title: whitespaces inside positive lookbehind regex
body: <p>wont work, how to fix this problem? I need to get the number.
  Foo can bee with space or without space.</p>
  
  <pre><code>(?&lt;=Foo:\s?&lt;/b&gt;).+\d
  &lt;b&gt;Foo:&lt;/b&gt;2013&lt;br&gt;&lt;b&gt;Bar:&lt;/b&gt;
  &lt;b&gt;Foo: &lt;/b&gt;2013&lt;br&gt;&lt;b&gt;Bar:&lt;/b&gt;
  </code></pre>