title: What is a vim regex for "abcd", where "bc" is optional?
body: <p>I want to search for occurrences of <code>ad</code> and <code>abcd</code> where the <code>bc</code> is optional.  How can I do that?  i.e.,</p>
  
  <pre><code>+-------+----------+
  | ad    | MATCH    |
  | abcd  | MATCH    |
  | abd   | NO match |
  | abbd  | NO match |
  | abced | NO match |
  | abcbcd| NO match |
  +-------+----------+
  </code></pre>