oh no why

regress is a backtracking regular expression engine implemented in Rust, which targets JavaScript regular expression syntax. See the crate documentation for more.

It's fast, Unicode-aware, has few dependencies, and has a big test suite. It makes fewer guarantees than the regex crate but it enables more syntactic features, such as backreferences and lookaround assertions.

The regress-tool binary can be used for some fun.

You can see how things get compiled with the dump-phases cli flag:

> cargo run 'x{3,4}' 'i' --dump-phases

You can run a little benchmark too, for example:

> cargo run --release -- 'abcd' 'i' --bench ~/3200.txt

This was my first Rust program so no doubt there is room for improvement.

There's lots of stuff still missing, maybe you want to contribute?

• An API for replacing a string while substituting in capture groups (e.g. with $1)
• An API for escaping a string to make it a literal
• Implementing std::str::pattern::Pattern

• Anchored matches like ^abc still perform a string search. We should compute whether the whole regex is anchored, and optimize matching if so.
• Non-greedy loops like .*? will eagerly compute their maximum match. This doesn't affect correctness but it does mean they may match more than they should.
• Pure literal searches should use Boyer-Moore or etc.
• There are lots of vectorization opportunities.