What's the main reason ripgrep is fast?

According to the README, ripgrep is fast for several interconnected reasons [README.md:L183-L210]:

1. Rust's regex engine: It's built on top of Rust's regex engine, which uses finite automata, SIMD, and aggressive literal optimizations to make searching very fast. PCRE2 support can be optionally enabled with the -P/--pcre2 flag.

2. Unicode support without performance loss: Rust's regex library maintains performance with full Unicode support by building UTF-8 decoding directly into its deterministic finite automaton engine.

3. Smart searching strategy: It supports searching with either memory maps (better for single files) or incrementally with an intermediate buffer (better for large directories), and ripgrep automatically chooses the best strategy for you.

4. Efficient ignore pattern matching: It applies your ignore patterns from .gitignore files using a RegexSet, which allows a single file path to be matched against multiple glob patterns simultaneously.

5. Parallel directory traversal: It uses a lock-free parallel recursive directory iterator, courtesy of the crossbeam and ignore crates.

The README notes [L183-L188] that detailed benchmarks with analysis are available on the author's blog, though a single benchmark is never enough to tell the full story.

FOLLOW_UPS:

• How does ripgrep handle PCRE2 regex patterns?
• What automatic filtering does ripgrep apply by default?
• How are .gitignore patterns efficiently matched?