re2js is a pure JavaScript port of Google's RE2 regular expression engine, providing linear-time O(n) matching and protection against ReDoS attacks. It implements a full compilation pipeline: parsing RE2 syntax into an AST, simplifying the expression, compiling into a bytecode instruction set (Prog), and executing via multiple engines (DFA, NFA, OnePass, Backtracker, Prefilter). The library exposes a Java-style Matcher API with find, matches, replaceAll, and supports named capture groups, lookbehinds (linear-time), and both JS and Java replacement string dialects. It is distributed as an npm package supporting ESM, CJS, and UMD, with TypeScript declarations, and is tested across Node.js, Bun, Deno, and browsers. The architecture follows Russ Cox's regexp articles and uses sparse sets for efficient NFA simulation. Unicode tables for categories, scripts, and case folding are generated from @unicode/unicode-17.0.0 and decoded lazily.