reaction: A daemon that scans program outputs for repeated patterns, and takes action

Before you tag it with vibecoding (as I almost did) make sure to look at that AGENTS.md. I won't spoil it.

I too was introduced to reaction recently by hearing about the FSF using it, they've also now written a blog post about that deployment.

I've been making heavy use of fail2ban with recent scraping but started to hit some scaling issues, so am interested in trying it out. Also a little relieved something like this already exists.

I find it a bit odd that it doesn't use the more conventional regex capture group (...) and $N substitution format in its syntax, but very well designed nonetheless.