pegjs/examples/json.pegjs

/* JSON parser based on the grammar described at http://json.org/. */

/* ===== Syntactical Elements ===== */

start
  = _ object:object { return object; }

object
  = "{" _ "}" _                 { return {};      }
  / "{" _ members:members "}" _ { return members; }

members
  = head:pair tail:("," _ pair)* {
      var result = {};
      result[head[0]] = head[1];
      for (var i = 0; i < tail.length; i++) {
        result[tail[i][2][0]] = tail[i][2][1];
      }
      return result;
    }

pair
  = name:string ":" _ value:value { return [name, value]; }

array
  = "[" _ "]" _                   { return [];       }
  / "[" _ elements:elements "]" _ { return elements; }

elements
  = head:value tail:("," _ value)* {
      var result = [head];
      for (var i = 0; i < tail.length; i++) {
        result.push(tail[i][2]);
      }
      return result;
    }

value
  = string
  / number
  / object
  / array
  / "true" _  { return true;   }
  / "false" _ { return false;  }
  // FIXME: We can't return null here because that would mean parse failure.
  / "null" _  { return "null"; }

/* ===== Lexical Elements ===== */

string "string"
  = '"' '"' _             { return "";    }
  / '"' chars:chars '"' _ { return chars; }

chars
  = chars:char+ { return chars.join(""); }

char
  // In the original JSON grammar: "any-Unicode-character-except-"-or-\-or-control-character"
  = [^"\\\0-\x1F\x7f]
  / '\\"'  { return '"';  }
  / "\\\\" { return "\\"; }
  / "\\/"  { return "/";  }
  / "\\b"  { return "\b"; }
  / "\\f"  { return "\f"; }
  / "\\n"  { return "\n"; }
  / "\\r"  { return "\r"; }
  / "\\t"  { return "\t"; }
  / "\\u" h1:hexDigit h2:hexDigit h3:hexDigit h4:hexDigit {
      return String.fromCharCode(parseInt("0x" + h1 + h2 + h3 + h4));
    }

number "number"
  = int_:int frac:frac exp:exp _ { return parseFloat(int_ + frac + exp); }
  / int_:int frac:frac _         { return parseFloat(int_ + frac);       }
  / int_:int exp:exp _           { return parseFloat(int_ + exp);        }
  / int_:int _                   { return parseFloat(int_);              }

int
  = digit19:digit19 digits:digits     { return digit19 + digits;       }
  / digit:digit
  / "-" digit19:digit19 digits:digits { return "-" + digit19 + digits; }
  / "-" digit:digit                   { return "-" + digit;            }

frac
  = "." digits:digits { return "." + digits; }

exp
  = e:e digits:digits { return e + digits; }

digits
  = digits:digit+ { return digits.join(""); }

e
  = e:[eE] sign:[+-]? { return e + sign; }

/*
 * The following rules are not present in the original JSON gramar, but they are
 * assumed to exist implicitly.
 *
 * FIXME: Define them according to ECMA-262, 5th ed.
 */

digit
  = [0-9]

digit19
  = [1-9]

hexDigit
  = [0-9a-fA-F]

/* ===== Whitespace ===== */

_ "whitespace"
  = whitespace*

// Whitespace is undefined in the original JSON grammar, so I assume a simple
// conventional definition consistent with ECMA-262, 5th ed.
whitespace
  = [ \t\n\r]