This is nifty, thanks for sharing! I had no idea that sed had a hold buffer, and it's very cool that you can swap it in and out within the sed command like that. It's funny, because I went essentially the opposite way that you did: I used to know sed and awk basics, but then I properly learned awk. Since then my sed has atrophied a bit, and I still only know the basics. I'll have to run through that tutorial you linked