Set variable equal to search match subsection

Question

Unnecessary Context:

I want to write a function that closes open HTML tags, because closetag.vim seems complicated, and I like to understand all the stuff I run in Vim.
My plan for this is to match the most recent previous <\w+ segment, grab whatever is \w+, and then output </\1> at the cursor location when I ran this function.

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Question:

How do I set a variable in my function equal to the most recent previous instance of \w\+ that comes after a <? I can do the search with ?<\(\w\+\), but nothing I've found will honor the capture group usage.

I have found matches and getmatches in :help (and searchpos and getline on SO), but it certainly feels like there should be an easier way than using two builtins together. I know about the search register, but that only contains the literal regex above, not what it has matched to.

It looks like a similar question was asked 5 months ago but the only answer is about matching tags on the same line, and I would like to do more. I found this question that says how to visually highlight the whole match, but can't handle capture groups, though it looks like the best method if I give up on the capture group limitation.

TL;DR: Is there a simple way to get the contents of a capture group search result into a variable?

Answer 1

I ended up combining these three answers to make my function.

No, there is not a way to get just a capture group from a search, but if your goal is just one capture group, you can use \zs and/or \ze as your group start and end points to determine highlighting and where your cursor lands.

" Ex: change <\(\w\+\) to to <\zs\w\+\ze (replace capture group with \zs and \ze)
function! GetMatch(pattern)
    let l:cursor_pos = getpos(".")
    let @/ = a:pattern
    " preserve old register content
    let savereg = @f
    " empty "f in case we don't find a match
    " if we skipped this, "f would keep whatever was in there before.
    let @f = ""
    normal "fygN
    let match = @f
    " restore register content
    let @f = l:savereg
    call setpos(".", l:cursor_pos)
    echo l:match
    return l:match
endfunction

I get the initial cursor_position and set the /search register.

From there, I can temporarily grab the results of my search into an arbitrary register (f for function is what I'm going with), then set a variable to hold the contents of that register and then reset the cursor_position to where it originally was.

I'm eventually going to hardcode the pattern to search for <\zs\w\+ but this was useful for testing purposes, and should showcase how to do this for anyone searching.