Improve support for subequations environment

[gssfaw]

Is your feature request related to a problem? Please describe it.
I've found that vimtex does not parse labels from the subequations environment of the asmath package if they contain non-Latin characters in them for the omnifunc completions.

Describe the solution you'd like
It would be great if the subequations environment got a better support. This is probably due to the fact that Unicode characters need special handling.

On this screenshot you can see that omnifunc (C-xC-o) does not show the number of the subequation if it contains a Cyrillic character, while the equation with Latin letters works as expected.

Here is a minimal .tex file with examples. The first one has Cyrillic letters, the former has Latin characters.

Minimal tex file

%! TeX program = lualatex
\documentclass[12pt]{article} 
\usepackage{polyglossia}
\setdefaultlanguage[babelshorthands=true, forceheadingpunctuation=false]{russian}
\setotherlanguages{english}
\usepackage{fontspec} 
\setmainfont{FreeSerif}
\setsansfont{FreeSans}
\setmonofont{FreeMono}
\usepackage{amsmath}   
\begin{document}
% subequations with cyrillic letters
\begin{subequations}
    \renewcommand{\theequation}{\theparentequation\asbuk{equation}}
    \begin{align}
        test 1 1\label{eq:test_1_1}\\ 
        test 1 2 \label{eq:test_1_2}
    \end{align}
\end{subequations}
% subequations with latin letters
\begin{subequations}
    \begin{align}
        test 2 1\label{eq:test_2_1}\\ 
        test 2 2 \label{eq:test_2_2}
    \end{align}
\end{subequations}

\eqref{eq:test_1_2} % Does not show the number 
\eqref{eq:test_2_2} % Does show the number

\end{document}

`main.aux` has all labels and they seem to look the same way.

main.aux

\relax 
\ProvideDocumentCommand \selectlanguage {sO{}m}{}
\catcode `"\active 
\@writefile{toc}{\selectlanguage *[babelshorthands=true, forceheadingpunctuation=false]{russian}}
\newlabel{eq:test_1_1}{{1а}{1}{}{}{}}
\newlabel{eq:test_1_2}{{1б}{1}{}{}{}}
\newlabel{eq:test_2_1}{{2a}{1}{}{}{}}
\newlabel{eq:test_2_2}{{2b}{1}{}{}{}}
\gdef \@abspage@last{1}

[lervag]

There was a regex that explicitly looked for simple lowercase latin characters. I've generalized it in a manner that looks safe and I believe this should resolve your issue.

[gssfaw]

It's working now, I really appreciate your help

[lervag]

Great; glad to hear it!