Possible syntax for parser combinators in Ruby.

grammar.rb

module SXP
  module Grammar extend Parallax::Grammar
    LPAREN  = char('(')
    RPAREN  = char(')')
    INTEGER = term(/^([+-]?\d+)/)  { |s| s.to_i }
    STRING  = term(/^"([^"]+)"/)   { |s| s }
    SYMBOL  = term(/^([\w\d_-]+)/) { |s| s.to_sym }

    Atom    = INTEGER | STRING | SYMBOL
    List    = LPAREN << +Atom >> RPAREN
    Nil     = LPAREN << RPAREN
    Expr    = Atom | Nil | List
    Top     = Expr*n
  end
end

input = %q((foo (bar 1 2 3) "Hello, world!"))

p Parallax::Parser.new(SXP::Grammar).parse(input)