gistfile1.html

<p>
  foo
</p>
<textarea>
hello world</textarea>
<p>
  hello world
</p>
<textarea>hello!</textarea>

temp.rb

text = File.read("temp.html")

def dumb_gsub(text)
  text.gsub(/^(?!\s+$)/m, "..")
end

def textarea_gsub(text)
  in_preserve = false
  text.split("\n").map do |line|
    line.gsub!(/^(?!\s+$)/m, "..") unless in_preserve
    in_preserve = true  if line =~ /<(textarea|pre)/
    in_preserve = false if line =~ /<\/(textarea|pre)/
    line
  end.join("\n")
end

puts textarea_gsub(text)