title: How to create category for external headers in clang-format?
body: <p>I want to configure clang-format to sort in C++ the included headers as follows:</p>
  
  <ul>
  <li>main header (associated with the current cpp file),</li>
  <li>local headers included via "",</li>
  <li>other headers included via &lt;>,</li>
  <li>headers from specific external libraries (e.g. boost, catch2),</li>
  <li>system/standard headers.</li>
  </ul>
  
  <p>I'm using clang-format 8.0.0 on macOS.
  My current configuration (snippet related only to includes) is as follows:</p>
  
  <pre><code>SortIncludes: true
  IncludeBlocks: Regroup
  IncludeCategories:
    # Headers in &lt;&gt; without extension.
    - Regex:           '&lt;([A-Za-z0-9\/-_])+&gt;'
      Priority:        4
    # Headers in &lt;&gt; from specific external libraries.
    - Regex:           '&lt;((\bboost\b)|(\bcatch2\b))\/([A-Za-z0-9.\/-_])+&gt;'
      Priority:        3
    # Headers in &lt;&gt; with extension.
    - Regex:           '&lt;([A-Za-z0-9.\/-_])+&gt;'
      Priority:        2
    # Headers in "" with extension.
    - Regex:           '"([A-Za-z0-9.\/-_])+"'
      Priority:        1
  </code></pre>
  
  <p>In this configuration I assume, that system/standard headers are without extension. It will not work for UNIX/POSIX headers. Main header is automatically detected and assigned the priority 0. So far, all seems working as expected, except for the category for external libraries. It looks like clang-format is assigning it to the priority 2. </p>
  
  <p>Expected result:</p>
  
  <pre><code>#include "test.h"
  
  #include &lt;allocator/region.hpp&gt;
  #include &lt;page.hpp&gt;
  #include &lt;page_allocator.hpp&gt;
  #include &lt;test_utils.hpp&gt;
  #include &lt;utils.hpp&gt;
  #include &lt;zone_allocator.hpp&gt;
  
  #include &lt;catch2/catch.hpp&gt;     // &lt;--------
  
  #include &lt;array&gt;
  #include &lt;cmath&gt;
  #include &lt;cstring&gt;
  #include &lt;map&gt;
  </code></pre>
  
  <p>Actual result:</p>
  
  <pre><code>#include "test.h"
  
  #include &lt;allocator/region.hpp&gt;
  #include &lt;catch2/catch.hpp&gt;     // &lt;--------
  #include &lt;page.hpp&gt;
  #include &lt;page_allocator.hpp&gt;
  #include &lt;test_utils.hpp&gt;
  #include &lt;utils.hpp&gt;
  #include &lt;zone_allocator.hpp&gt;
  
  #include &lt;array&gt;
  #include &lt;cmath&gt;
  #include &lt;cstring&gt;
  #include &lt;map&gt;
  </code></pre>
  
  <p>How to configure priority 3 to have the expected result?</p>