg-run (peg number)))
@end example

There must be values on the stack before they can be popped and
returned -- if there aren't enough stack values to bind to an action's
left-hand terms, they will be bound to @code{nil}.  An action with
only right-hand terms will push values to the stack; an action with
only left-hand terms will consume (and discard) values from the stack.
At the end of parsing, stack values are returned as a flat list.

To return the string matched by a @acronym{PEX} (instead of simply
moving point over it), a rule like this can be used:

@example
(one-word
  `(-- (point))
  (+ [word])
  `(start -- (buffer-substring start (point))))
@end example

The first action pushes the initial value of point to the stack.  The
intervening @acronym{PEX} moves point over the next word.  The second
action pops the previous value from the stack (binding it to the
variable @code{start}), and uses that value to extract a substring
from the buffer and push it to the stack.  This pattern is so common
that @acronym{PEG} provides a shorthand function that does exactly the
above, along with a few other shorthands for common scenarios:

@table @code
@item (substring @var{E})
Match @acronym{PEX} @var{E} and push the matched string to the stack.

@item (region @var{E})
Match @var{E} and push the start and end positions of the matched
region to the stack.

@item (replace @var{E} @var{replacement})
Match @var{E} and replaced the matched region with the string @var{replacement}.

@item (list @var{E})
Match @var{E}, collect all values produced by @var{E} (and its
sub-expressions) into a list, and push that list to the stack.  Stack
values are typically returned as a flat list; this is a way of
``grouping'' values together.
@end table

@node Writing PEG Rules
@section Writing PEG Rules

Something to be aware of when writing PEG rules is that they are
greedy.  Rules which can consume a variable amount of text will always
consume the maximum amount possible, even if that causes a rule that
might otherwise have matched to fail later on -- there is no
backtracking.  For instance, this rule will never succeed:

@example
(forest (+ "tree" (* [blank])) "tree" (eol))
@end example

The @acronym{PEX} @code{(+ "tree" (* [blank]))} will consume all
repetitions of the word ``tree'', leaving none to match the final
@code{"tree"}.

In these situations, the desired result can be obtained by using
predicates and guards -- namely the @code{not}, @code{if} and
@code{guard} expressions -- to constrain behavior.  For instance:

@example
(forest (+ "tree" (* [blank])) (not (eol)) "tree" (eol))
@end example

The @code{if} and @code{not} operators accept a parsing expression and
interpret it as a boolean, without moving point.  The contents of a
@code{guard} operator are evaluated as regular Lisp (not a
@acronym{PEX}) and should return a boolean value.  A @code{nil} value
causes the match to fail.

Another potentially unexpected behavior is that parsing will move
point as far as possible, even if the parsing ultimately fails.  This
rule:

@example
(end-game "game" (eob))
@end example

when run in a buffer containing the text ``game over'' after point,
will move point to just after ``game'' then halt parsing, returning
@code{nil}.  Successful parsing will always return @code{t}, or the
contexts of the parsing stack.