Support capture groups with the RE/flex regex matcher?

Hello,

First off, I'm a huge fan of RE-flex for my projects and ugrep is by far my most valuable and most used tool at my workplace. I deal with a lot of text.

Out of all that usage, I consistently miss the capture group feature. I know it is supported with other regex libraries that RE-flex (and therefore ugrep) can use, but that is not the same as RE-flex supporting it directly. I love ugrep and RE-flex's speed and features, but it always hurts me that I have to compile with PCRE2 mode (-P) to get capture groups with ugrep and use Boost.Regex when directly using RE-flex. They're such a common thing to need--why does RE-flex not support them? I'd like to use it and only it so that I do not have to bother with Boost or PCRE2, and also for the performance benefits.

I know POSIX and other compatibility things are at play, but I don't see how supporting capture groups would violate them. The docs say that RE-flex supports the lazy quantifiers, which is not part of POSIX, and it also supports non-capturing groups, but not capture groups. Why is this? It seems like it would be easy to add support for capturing groups considering that non-capturing groups are supported. It makes me think there is some sort of design decision that has already been made which does not allow them, but I don't understand enough of the library to know what it is.

Thanks in advance for your patience if I am making an incorrect assumption or unaware of some functionality that would do this. I'm also sorry if this has already been addressed in another issue or in the docs, but I could not find it either here, ugrep's github project, or the docs.

Start with the RE-flex regex matcher and its documentation, focusing on the existing non-capturing-group behavior and the stated compatibility considerations. Determine how direct capture-group support should be exposed and what behavior would demonstrate that RE-flex, without Boost.Regex or PCRE2, supports it.