regress 0.2.0

A regular expression engine targeting EcmaScript syntax

regress - REGex in Rust with EcmaScript Syntax

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.


Add this to your Cargo.toml:

regress = "0.1"

Fun Tools

The tester 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

Want to contribute?

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?

Currently Missing Syntax

  • Named capture groups like (?<count>\d+)
  • Unicode property escapes like \p{Sc}

Currently Missing Features

  • 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

Missing Performance Optimizations

  • 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.
  • Case-insensitive literals should compute the "preimage" (i.e. characters which fold together) instead of folding. In particular if the preimage is only that character this will accelerate matching.
  • Pure literal searches should use Boyer-Moore or etc.
  • The fold table should be bitpacked more tightly, e.g. using 24 bits for a code point.
  • There are lots of vectorization opportunities.