RE#

A high-performance, automata-based regex engine with first-class support for intersection (&), complement (~). Non-backtracking with linear-time matching. Shines on complex patterns (large alternations, lookarounds, boolean combinations) where traditional engines degrade or fall back to slower paths.

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

Quick start

cargo add resharp

// 8+ alphanumeric & contains digit & contains uppercase
let re = resharp::Regex::new(r"[A-Za-z0-9]{8,}&_*[0-9]_*&_*[A-Z]_*").unwrap();

let found = re.is_match(b"Hunter2024").unwrap();
let matches = re.find_all(b"try Hunter2024 or password1").unwrap();

When to use RE# over regex

RE# operates on &[u8] / UTF-8 and aims to match regex crate throughput on standard patterns. Reach for RE# when you need:

• intersection (&), complement (~), or lookarounds
• large alternations with high throughput (at the cost of memory)
• fail-loud behavior: capacity / lookahead overflow returns Err instead of silently degrading

RE# is designed around is_match and find_all. It doesn't provide find or captures, but for simple cases you can often substitute find_anchored, or emulate a capture group with lookarounds. For example, a(b)c becomes (?<=a)b(?=c). For anything more involved, use the regex crate instead.

Syntax extensions

RE# supports standard regex syntax plus three extensions: _ (any byte), & (intersection), and ~(...) (complement). _* 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.

Differences from PCRE / regex

• Leftmost-longest, not leftmost-greedy. y|yes on "yes" matches yes. Branch order is irrelevant.
• Multiline on by default. ^/$ match start/end of line; disable with (?-m). \A/\z always anchor to input.
• \w defaults to 2-byte UTF-8. See UnicodeMode.

Lazy quantifiers (*?, +?, ...) are parse errors; rewrite with complement when possible: <div>.*?</div> -> <div>~(_*</div>_*)</div>. See syntax.md for the rest.

Configuration

let opts = resharp::RegexOptions {
    max_dfa_capacity: 65535,    // max automata states (default: u16::MAX)
    lookahead_context_max: 800, // max lookahead context distance (default: 800)
    hardened: false,            // linear find_all worst-case (~10x slower average)
    unicode: resharp::UnicodeMode::Default, // Ascii | Default | Full | Javascript
    ..Default::default()
};
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, WASM). Run with cargo bench -- 'readme/' --list.

AMD Ryzen 7 5800X (105W TDP)

Notes:

• Sparse matches (~15 in 900KB): roughly tied. Everyone spends most of their time scanning past non-matching bytes in a tiny purpose-built automaton, the DFA size does not matter at all!
• Dense matches (~2678 in 944KB): the other engines degrade sharply because they must construct more of the lazy state machine. RE# holds at 535 MiB/s vs 58 MiB/s for regex on x86.
• (?i) case-insensitive: regex falls back to a slower engine and drops to 0.01 MiB/s. RE# folds case into the DFA and keeps full speed.
• Lookarounds: RE# compiles them directly into the automaton. regex doesn't support them (except anchors); fancy-regex backtracks, which can be orders of magnitude slower.
• See also the rebar comparison: not apples-to-apples (different match semantics, short repeated inputs), but a useful ballpark.
• Found a pattern where RE# is >5x slower than regex or fancy-regex? Please open an issue.