regex and regex_lite disagree on whether repetition operators associate with flag-setters

[orlp]

What version of regex are you using?

regex == 1.12.3
regex_lite == 0.1.9

Describe the bug at a high level.

regex and regex_lite both support "flag-setters". They have the following syntax: (?flags). This looks superficially like a group but only sets a flag.

However the two tools disagree on whether this element of the syntax associates with a repetition operator.

What are the steps to reproduce the behavior?

let pattern = "a(?u)*";
let haystack = "aaa";
dbg!(regex_lite::Regex::new(pattern).unwrap().find_iter(haystack).collect::<Vec<_>>());
dbg!(regex::Regex::new(pattern).unwrap().find_iter(haystack).collect::<Vec<_>>());

What is the actual behavior?

regex_lite does not associate the repetition operator with the flag setter (?u), so it will simply behave equivalent to a* and match the three a's.

regex does associate the repetition operator with the flag setter, and gives an error:

regex parse error:
    a(?u)*
         ^
error: repetition operator missing expression

What is the expected behavior?

I can see something for both approaches but they should probably be equivalent.