automata: fix bug where reverse NFA lacked an unanchored prefix

Previously, when compiling a Thompson NFA, we were omitting an unanchored prefix when the HIR contained a `^` in its prefix. We did this because unanchored prefix in that case would never match because of the requirement imposed by `^`. The problem with that is it's incorrect when compiling a reverse automaton. For example, in the case of building a reverse NFA for `^Qu`, we should sitll include an unanchored prefix because the `^` in that case has no conflict with it. It would be like if we omitted an unanchored prefix for `Qu$` in a forward NFA, which is obviously wrong. The fix here is pretty simple: in the reverse case, check for `$` in the suffix of the HIR rather than a `^` in the prefix. Fixes #1169

Author: BurntSushi

regex-automata/src/nfa/thompson/compiler.rs

@@ -961,10 +961,12 @@ impl Compiler {
  // for all matches. When an unanchored prefix is not added, then the
  // NFA's anchored and unanchored start states are equivalent.
  let all_anchored = exprs.iter().all(|e| {
- e.borrow()
- .properties()
- .look_set_prefix()
- .contains(hir::Look::Start)
+ let props = e.borrow().properties();
+ if self.config.get_reverse() {
+ props.look_set_suffix().contains(hir::Look::End)
+ } else {
+ props.look_set_prefix().contains(hir::Look::Start)
+ }
  });
  let anchored = !self.config.get_unanchored_prefix() || all_anchored;
  let unanchored_prefix = if anchored {
@@ -1928,6 +1930,11 @@ mod tests {
  State::Sparse(SparseTransitions { transitions })
  }
 
+ fn s_look(look: Look, next: usize) -> State {
+ let next = sid(next);
+ State::Look { look, next }
+ }
+
  fn s_bin_union(alt1: usize, alt2: usize) -> State {
  State::BinaryUnion { alt1: sid(alt1), alt2: sid(alt2) }
  }
@@ -1978,6 +1985,80 @@ mod tests {
  );
  }
 
+ #[test]
+ fn compile_no_unanchored_prefix_with_start_anchor() {
+ let nfa = NFA::compiler()
+ .configure(NFA::config().which_captures(WhichCaptures::None))
+ .build(r"^a")
+ .unwrap();
+ assert_eq!(
+ nfa.states(),
+ &[s_look(Look::Start, 1), s_byte(b'a', 2), s_match(0)]
+ );
+ }
+
+ #[test]
+ fn compile_yes_unanchored_prefix_with_end_anchor() {
+ let nfa = NFA::compiler()
+ .configure(NFA::config().which_captures(WhichCaptures::None))
+ .build(r"a$")
+ .unwrap();
+ assert_eq!(
+ nfa.states(),
+ &[
+ s_bin_union(2, 1),
+ s_range(0, 255, 0),
+ s_byte(b'a', 3),
+ s_look(Look::End, 4),
+ s_match(0),
+ ]
+ );
+ }
+
+ #[test]
+ fn compile_yes_reverse_unanchored_prefix_with_start_anchor() {
+ let nfa = NFA::compiler()
+ .configure(
+ NFA::config()
+ .reverse(true)
+ .which_captures(WhichCaptures::None),
+ )
+ .build(r"^a")
+ .unwrap();
+ assert_eq!(
+ nfa.states(),
+ &[
+ s_bin_union(2, 1),
+ s_range(0, 255, 0),
+ s_byte(b'a', 3),
+ // Anchors get flipped in a reverse automaton.
+ s_look(Look::End, 4),
+ s_match(0),
+ ],
+ );
+ }
+
+ #[test]
+ fn compile_no_reverse_unanchored_prefix_with_end_anchor() {
+ let nfa = NFA::compiler()
+ .configure(
+ NFA::config()
+ .reverse(true)
+ .which_captures(WhichCaptures::None),
+ )
+ .build(r"a$")
+ .unwrap();
+ assert_eq!(
+ nfa.states(),
+ &[
+ // Anchors get flipped in a reverse automaton.
+ s_look(Look::Start, 1),
+ s_byte(b'a', 2),
+ s_match(0),
+ ],
+ );
+ }
+
  #[test]
  fn compile_empty() {
  assert_eq!(build("").states(), &[s_match(0),]);