gistfile1.rb

#!/usr/bin/env ruby

require 'rubygems'
require 'activesupport'

class StringReplacer
  NEWLINE = "AijQA6tD1wkWqgvLzXD"
  START_MARKER = '# START StringReplacer %s -- DO NOT MODIFY'
  END_MARKER = "# END StringReplacer %s -- DO NOT MODIFY#{NEWLINE}"
  
  attr_accessor :path
  def initialize(path)
    @path = path
  end
  
  def replace!(replacement, id = 1)
    new_path = "#{path}.new"
    backup_path = "#{path}.bak"
    current_start_marker = START_MARKER % id.to_s
    current_end_marker = END_MARKER % id.to_s
    replacement_with_markers = current_start_marker + NEWLINE + replacement + NEWLINE + current_end_marker
    text = IO.read(path).gsub("\n", NEWLINE)
    if text.include? current_start_marker
      text.gsub! /#{Regexp.escape current_start_marker}.*#{Regexp.escape current_end_marker}/, replacement_with_markers
    else
      text << NEWLINE << replacement_with_markers
    end
    text.gsub! NEWLINE, "\n"
    File.open(new_path, 'w') { |f| f.write text }
    FileUtils.mv path, backup_path
    FileUtils.mv new_path, path
  end
end

r = StringReplacer.new File.expand_path("~/Desktop/foo.txt")
r.replace! "Foo Bar"

puts IO.read("~/Desktop/foo.txt")