How to insert a string into a textfile

Question

I have a configuration file to which I want to add a string, that looks e.g. like that:

line1
line2
line3
line4

The new string should not be appended but written somewhere into the middle of the file. Therefore I am looking for a specific position (or string) in the file and when it has been found, I insert my new string:

file = File.open(path,"r+")
while (!file.eof?)
  line = file.readline
  if (line.downcase.starts_with?("line1"))
    file.write("Some nice little sentence")
  end
end

The problem is that Ruby overwrites the line in that position with the new text, so the result is the following:

line1
Some nice little sentence
line3
line4

What I want is a "real" insertion:

line1
Some nice little sentence
line2
line3
line4

How can this be achieved?

Answer 1

def replace(filepath, regexp, *args, &block)
  content = File.read(filepath).gsub(regexp, *args, &block)
  File.open(filepath, 'wb') { |file| file.write(content) }
end


replace(my_file, /^line2/mi) { |match| "Some nice little sentence"}

line1
Some nice little sentence
line2
line3
line4

and if you want to append to the existing...

replace(my_file, /^line2/mi) do |match| 
  "#{match} Some nice little sentence"
end

line1
line2 Some nice little sentence
line2
line3
line4