Reconocimiento de Lenguajes con PEGjs

PEGs versus Gramáticas

Una gramática y un PEG con las mismas reglas no definen el mismo lenguaje. Véase este ejemplo:

[~/srcPLgrado/pegjs/examples(master)]$ cat 
#!/usr/bin/env coffee
PEG = require 'pegjs'
coffee = require 'pegjs-coffee-plugin'
grammar = """
a =  b 'c'           
b = 'b' / 'b' 'a'   
parser = PEG.buildParser grammar, plugins: [coffee]
r = parser.parse "bc"
console.log("r = #{r}")
r = parser.parse "bac"
console.log("r = #{r}")
[~/srcPLgrado/pegjs/examples(master)]$ coffee 
r = b,c
SyntaxError: Expected "c" but "a" found.

Obsérvese que la correspondiente gramática genera el lenguaje:

{ 'bc', 'bac' }
Mientras que el PEG acepta el lenguaje 'bc'.

Dangling else: Asociando un else con su if mas cercano

The dangling else is a problem in computer programming in which an optional else clause in an If–then(–else) statement results in nested conditionals being ambiguous.

Formally, the reference context-free grammar of the language is ambiguous, meaning there is more than one correct parse tree.

In many programming languages one may write conditionally executed code in two forms:

the if-then form, and the if-then-else form – the else clause is optional:

              if a then s
              if a then s1 else s2

This gives rise to an ambiguity in interpretation when there are nested statements, specifically whenever an if-then form appears as s1 in an if-then-else form:

              if a then if b then s else s2
In this example, s is unambiguously executed when a is true and b is true, but one may interpret s2 as being executed when a is false In other words, one may see the previous statement as either of the following expressions:
if a then (if b then s) else s2
if a then (if b then s else s2)

This is a problem that often comes up in compiler construction, especially scannerless parsing.

The convention when dealing with the dangling else is to attach the else to the nearby if statement.

Programming languages like Pascal and C follow this convention, so there is no ambiguity in the semantics of the language, though the use of a parser generator may lead to ambiguous grammars. In these cases alternative grouping is accomplished by explicit blocks, such as begin...end in Pascal and {...} in C.

Here follows a solution in PEG.js:


$ cat danglingelse.pegjs 
S ← 'if' C 'then' S 'else' S / 'if' C 'then' S

S =   if C:C then S1:S else S2:S { return [ 'ifthenelse', C, S1, S2 ]; }
    / if C:C then S:S            { return [ 'ifthen', C, S]; }
    / O                          { return 'O'; }
_ = ' '*
C = _'c'_                        { return 'c'; }
O = _'o'_                        { return 'o'; }
else = _'else'_                 
if = _'if'_
then = _'then'_


$ cat use_danglingelse.js 
var PEG = require("./danglingelse.js");
var r = PEG.parse("if c then if c then o else o");


$ ../bin/pegjs danglingelse.pegjs 
$ node use_danglingelse.js 
[ 'ifthen', 'c', [ 'ifthenelse', 'c', 'O', 'O' ] ]


Invirtiendo el orden de las Alternativas

Si invertimos el orden de las alternativas:

[~/srcPLgrado/pegjs/examples(master)]$ cat danglingelse2.pegjs 
S ← 'if' C 'then' S 'else' S / 'if' C 'then' S

S =   if C:C then S:S            { return [ 'ifthen', C, S]; }
    / if C:C then S1:S else S2:S { return [ 'ifthenelse', C, S1, S2 ]; }
    / O                          { return 'O'; }
_ = ' '*
C = _'c'_                        { return 'c'; }
O = _'o'_                        { return 'o'; }
else = _'else'_                 
if = _'if'_
then = _'then'_
el lenguaje reconocido cambia (vease el ejemplo en la sección 4.8.1):
[~/srcPLgrado/pegjs/examples(master)]$ pegjs danglingelse2.pegjs 
[~/srcPLgrado/pegjs/examples(master)]$ cat use_danglingelse2.js 
var PEG = require("./danglingelse2.js");
var r = PEG.parse("if c then if c then o else o");

[~/srcPLgrado/pegjs/examples(master)]$ node use_danglingelse2.js 

      throw peg$buildException(null, peg$maxFailExpected, peg$maxFailPos);
SyntaxError: Expected " " or end of input but "e" found.

Not Predicate: Comentarios Anidados

The following recursive PEG.js program matches Pascal-style nested comment syntax:
(* which can (* nest *) like this *)


[~/srcPLgrado/pegjs/examples(master)]$ cat pascal_comments.pegjs 
/* Pascal nested comments */

P     =   prog:N+                       { return prog; }
N     =   chars:$(!Begin .)+            { return chars;}
        / C
C     = Begin chars:$T* End             { return "C: "+chars; }
T     =   C 
        / (!Begin !End char:.)          { return char;}
Begin = '(*'
End   = '*)'


$ cat use_pascal_comments.js 
var PEG = require("./pascal_comments.js");
var r = PEG.parse(
  "not bla bla (* pascal (* nested *) comment *)"+
  " pum pum (* another comment *)");


$ ../bin/pegjs pascal_comments.pegjs 
$ node use_pascal_comments.js 
[ 'not bla bla ',
  ' pascal  nested  comment ',
  ' pum pum ',
  ' another comment ' ]


Un Lenguaje Dependiente del Contexto

El lenguaje $ \{ a^n b^n c^n / n \in \mathcal{N} \}$ no puede ser expresado mediante una gramática independiente del contexto.

[~/srcPLgrado/pegjs/examples(master)]$ cat anbncn.pegjs 
  The following parsing expression grammar describes the classic 
  non-context-free language : 
               { a^nb^nc^n / n >= 1 }

S = &(A 'c') 'a'+ B:B !.  { return B; }
A = 'a' A:A? 'b' { if (A) { return A+1; } else return 1; }
B = 'b' B:B? 'c' { if (B) { return B+1; } else return 1; }

Este ejemplo puede ser obtenido desde GitHub:

[~/Dropbox/src/javascript/PLgrado/pegjs/examples(master)]$ git remote -v
dmajda (fetch)
dmajda (push)
origin (fetch)
origin (push)

Veamos un ejemplo de uso:

[~/srcPLgrado/pegjs/examples(master)]$ cat use_anbncn.js 
#!/usr/bin/env node
var PEG = require("./anbncn.js");

if (process.argv.length > 2) {
  try {
    var r = PEG.parse(process.argv[2]);
    console.log("ok "+JSON.stringify(r));
  catch (e) {
var inputs = ["aabbcc", 
              "aabbc",     // error
              "aaaabbbccc"  // not accepted

for(var i = 0; i < inputs.length; i++) {
  var input = inputs[i];
  try {
    var r = PEG.parse(input);
    console.log("ok "+JSON.stringify(r));
  catch (e) {


[~/srcPLgrado/pegjs/examples(master)]$ node use_anbncn.js
ok 2
Grr....SyntaxError: Expected "c" but end of input found.
ok 3
Grr....SyntaxError: Expected undefined but "a" found.

Casiano Rodríguez León