There should be some way to highlight the inside of a region

[Screwtapello]

The Markdown highlighter contains support for highlighting different languages in fenced code blocks, like this:

```rust
fn main() {
    println!("hello, world");
}
```

The actual Kakoune script it generates looks something like this (at least, after PR #2400 lands):

add-highlighter shared/markdown/rust region -match-capture ^(\h*)```\h*rust\\b   ^(\h*)``` regions
add-highlighter shared/markdown/rust/ default-region fill meta
add-highlighter shared/markdown/rust/inner region \A```[^\\n]*\K (?=```) ref rust

As I understand it, this means:

• A Rust region starts from a line beginning with triple-backtick followed by rust, and ends with a line with the same indent containing a triple-quote.
• That entire region, including the opening and closing markers, gets filled with the "meta" face.
• The inner region, after the opening marker and before the closing marker, gets highlighted according to Rust rules.

There's just one problem: the inner region doesn't necessarily end before the closing marker, it ends before the first triple-backtick. So if I have a code-block like this:

```rust
fn main() {
    // some comment with ``` in it
    println!("hello, world");
}
```

...the Rust highlighting ends in the middle of the second line, instead of continuing until the end. The obvious solution is to change the region-end regex from (?=```) to (?=```\z) since we want to end the region when we hit the closing marker, not any old triple-backtick. Unfortunately, that solution produces an error:

'add-highlighter' regex parse error: Lookaround can only contain literals, any chars or character classes at (?=```\z)<<<HERE>>>

I know that Kakoune's non-backtracking regex-engine implies certain limitations about lookahead and lookbehind, but I vaguely recall the current implementation is overly restrictive. If this particular restriction is not entirely necessary, it would be great if it could be lifted.

Other alternatives that I can think of:

• If there were some way to automatically apply a highlighter to the inside of a region (that is, between the text matched by the opening and closing regexes, without including them), that would be fine.
• An opposite version of \K, that resets the end position of capture group 0 to the current position.

[pickfire]

Is this related to adding rust highlighting in rust doc comments?

Also, is this markdown related?

> ```python
> print("Hello world")
> ```

```python
print("Hello world")

Currently, the quote part is not highlighted.

I believe the same goes for rust.

//! Hello world
//! ```
//! println!("hello world");
//! ```

[lenormf]

Related to region highlighting issues: #3055 #3024 #3375 #3056 #1670
Dupe #3121

[pickfire]

Rust doc blocks already work (I forgot when but quite some time ago), I ported a similar version from rust.vim. It even checks checks for the correct term, like edition2018,no_run and also support # for hiding code. But I didn't add support for other languages.