RegexSet and Regex give different results for the same pattern in 1.9

[jorendorff]

What version of regex are you using?

1.9.3. The issue is present in regex 1.9.0 and later.

Describe the bug at a high level.

RegexSet::new([r"(?m)^ *v [0-9]"]).unwrap().is_match("v 0") incorrectly returns false in version 1.9.0 and later.

It returns true in 1.8.4.

It returns true if I use a Regex instead of a RegexSet.

What are the steps to reproduce the behavior?

fn main() {
    let pattern = r"(?m)^ *v [0-9]";
    let text = "v 0";

    let re = regex::Regex::new(pattern).unwrap();
    println!("re is: {re:?}");
    println!("{}", re.is_match(text)); // true (correct)

    let rs = regex::RegexSet::new([pattern]).unwrap();
    println!("rs is: {rs:?}");
    println!("{}", rs.is_match(text)); // false (incorrect)
}

(playground link)

What is the actual behavior?

re is: Regex("(?m)^ *v [0-9]")
true
rs is: RegexSet(["(?m)^ *v [0-9]"])
false

What is the expected behavior?

The last line should be true.

[BurntSushi]

One interesting bit here is that while is_match returns false, matches(..).matched_any() returns true!

fn main() {
    env_logger::init();
    let pattern = r"(?m)^ *v [0-9]";
    let text = "v 0";

    let rs = regex::RegexSet::new([pattern]).unwrap();
    println!("rs is: {rs:?}");
    println!("{}", rs.is_match(text)); // false (incorrect)
    println!("{}", rs.matches(text).matched_any()); // true!
}

[BurntSushi]

This is fixed in regex 1.9.4 and regex-automata 0.3.7 on crates.io. See #1072 for the details about what caused this. Nice find and thank you for the report!

[jorendorff]

obviously, killing it here. just wow. i don't have the words