Parse allowing nested parentheses in nom - rust

Question

I'm using nom. I'd like to parse a string that's surrounded by parentheses, and allowing for additional nested parentheses within the string.
So (a + b) would parse as a + b, and ((a + b)) would parse as (a + b)
This works for the first case, but not the nested case:
pub fn parse_expr(input: &str) -> IResult<&str, &str> {
// TODO: this will fail with nested parentheses, but `rest` doesn't seem to
// be working.
delimited(tag("("), take_until(")"), tag(")"))(input)
}
I tried using rest but this doesn't respect the final ):
pub fn parse_expr(input: &str) -> IResult<&str, &str> {
delimited(tag("("), rest, tag(")"))(input)
}
Thanks!

Answer 1

I found a reference to this in the nom issue log: https://github.com/Geal/nom/issues/1253
I'm using this function, from parse_hyperlinks — basically a hand-written parser for this https://docs.rs/parse-hyperlinks/0.23.3/src/parse_hyperlinks/lib.rs.html#41 :
pub fn take_until_unbalanced(
opening_bracket: char,
closing_bracket: char,
) -> impl Fn(&str) -> IResult<&str, &str> {
move |i: &str| {
let mut index = 0;
let mut bracket_counter = 0;
while let Some(n) = &i[index..].find(&[opening_bracket, closing_bracket, '\\'][..]) {
index += n;
let mut it = i[index..].chars();
match it.next().unwrap_or_default() {
c if c == '\\' => {
// Skip the escape char `\`.
index += '\\'.len_utf8();
// Skip also the following char.
let c = it.next().unwrap_or_default();
index += c.len_utf8();
}
c if c == opening_bracket => {
bracket_counter += 1;
index += opening_bracket.len_utf8();
}
c if c == closing_bracket => {
// Closing bracket.
bracket_counter -= 1;
index += closing_bracket.len_utf8();
}
// Can not happen.
_ => unreachable!(),
};
// We found the unmatched closing bracket.
if bracket_counter == -1 {
// We do not consume it.
index -= closing_bracket.len_utf8();
return Ok((&i[index..], &i[0..index]));
};
}
if bracket_counter == 0 {
Ok(("", i))
} else {
Err(Err::Error(Error::from_error_kind(i, ErrorKind::TakeUntil)))
}
}
}