Enum ScannerEngine 

#[non_exhaustive]
pub enum ScannerEngine {
    Meta,
    Lazy,
    Dense,
}

Which concrete regex-automata engine drives the fast (look-around-free) partition. Provisional knob (see LarkOptions::scanner_engine); the merge semantics (MatchKind::LeftmostFirst, anchored-at-pos span search) and the resulting token stream are byte-identical across all three — only the build-time / warm-scan / resident-memory trade differs.

Variants (Non-exhaustive)

Non-exhaustive enums could have additional variants added in future. Therefore, when matching against variants of non-exhaustive enums, an extra wildcard arm must be added to account for any future variants.

Meta

regex-automata meta Regex — internally a lazy/hybrid DFA with an optimizer-picked engine and a built-in cache pool. Cheap construction, WASM-friendly. Opt-in fallback since the tail-risk bench flipped the default to Lazy: on every measured grammar including a deliberately cache-thrashing adversarial one, Lazy was faster, and Meta degraded worse under the stress that was supposed to be its advantage.

Lazy

regex-automata hybrid (lazy) DFA driven directly, with a per-scanner Cache pool (8 MiB cap) for &self access. Skips the meta optimizer’s per-search engine choice; construction stays cheap (the DFA is built lazily during the scan) at the cost of a cache-fill transient. The shipping default — 8-25 % faster warm than Meta with neutral-to-lower resident memory.

Dense

regex-automata dense DFA (fully materialized at build time). Fastest warm scan on small alphabets, but determinization over \w-class terminals blows up construction time and resident memory on large grammars (python).

Implementations

impl ScannerEngine

pub fn from_env() -> ScannerEngine

The engine selected by the HL_ENGINE environment hook (meta|lazy| dense), or the enum default (ScannerEngine::Lazy) when unset/unrecognised. Public so out-of-façade harnesses (the conformance corpus A/B, benches) exercise the same engine the process default resolves to. HL_ENGINE is sampled once per process and cached, so a later env change is ignored; pass an explicit engine to Scanner::new_with_engine for a deterministic choice.

Trait Implementations

impl Clone for ScannerEngine

fn clone(&self) -> ScannerEngine

Returns a duplicate of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl Copy for ScannerEngine

impl Debug for ScannerEngine

fn fmt(&self, f: &mut Formatter<'_>) -> Result

Formats the value using the given formatter.

impl Default for ScannerEngine

fn default() -> ScannerEngine

Returns the “default value” for a type.

impl Eq for ScannerEngine

impl PartialEq for ScannerEngine

fn eq(&self, other: &ScannerEngine) -> bool

Equality operator ==.

fn ne(&self, other: &Rhs) -> bool

Inequality operator !=.

impl StructuralPartialEq for ScannerEngine