module Regexp::Parser

def self.meta(token)
  case token.token
  when :dot
    @node << CharacterType::Any.new(token)
  when :alternation
    unless @node.token == :alternation
      unless @node.last.is_a?(Alternation)
        alt = Alternation.new(token)
        seq = Sequence.new
        while @node.expressions.last
          seq.insert @node.expressions.pop
        end
        alt.alternative(seq)
        @node << alt
        @node = alt
        @node.alternative
      else
        @node = @node.last
        @node.alternative
      end
    else
      @node.alternative
    end
  else
    raise UnknownTokenError.new('Meta', token)
  end
end