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Fix onepass DFA panic with zero-repetition capture groups#1

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copilot/fix-search-slots-panics
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Fix onepass DFA panic with zero-repetition capture groups#1
Copilot wants to merge 2 commits intomasterfrom
copilot/fix-search-slots-panics

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Copilot AI commented Jan 27, 2026 •
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search_slots panicked when using regexes with zero-repetition capture groups (e.g., (abc){0}) and switching between regexes with different capture group counts.

let expr = regex_automata::meta::Regex::new(r"(abc)(ABC){0}").unwrap();
let expr2 = regex_automata::meta::Regex::new(r"(abc)(ABC)").unwrap();
let mut slots: Vec<Option<NonMaxUsize>> = vec![None; 6];

expr.search_slots(&input, &mut slots);   // Works
expr2.search_slots(&input, &mut slots);  // Works
expr.search_slots(&input, &mut slots);   // Panicked: range end index 4 out of range for slice of length 2

Changes

  • Cache::setup_search: Clamp explicit_slot_len to actual explicit_slots vector size to prevent out-of-bounds access
  • Slot copying logic: Copy only the minimum available slots from cache instead of assuming size match between cache and caller's array
  • Test coverage: Added regression test for zero-repetition capture groups with varying slot array sizes

Zero-repetition capture groups now correctly populate slots with None values as expected.

Original prompt

This code panics when the documentation states that search_slots should accept any size Vec. Let's make the necessary fixes to the code so it doesn't panic and the documentation becomes true. Zero repetition on a capture group means the capture group should be None...

use regex_automata::util::primitives::NonMaxUsize; use regex_automata::Anchored; use regex_automata::Input; fn main() { let expr = regex_automata::meta::Regex::new(r"(abc)(ABC){0}").unwrap(); let s = "abcABC"; dbg!(expr.is_match(s)); let mut inner_slots: Vec<Option<NonMaxUsize>> = Vec::new(); let input = Input::new(s).span(0..s.len()).anchored(Anchored::Yes); inner_slots.resize(4, None); dbg!(expr.search_slots(&input, &mut inner_slots)); dbg!(&inner_slots); inner_slots.resize(6, None); let expr2 = regex_automata::meta::Regex::new(r"(abc)(ABC)").unwrap(); dbg!(expr2.search_slots(&input, &mut inner_slots)); dbg!(&inner_slots); dbg!(expr.search_slots(&input, &mut inner_slots)); dbg!(&inner_slots); }

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Copilot AI changed the title [WIP] Fix panic issue in search_slots for any size Vec Fix onepass DFA panic with zero-repetition capture groups Jan 27, 2026
Copilot AI requested a review from keith-hall January 27, 2026 02:37
@keith-hall keith-hall marked this pull request as ready for review January 27, 2026 03:11
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