f = IO.open("file.txt").split(/\n/)
for i in (0..f.length)
	if f[i].to_s.match(/regexp_a/) then
		f[i] = f[i].gsub(/regexp_b/,/regexp_c/)
	end
end

output = File.open("file.txt","w")
f.each do |lines|
  output.write(lines + "\n")
end
output.close