title: Org Agenda regexp search Categories
body: <p>I like to structure my org-mode projects using the <code>:CATEGORY:</code> property, however categories seem not to be recognised by <code>org-agenda-filter-by-regexp</code> (bound to <code>=</code>). When viewing a rather large list of TODO items, it would be helpful to be able to narrow the list to matching categories.</p>
  
  <p>I know that I can use <code>org-agenda-filter-by-category</code> (<code>&lt;</code>), but this command must be executed on the category entry.</p>
  
  <p>Is there a way to regexp filter the Org Agenda <em>including</em> categories?</p>
  
  <p>The function below performs what I'd like, however I can't get it to work from the <code>org-agenda</code> dispatcher as a custom agenda command.</p>
  
  <pre><code>(defun rnkn/org-category-match (str)
    "Org Agenda match TODO headlines matching :CATEGORY: property STR."
    (interactive "sCategory: ")
    (org-tags-view t (concat "CATEGORY={" str "}")))
  </code></pre>