A ruby snippet found on SO for doing a find and replace in a text file. Slightly modified for a recent use case.

file_find_replace.rb

# Inspired this question/answer (up-vote him!):
# http://stackoverflow.com/a/1274631/1598965

file_names = ['foo.txt', 'bar.txt']

file_names.each do |file_name|
  text = File.read(file_name).gsub(/search_regexp/, "replace string")
  # This will overwrite the existing file with the above gsub output!
  File.open(file_name, "w+") {|file| file.puts text}
end