Racc Samples

json_parse.ry.rb

class JsonParser
options no_result_var
rule
  json_text       : ws object ws
                    { val[1] }
                  | ws array ws
                    { val[1] }

  ws              :
                  | ws SPACE

  value           : FALSE
                  | NULL
                  | TRUE
                  | object
                  | array
                  | number
                  | STRING

  number          : INTEGER
                    { val[0].to_i }
                  | FLOAT
                    { val[0].to_f }

  object          : '{' ws '}'
                      { {} }
                  | '{' ws members ws '}'
                      { val[2].to_h }

  members         : member
                      { [ val[0] ] }
                  | members ws ',' ws member
                      { val[0] << val[4] }

  member          : STRING ws ':' ws value
                      { [ val[0], val[4] ] }

  array           : '[' ws ']'
                      { [] }
                    | '[' ws values ws ']'
                      { val[2] }

  values          : value
                    { [ val[0] ] }
                  | values ws ',' ws value
                    { val[0] << val[4] }
end

---- header

require 'strscan'

---- inner

R_SPACE    = /\A[ \t\n\r]+/
R_RESERVED = /\A[\{\}\[\],:]/
R_INTEGER  = /\A-?(?:0|[1-9]\d*)/
R_FLOAT    = /\A-?(?:0|[1-9]\d*)(?:\.\d+)/
R_STRING   = /\A"(?:\\["\\\/bfnrt]|\\u[\dA-Za-z]{4}|[^\\"])*"/
R_FALSE    = /\Afalse/
R_NULL     = /\Anull/
R_TRUE     = /\Atrue/

def initialize(obj)
  src = obj.is_a?(IO) ? obj.read : obj.to_s
  @s = StringScanner.new(src)
end

def parse
  piece = nil
  @q = []

  until @s.eos?
    if (piece = @s.scan R_SPACE)
      @q << [:SPACE, piece]
    elsif (piece = @s.scan R_RESERVED)
      @q << [piece, piece]
    elsif (piece = @s.scan R_FLOAT)
      @q << [:FLOAT, piece]
    elsif (piece = @s.scan R_INTEGER)
      @q << [:INTEGER, piece]
    elsif (piece = @s.scan R_STRING)
      @q << [:STRING, piece.gsub(/\A"(.*)"/, "\\1")]
    elsif @s.scan R_FALSE
      @q << [:FALSE, false]
    elsif @s.scan R_NULL
      @q << [:NULL, nil]
    elsif @s.scan R_TRUE
      @q << [:TRUE, true]
    else
      raise 'Error'
    end
  end

  do_parse
end

def next_token
  @q.shift
end

---- footer
if __FILE__ == $0
  parser = JsonParser.new(ARGV[0])

  begin
    p parser.parse
  rescue Racc::ParseError => e
    $stderr.puts e
  end
end