dfa: fix bug in how the reverse DFA is called

In turns out that in *some* calls to Fsm::reverse, we were passing an
incorrect start offset. Namely, the haystack we pass is sub-sliced at
`&text[start..]`, but in some places, we were passing `text.len()` as
the start offset of the reverse search. But of course, it should be
`text.len() - start`. This was indeed the case in most places, but it
looks like it needed to be corrected in two additional places.

I've also added this test to regex-automata's set of regression tests
and can confirm that it doesn't happen there. (regex-automata is far
more principled about handling offsets like this.)

Fixes #969

src/exec.rs

@@ -459,7 +459,7 @@ impl<'c> RegularExpression for ExecNoSync<'c> {
                     self.cache.value(),
                     true,
                     &text[start..],
-                    text.len(),
+                    text.len() - start,
                 ) {
                     dfa::Result::Match(_) => Some(text.len()),
                     dfa::Result::NoMatch(_) => None,
@@ -511,7 +511,7 @@ impl<'c> RegularExpression for ExecNoSync<'c> {
                     self.cache.value(),
                     true,
                     &text[start..],
-                    text.len(),
+                    text.len() - start,
                 ) {
                     dfa::Result::Match(_) => true,
                     dfa::Result::NoMatch(_) => false,

tests/test_default.rs

@@ -220,3 +220,13 @@ fn empty_alt_regex_fails() {
     let result = Regex::new(r"(?:|){4294967295}");
     assert!(result.is_err());
 }
+
+// Regression test for: https://github.com/rust-lang/regex/issues/969
+#[test]
+fn regression_i969() {
+    use regex::Regex;
+
+    let re = Regex::new(r"c.*d\z").unwrap();
+    assert_eq!(Some(6), re.shortest_match_at("ababcd", 4));
+    assert_eq!(Some(6), re.find_at("ababcd", 4).map(|m| m.end()));
+}