ming li

I actually hadn't thought of your solution and its existence probably eliminates a lot of use cases of `peeking_fold_while`. I suppose one benefit is that `peeking_fold_while` enables folding the same...

Given that there is precedence with [`peeking_take_while`](https://docs.rs/itertools/0.10.0/itertools/trait.Itertools.html#method.peeking_take_while) is this something we should revisit?

Is this an issue with how `Hir` is rendered? Looking at `Writer::visit_pre` https://github.com/rust-lang/regex/blob/04e025b86144bbdf41425fef4a1d06161dc645d7/regex-syntax/src/hir/print.rs#L86-L91 and `Writer::visit_post` https://github.com/rust-lang/regex/blob/04e025b86144bbdf41425fef4a1d06161dc645d7/regex-syntax/src/hir/print.rs#L173-L180 repetitions should probably be wrapped in parentheses.

It looks like alternations have the same issue: ```rust use regex_syntax::hir; fn main() { // Is this: xab+y or x(ab)+y? let xabplusy = hir::Hir::concat(vec![ hir::Hir::literal(hir::Literal::Unicode('x')), hir::Hir::repetition(hir::Repetition { kind: hir::RepetitionKind::OneOrMore, greedy:...