C-loudbleed, a C pointer arithmetic bug in Cloudflare

https://blog.cloudflare.com/incident-report-on-memory-leak-caused-by-cloudflare-parser-bug/

>>22
Actually, ragel is a state machine compiler, and this brings me to, I think, an interesting point.

According to the ragel homepage,

Ragel compiles executable finite state machines from regular languages

HTML is not a regular language, so how could they have written a non-buggy parser in the first place?