RE#

A high-performance, automata-based regex engine with first-class support for intersection and complement operations. RE#'s main strength is complex patterns - large lists of alternatives, lookarounds, and boolean combinations - where traditional engines degrade or fall back to slower paths.

RE# compiles patterns into deterministic automata. All matching is non-backtracking with guaranteed linear-time execution. RE# extends standard regex syntax with intersection (&), complement (~), and a universal wildcard (_), enabling patterns that are impossible or impractical to express with standard regex.

paper | blog post | syntax docs | dotnet version and web playground

Install

cargo add resharp

Usage

let re = resharp::Regex::new(r".*cat.*&.*dog.*&.{8,15}").unwrap();

let matches = re.find_all(b"the cat and the dog");
let found = re.is_match(b"the cat and the dog");

Syntax extensions

RE# supports standard regex syntax plus three extensions: _ (universal wildcard), & (intersection), and ~(...) (complement). _ matches any character including newlines, so _* means "any string".

_*              any string
a_*             any string that starts with 'a'
_*a             any string that ends with 'a'
_*a_*           any string that contains 'a'
~(_*a_*)        any string that does NOT contain 'a'
(_*a_*)&~(_*b_*)  contains 'a' AND does not contain 'b'
(?<=b)_*&_*(?=a)  preceded by 'b' AND followed by 'a'

You combine all of these with & to get more complex patterns. RE# also supports lookarounds ((?=...), (?<=...), (?!...), (?<!...)), compiled directly into the automaton with no backtracking.

NOTE: RE# is not compatible with some regex crate features, eg. lazy quantifiers (.*?). See the full syntax reference for details.

Differences from resharp-dotnet and rust regex

Written from scratch in rust - operates on &[u8] / UTF-8 rather than UTF-16, uses regex-syntax as a parser base. When to use this over the regex crate:

• intersection, complement, or lookarounds
• large alternatives with high performance (at the expense of memory)
• leftmost longest matches rather than leftmost first
• find_anchored and find_all (no find or captures)

Matching returns Result<Vec<Match>, Error> - capacity or lookahead overflow will fail outright rather than silently degrade. EngineOptions controls precompilation threshold, capacity, and lookahead context:

let opts = resharp::EngineOptions {
    dfa_threshold: 100,           // eagerly compile up to N states
    max_dfa_capacity: 65535,       // max automata states (default: u16::MAX)
    lookahead_context_max: 800,    // max lookahead context distance (default: 800)
};
let re = resharp::Regex::with_options(r"pattern", opts).unwrap();

Benchmarks

Throughput comparison with regex and fancy-regex, compiled with --release. Compile time is excluded; only matching is measured. Uses SIMD intrinsics (AVX2, NEON) with possibly more backends in the near future. Run with cargo bench -- 'readme/' --list.

AMD Ryzen 7 5800X (105W TDP)

Rockchip RK3588 ARM (5-10W TDP)

Notes on the results:

• The first dictionary row is roughly tied - the prose haystack only contains ~15 matches, so the lazy DFA barely explores any states. RE#'s advantage is that its full DFA is smaller, but this isn't visible when most states are never materialized.
• On longer inputs or denser matches, the other engines will degrade - take lazy-dfa benchmarks with a grain of salt, you will not be matching the exact same string over and over in the real world. The seeded dictionary row confirms this: with ~2678 matches, RE# holds at 449 MiB/s vs 58 MiB/s for regex on x86, and 229 MiB/s vs 25 MiB/s on ARM.
• The (?i) row shows what happens when the pattern forces regex to fall back from its DFA to an NFA: throughput drops to 0.03 MiB/s (x86) / 0.01 MiB/s (ARM). RE# handles case folding in the DFA and maintains full speed. You can increase regex's DFA threshold to avoid this fallback, but only up to a point.
• RE# compiles lookarounds directly into the automaton - no back-and-forth between forward and backward passes. regex doesn't support lookarounds except for anchors; fancy-regex handles them via backtracking, which is occasionally much slower.
• The same patterns that win on x86 also win on ARM - the full DFA approach scales down well.
• If you encounter a bug or a pattern where RE# is >5x slower than regex or fancy-regex, please open an issue - it would help improve the library. Note that regex returns leftmost-first matches while RE# returns leftmost-longest, so match results may differ. The performance profile also differs - RE# works right to left while regex works left to right.

Crate structure

And most importantly, have fun! :)