class Parser
  def initialize(pattern)
    @pattern = pattern
  end

  def parse(stream)
    a = stream.chars
    b = a.shift
    if b =~ @pattern
      return [b, parse(a.join)]
    end
  end
end

parser = ->(str) {
  parser.call [str[1].flatten.chars.first, str[1].flatten.chars.drop(1)]
}

p parser.call ["", "Hello, World"]