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.

Syntax

regress targets the EcmaScript 2018 standard regexp syntax, including support for gnarly cases such as variable-width lookbehind assertions containing capture groups. Note that subsequent standard have mostly left regexp syntax untouched, with a few exceptions such as the 'v' flag, which is supported.

That darn 'u' flag

You will be sad to learn that JavaScript does not use UTF-8.

Originally JavaScript was designed for UCS-2, with 16-bit characters. Later UCS-2 was supplanted with UTF-16; however this was not automatic for regexps, but instead required opt-in for each regexp, with the 'u' flag. For example, the grinning face emoji U+1F600 is represented in a JavaScript string as a surrogate pair U+D83D and U+DE00. In a regexp without the 'u' flag, these surrogate pairs are matched as distinct characters:

const s = "😀";
const re = /./;
const m = s.match(re);
console.log(m); // returns \uD83D, high surrogate

This behavior is almost never desired but is required by the ES spec. It's also super-awkward to express in Rust, which uses UTF-8 extensively. See below for how regress handles this.

The 'u' flag doesn't just modify character sets; it also affects other behaviors such as how case-insensitive matching works, and (most bizarrely) the behavior of backreferences like \2. For example, in non-Unicode mode, \2 is a backreference if there are at least two capture groups; otherwise it is an octal escape (!).

regress mostly ignores the 'u' flag for character decoding - that's instead given by the call site (see below). regress attempts to implement the other behaviors faithfully.

Character sets

tl;dr use UTF-8 (or ASCII) input and the 'u' flag, unless you are implementing a JavaScript engine and care about strict conformance.

To support JavaScript pre-Unicode semantics, regress supports multiple input forms on the Regex object. These are:

UTF-8. The default (unsuffixed) form. Input is &str. This always decodes whole characters from the input string.
ASCII. Use the *_ascii family of functions on Regex if you know your input is ASCII. Input is still &str.
UTF-16. Use the *_utf16 family of functions. Input is &[u16]. Characters are always decoded as UTF-16.
UCS-2. OG JavaScript. Use *_ucs2 functions. Input is &[u16]. Surrogate pairs are split freely. Only use if you want to implement strict JS semantics.

Both the UTF-16 and UCS-2 forms require the Rust feature 'utf16' to be enabled. It is off by default.

Fun Tools

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' --flags 'i' --bench ~/3200.txt

OxideJS fork note

This directory is a vendored copy of upstream regress 0.11.1 (source at https://github.com/ridiculousfish/regress, crate files from the crates.io registry, licenses preserved in LICENSE-APACHE / LICENSE-MIT), consumed via the workspace root [patch.crates-io] instead of the registry copy.

Deviations from upstream (the remaining vendored files are bit-for-bit upstream 0.11.1, other than this note; upstream repo-only files — tests/, .github/, Cargo.toml.orig, perf.md, regress_dfa_plan.txt, rustfmt.toml — are not vendored):

src/unicodetables.rs gains the Unknown (Zzzz) script value — script and script-extensions interval sets covering the unassigned code points — plus its name/alias arm in the script-value parser. Purely additive: no upstream line is modified. Upstream 0.11.1 lacks the value, which rejects spec-legal patterns such as \p{Script=Unknown}; upstream 0.12.0 has it, but the 0.11.1 → 0.12.0 delta changes broader parser/matcher behavior this project does not want to absorb.
The byte-literal optimization gate moves from compile-time cfg (the utf16 feature) to a per-compilation runtime flag, so enabling utf16 no longer disables the byte passes on the str path:
- src/optimizer.rs — the literal-byte pass is gated by the new optimize_with_byte_literals(ire, byte_literals) entry; optimize keeps the previous behavior as its wrapper.
- src/classicalbacktrack.rs — the start-predicate dispatch in the matcher's next-match loop switches on input encoding at runtime: byte inputs (UTF-8/ASCII) keep the anchored/byte-positioning fast paths, unit inputs (UTF-16/UCS-2) fall back to position-by-position tries.
- src/api.rs — Regex retains the pattern's source code-point sequence plus a lazily materialized unit-side IR (OnceLock), re-parsed with the byte-literal pass disabled on first unit-input entry; the unit entry points compile through that IR.
Cargo.toml — the upstream workspace-members table (for the repo's regress-tool/gen-unicode subdirectories, not vendored here) is replaced by a standalone empty [workspace] table, and the utf16 feature comment is refreshed to describe the runtime gate.