Treesitter query question, matching only substring of a node

[Danny Freeman <danny@dfreeman.email>]

Hello,

I am currently seeing what it will look like to add treesitter support
for clojure-mode. I am using this grammer:
https://github.com/sogaiu/tree-sitter-clojure
and it is working great so far.

It has definitions for nodes that match full namespaced keywords and
symbols, such that

:plain-keyword      matches (kwd_lit) node
:namespace/keyword  matches (kwd_lit) node

and similarly for symbols

'plain-symbol       matches (sym_lit) node
'namespaced/symbol  matches (sym_lit) node

In clojure, the part before the / character is the NAMESPACE, and the
part after the / character is the NAME.

The current clojure-mode highlights the namespaced part with a different
face than the name part.

I am trying to use this existing grammer to apply different faces to the
namespaced and named part. I can write a query that matches the whole
keyword easily, using `treesit-font-lock-rules`

```
  (treesit-font-lock-rules
   ...
   :feature 'keyword
   :language 'clojure
   '((kwd_lit) @clojure-keyword-face)
   ...
   )
```
This works fine.

What I'm trying to do now is capture a substring of that node, the
namespaced part.

I can use the `:match` predicate to identify the namespace part, but the
matched group doesn't get captured by anything

```
(defvar kw-query
  `((((kwd_lit) @kw)
     (:match "^:.*/" @kw) ;; Can I capture this??
     )))

(treesit-query-string ":namespaced/keyword" kw-query 'clojure)
```

Is something like this possible with the treesitter query engine in
Emacs? Or do I need to handle this at the grammer level? Perhaps with a
field?

There is an issue that was closed in the grammer's repository asking
about this, but their advise was to do some substring matching in the
editor, so here I am
https://github.com/sogaiu/tree-sitter-clojure/issues/28 

-- 
Danny Freeman