It’s quite literally parsing xml using regular expressions. It’s an excellent case study for why you shouldn’t do that.
Projects didn’t start using Nokogiri for performance. They used it because it’s correct.
It's like a textbook example no? Don't parse non-regular languages with regular expressions.
I was testing o3 recently and it kept changing the library used by a block of code every time it tried to fix an issue in the block that was unrelated to the library used (haven't seen that happen with Sonnet)
Easy to see how issues could creep in because a modification is made that switches to an inferior library/gem that exists in the code base or standard library so still passes tests etc. but doesn't need a Gemfile change