Details of the Cloudflare outage on July 2, 2019

One thing that was interesting to me:

The outage was caused by a regex that ended up doing a lot of backtracking, which caused PCRE, the regex engine, to essentially handle a runaway expression.

This reminded me of a HN post from a couple months back by the author of Google Code Search, and how it worked: https://swtch.com/~rsc/regexp/regexp4.html . Interestingly, he wrote his own regex engine, RE2, specifically because PCRE and others did not use real automata and he needed a way to do arbitrary regex search safely.

The problem is that a deterministic regex engine (deterministic finite automata or DFA) is strictly less powerful than a non-deterministic one (NFA). DFA's can't backtrack, for example. In addition, DFA's can be quite a bit slower for certain inputs and matches.