GHSA-gg6h-894w-4m9c - RE2JS: Quadratic-time denial of service in re2js lookbehind matching

### Summary Using the opt-in `LOOKBEHINDS` flag together with any repeated-matching API (`matcher().find()` in a loop, `matchAll()`, `split()`, `matcher().replaceAll()`) causes quadratic-time (Θ(n²)) matching, contradicting the library's own "linear time matching" guarantee. A single call with a 16,000-character input takes **36.7–38.7 seconds of CPU**. ### Description #### Root cause In `build/index.cjs`, `Machine.match`: ```js let currentPos = this.prog.numLb > 0 ? 0 : pos; // ~line 1907 ... if (this.ncap === 0 && this.matched) break; // ~line 1953 ``` Any pattern containing a lookbehind sets `prog.numLb > 0`, which forces the scan to **always restart at offset 0**, regardless of the requested start position — the lookbehind automaton is re-seeded and the whole input is re-walked on every single `find()` call. The early-exit optimization that would otherwise stop at the first match only fires when `ncap === 0`, but every `Matcher`-based API (`find`/`matchAll`/`split`/`replaceAll`) always requests `ncap = 2` (one capture group slot) **even for a captureless lookbehind pattern** — so the early exit never triggers through those APIs, and the scan continues to the end of the input on every find. A find-loop over *k* matches in an *n*-character string therefore costs *k* × Θ(*n*); for dense matches *k* ∝ *n*, giving **Θ(n²)** total. #### Isolating control Identical find-loop, identical input sizes, same 2-character literal pattern *shape* — but **without** the `LOOKBEHINDS` flag: | n | with `LOOKBEHINDS` (lookbehind pattern) | without (plain literal) | native JS `RegExp` lookbehind | |---|---|---|---| | 2,000 | 204–228 ms | 3 ms | 0 ms | | 4,000 | 877–899 ms | 4 ms | 0 ms | | 8,000 | 5,746–6,599 ms | 7 ms | 1 ms | | 16,000 | 36,667–38,732 ms | 6 ms | 1 ms | This isolates the lookbehind machinery itself — not pattern length, not match density in the abstract — as the exact and sole cause. #### Impact - `re2js`'s entire value proposition is being an