Matching ^ and $ with Regex::captures_at

[ClementNerma]

Hi there!

I'm currently building a tiny TextMate-like syntax highlighter using regexes at its core.

One thing I need to do is to match the beginning of a substring. For instance:

let regex = Regex::new("^@direct");
let input = "call(@direct cmd)";

regex.captures_at(input, 5); // Must match

Unfortunately, this doesn't work due to captures_at being aware of the input context, which means ^ will only match if we're actually at the beginning of the whole input, not just the part we are matching against.

What workaround could be used for this use case? I need something that will allow the regex to match in the exact same conditions (full input, provided position).

EDIT: To be clearer, I need a way to specify in the regex that I want to match the beginning or end of the portion of string it is matched against, not the full string.

Thanks in advance for your help!

[BurntSushi]

I think the direct and most straight-forward answer to your specific question is regex.captures(&input[5..]).

Otherwise, if you want to start a search at an arbitrary position while taking look-behind assertions into account but want to limit your matches to those that start at the same position that you started your search, then you'll have to drop down to regex-automata and use its anchored option. The example there uses the PikeVM, but it will also work with the meta regex engine which is probably what you want to use. (The regex crate is itself just a wrapper around the meta regex engine.)