title: Sub-match syntax highlighting in Vim
body: <p>First, I'll show the specific problem I'm having, but I think the problem can be generalized.</p>
  
  <p>I'm working with a language that has explicit parenthesis syntax (like Lisp), but has keywords that are only reserved against the left paren. Example:</p>
  
  <pre><code>(key key)
  </code></pre>
  
  <p>the former is a reserved word, but the latter is a reference to the variable named "key"</p>
  
  <p>Unfortunately, I find highlighting the left paren annoying, so I end up using</p>
  
  <pre><code>syn keyword classification key
  </code></pre>
  
  <p>instead of</p>
  
  <pre><code>syn keyword classification (key
  </code></pre>
  
  <p>but the former triggers on the variable uses as well.</p>
  
  <p>I'd take a hack to get around my problem, but I'd be more interested in a general method to highlight just a subset of a given match.</p>