if you run this algorithm left to right, it will mark the position of every valid end of a match. if you run it right to left, it will mark the position of every valid start of a match. and if you run it in both directions, you can combine the results to get the leftmost-longest match, which is what you intuitively expect when you think of regex matching. this is a bit more work to set up, but it doesn’t change the core algorithm at all.
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[Text] The Onion Architecture, Jeffrey Palermo https://jeffreypalermo.com/2008/07/the-onion-architecture-part-1/
almost every regex engine today descends from one of two approaches: Thompson’s NFA construction (1968) or backtracking (1994). Thompson-style engines (grep, RE2, Rust’s regex) give you linear-time guarantees but only support the “standard” fragment - | and *. backtracking engines (the rest, 95% chance the one you’re using) give you a mix of advanced features like backreferences, lookarounds.., but are unreliable, and can blow up to exponential time on adversarial inputs, which is a real security concern known as ReDoS. to be more precise, this exponential behavior is not the only problem with backtracking engines - they also handle the OR (|) operator much slower, but let’s try to start with the big picture.