'Overlapping' empty match suppression algorithm hides real non-empty matches

[orlp]

What version of regex are you using?

regex == 1.12.3

Describe the bug at a high level.

regex suppresses empty matches in find_iter() that start at the end of the previous match because it considers them overlapping with the previous match. I disagree with this interpretation, but let's assume that's the case.

regex however still remembers the suppressed empty match and uses it to suppress otherwise completely non-overlapping non-empty matches as well.

What are the steps to reproduce the behavior?

let pattern = "a?|b";
let haystack = "aba";
dbg!(regex::Regex::new(pattern).unwrap().find_iter(haystack).collect::<Vec<_>>());

What is the actual behavior?

We see the following matches:

[
    Match {
        start: 0,
        end: 1,
        string: "a",
    },
    Match {
        start: 2,
        end: 3,
        string: "a",
    },
]

What is the expected behavior?

I would expect a match of "" or "b" (or both) at start: 1, but not that there is zero matches starting at index 1.

[orlp]

To further highlight the silliness of the current behavior, keep the pattern as a?|b and change the haystack to abbba. The outer two as match, of course, but for the inner three bs we only get two matches (with empty strings).