Future: Integrate RE2 as preferred regex engine (with Boost and std::regex fallbacks)

[BrunoO]

Goal

Add RE2 as the preferred regex engine when available, with fallbacks:

1. RE2 (when available and pattern-compatible)
2. Boost.Regex (when available)
3. std::regex (final fallback)

Expected benefits of RE2

• Performance – Typically 5–10× faster than std::regex for complex patterns and long text (DFA/NFA hybrid, no backtracking).
• Predictable complexity – Linear time O(n) in input size; no exponential blow-up on pathological patterns.
• ReDoS mitigation – Avoids catastrophic backtracking, reducing risk of regex denial-of-service from user or API-supplied patterns.
• Thread safety – RE2 objects are safe to share across threads; no extra locking in the regex layer.
• Bounded memory – Configurable limits; no unbounded memory growth during matching.
• Production use – Widely used in production; BSD-3-Clause license.

Patterns that need lookahead/lookbehind or backreferences will continue to use Boost or std::regex via the fallback path.

Approach (Option B – package managers)

• macOS: Homebrew (brew install re2)
• Linux: vcpkg (vcpkg install re2) alongside existing Boost
• Windows: initially no RE2; optional vcpkg later for parity

Implementation order

Benchmark strengthening (before RE2)

1. Regex microbenchmark – New regex_benchmark executable to measure compile + match time for literal, simple, and complex patterns on filename/content corpora. Establish baseline with current std/Boost implementation.
2. SearchBenchmark extensions – Add --regex-engine=auto|re2|boost|std and regex-heavy reference configs to measure end-to-end impact.

RE2 integration

3. Build plumbing – CMake detection (find_package(re2 CONFIG QUIET)), HAVE_RE2 / RE2_REGEX_AVAILABLE, CI steps to install RE2 on macOS and Linux. No runtime behavior change.
4. Unified regex wrapper – Single API with engine priority (RE2 → Boost → std), pattern checks for RE2-unsupported features (lookahead/lookbehind, backrefs), caching.
5. Tests & benchmarks – Unit tests for selection/fallback; re-run regex_benchmark and search_benchmark to quantify improvement.
6. Windows RE2 (optional) – vcpkg on Windows, PGO rules for RE2 target.
7. Documentation – Engine order, CMake options, pattern limitations, how to run benchmarks.

References

• RE2 feasibility and pattern fallbacks: internal-docs/archive/RE2_FEASIBILITY_STUDY.md (in main repo)
• Implementation phases and benchmark plan: internal-docs/plans/2026-03-15_RE2_AND_BENCHMARK_PHASES.md (in main repo)

This issue is a placeholder for tracking the above work; no code changes required until implementation starts.