formatted_string.rb

class FormattedString < String
  PATTERN = /:(\w+)/
  DEFER   = lambda { |match| match }

  def initialize str = '', pattern = nil
    super str

    pattern ||= PATTERN
    @compiler = DEFER

    @orig = str
    @gsub = gsub! pattern
  end

  def compiler &block
    @compiler = block if block
    @compiler
  end

  def compiler= block
    @compiler = block || DEFER
  end

  def compile!
    @gsub.each &compiler
  end

  def revert!
    replace @orig
  end
end