replace.rb

  lines = File.readlines('file.rb')

  File.open('file.rb', 'r+') do |f|
    f.each_line do |line|
      if line.start_with?('dog', 'cat')
        line.gsub!(/food/, '')
      end
    end
  end


# example this should become

# cat's are really nice sometimes, we give them good cat food.
# dog's are nice too

# should become

# cat's are really nice sometimes, we give them good cat .
# dog's are nice too