Nested highlighting

Question

In html eruby files, I would like to highlight the <% ... %> text in dark green only when it is inside double quotes. I.e. in in this example:

I want it dark green instead of white.

I know that <% ... %> and <%= ... %> are respectively defined as erubyBlock and erubyExpression in terms of syntax highlighting. I could highlight them with hi erubyBlock guifg=... etc, but I would like to do so only when they are inside double quotes (which have the htmlString syntax definition).

So basically I would like to do something like hi htmlString>erubyBlock guifg=... (if this syntax existed).

I have tried looking at the Vim help but could not find anything. Is there a way to do this?

Answer 1

Ok, this is probably not the best solution because it is pretty invasive. But, it's all I could come up with so I will give it to you.

One thing we can do is basically manually do the highlighting ourselves. So we could easily do something like:

   syn match quotedErubyBlock '"<%.\{-}%>"'

This will define a new highlighting section that matches "<% followed by any text, followed by %>". This should get us what we want, but the problem with this is that it will only affect strings that are not contained in any other syntax region. Clearly in your example this is going to show up in an htmlTag region, and so we need to get a bit more complicated. What I've come up with is the following:

autocmd BufReadPost *.html syn match quotedErubyBlock '"<%.\{-}%>"' contained
autocmd BufReadPost *.html syn region  htmlTag start=+<[^/]+   end=+>+ fold contains=htmlTagN,htmlString,htmlArg,htmlValue,htmlTagError,htmlEvent,htmlCssDefinition,quotedErubyBlock,@htmlPreproc,@htmlArgCluster
autocmd BufReadPost *.html hi link quotedErubyBlock Constant

Ok, that's a lot to swallow. Let's go through it.

1. The first line defines our new highlighting section quotedErubyBlock and also specifies that it will be contained in some other region.
2. The next line is the invasive part. I've stolen this line from vim's html.vim file. Basically we are redefining what an htmlTag is. The only thing I changed is I added our quotedErubyBlock to the contains= list. This way vim can "see" our new highlighting section within an htmlTag block.
3. This line just tells vim what color to highlight our new section with. I've put Constant because I think that will give you what you are looking for, but feel free to change it to whatever works best for you.

As a last note, the autocmd BufReadPost *.html is an autocommand that will do our highlighting whenever a file is loaded with the extension html. I'm not very familiar with eruby, so you may need to change this if that is not the proper filetype.

See the vimwiki's page on creating your own syntax file for more information.

Hope this helps.