Class UserRegex

packages/just-bash/src/regex/user-regex.ts:73–499

 * Uses RE2 for linear-time matching, providing ReDoS protection.
 */
export class UserRegex implements RegexLike {
  private readonly _re2: RE2JS;
  private readonly _pattern: string;
  private readonly _flags: string;
  private readonly _global: boolean;
  private readonly _ignoreCase: boolean;
  private readonly _multiline: boolean;
  private _lastIndex = 0;
  // Cache native RegExp for compatibility - created lazily
  private _nativeRegex: RegExp | null = null;
  // Reusable RE2 Matcher to avoid per-call allocation in tight grep loops.
  // Matcher allocation dominates regex.test/exec cost when called once per line
  // across thousands of lines. We mutate charSequence in-place (not resetMatcherInput,
  // which is broken in re2js 1.2.1 — see acquireMatcher).
  private _matcher: ReturnType<RE2JS["matcher"]> | null = null;
  private _matcherInput: string | null = null;

  private acquireMatcher(input: string): ReturnType<RE2JS["matcher"]> {
    if (this._matcher === null) {
      this._matcher = this._re2.matcher(input);
      this._matcherInput = input;
      return this._matcher;
    }
    if (this._matcherInput !== input) {
      // Swap the cached Utf16MatcherInput's charSequence in-place to avoid
      // allocating a new Matcher per call. RE2JS's resetMatcherInput is not
      // safe with raw strings (the constructor wraps strings via
      // MatcherInput.utf16, but resetMatcherInput assigns its argument
      // directly and then calls .length() as a method, which throws on a
      // raw string). MatcherInput is not exported, so we mutate the existing
      // wrapper's charSequence field — Matcher.reset() reads matcherInput.length()
      // afterwards, so the new length is picked up correctly.
      // biome-ignore lint/suspicious/noExplicitAny: reaching into re2js internals
      (this._matcher as any).matcherInput.charSequence = input;
      this._matcherInput = input;
    }
    this._matcher.reset();
    return this._matcher;
  }

  constructor(pattern: string, flags = "") {
    this._pattern = pattern;
    this._flags = flags;
    this._global = flags.includes("g");
    this._ignoreCase = flags.includes("i");
    this._multiline = flags.includes("m");

    try {
      const translatedPattern = translatePattern(pattern);
      const re2Flags = convertFlags(flags);
      this._re2 = RE2JS.compile(translatedPattern, re2Flags);
    } catch (e) {
      if (e instanceof RE2JSSyntaxException) {
        // Provide helpful error messages for unsupported RE2 features
        const msg = e.message || "";
        let explanation = "";

        if (