Missing method Regex::captures_at

[lilyball]

The Regex type has _at variants of most searching methods, except it doesn't have captures_at. This seems like an oversight.

[BurntSushi]

It wasn't an oversight. The intent was for folks to use captures_read_at instead. The thinking here is that the _at variants already represent niche use cases, so it didn't seem necessary to add the routine. Do you have a specific use case in mind here?

[lilyball]

I'm currently writing a Rust equivalent of Apple's Scanner, because I want something that lets me quickly do ad-hoc parsing of input for Advent of Code, and one of the things I'm adding to it is a scan_regex() method. I wanted to be able to return a regex::Captures object, but also I thought it would be nice to allow the regex to know about surrounding context (since I'm matching it at an offset into the source string).

That said, since creating this issue, I realized this crate doesn't support look-around assertions, so the ability to see context pretty much just affects the behavior of ^ and \A, so ultimately it's not really very important.

But the core idea here is I'm taking a caller-supplied Regex and matching it in a substring of a caller-supplied larger string, and I want to return a regex::Captures to my caller, and it seems I can't do that right now while allowing the regex to see the surrounding context.

[BurntSushi]

Gotya. Yeah, that makes sense. A PR that adds it to ::Regex and ::bytes::Regex with a test or two would be welcome. :-) Otherwise, I can try to add it when I get around to it!

[rauschma]

This could work: https://docs.rs/fancy-regex/latest/fancy_regex/struct.Regex.html#method.captures_from_pos

[rauschma]

I have an additional requirement: I’d like to match at exactly a given index. That could be achieved via either a flag (JavaScript has y for this purpose) or a special anchor that matches at the end of the previous match.

My use case is tokenization.

UPDATE: fancy-regex (method captures_from_pos) plus \G do what I need: https://users.rust-lang.org/t/matching-a-regex-at-a-particular-index/86070/2

[BurntSushi]

That is a separate issue entirely and is unlikely to be supported in the regex crate API. It will be available in regex-automata once #656 is done though, assuming I'm understanding you correctly. A concrete example with inputs, actual output and desired output would be much clearer.

[BurntSushi]

a special anchor that matches at the end of the previous match.

I personally consider this a very nutty thing to do.

regex-automata will support this by simply making it possible to execute an anchored search anywhere. That is, ^ only matches at position 0, but an anchored search is only required to produce matches starting at the place where the search started. They are orthogonal things, but it is what it sounds like you're after here.