Highlight syntax in start and end delimiters of region

Question

I'm not sure I'm using the right tool for the job here, but I'm trying to embed Lua syntax highlighting inside a block syntax. The only way I can make this work is with a syn region. I can find the block easily enough with a syn match, but trying to embed another syntax in there (e.g. with contains=@Lua just ends up bleeding all other the place.

This is for the vim-sile syntax plugin. I've managed to do something similar embedding Lua for an inline syntax where the region end actually is just a delimiter, so that is hilghlighted correctly. See for example:

\begin[papersize=a5]{document}
\nofolios
\font[family=Libertinus Serif]
\begin{script}
-- Lua comment
.myvar = "mod4=" .. 4 % 2
\end{script}

\begin{raggedright}
% sile comment
Lorem ipsum...
\end{raggedright}

\script{myvar = "mod4=" .. 4 % 2}

lorem \emph{ipsum}...
\end{document}

Note how the Lua code inside the \script{...} command works great, but the same code in the \begin{script}...\end{script} block does not.

I have matches already setup for parsing the details of \begin{<command>} and those are nicely highlighted for other possible values, but specifically for script when Lua is going to be embedded I can't figure out how to use my regular highlighting for the delimiters and use them in the start/end.

syn region sileBlockLua matchgroup=Delimiter start="\\begin{script}"    end="\\end{script}" contains=@LUA

This finds the blocks and does the Lua embedding right, but the start and end text ends up as Delimiter.

How can I capture a region to embed another syntax, but also capture and highlgiht the start and end delimiters?

Answer 1

This syntax rule works to highlight a Lua script inside a \begin{script} block:

syn region sileBlockLua
    \ start="\\begin{script}" end="\\end{script}"
    \ contains=sileBlock,@LUA keepend

You can add this rule to a file named after/syntax/sile.vim, for instance under your ~/.vim directory.

The parts in bold are the ones that make it work.

With contains=sileBlock,@LUA, you're allowing the rules that match \begin and \end (including the following {...} with a command) within this region, so this will end up highlighting the delimiters appropriately.

With keepend, you're preventing the syntax engine from parsing Lua syntax past the end delimiter. This directive makes the match on the end delimiter stronger than the contained parts.

---

UPDATE: An additional improvement is to remove the end tag from the region, so that incomplete Lua syntax doesn't affect it. That can be done with the further addition of:

syn region sileBlockLua
    \ start="\\begin{script}" end="\\end{script}"me=s-1
    \ contains=sileBlock,@LUA keepend

Which indicates that the match end for that tag will be 1 character before the start of where the regular expression matches.

By having that match outside of the region, it will be unaffected by the Lua syntax rules.

(Thanks @Caleb for incorporating my suggestions into an upstream commit!)