title: Replace hashtags in a single pass with regex
body: <p>I want to replace all hashtags in a string with their equivalent tag in Java. Examples:</p>
  
  <pre><code>This is a #foo_bar #document about #nothing_but_tags!
  </code></pre>
  
  <p>will result in:</p>
  
  <pre><code>This is a foo bar document about nothing but tags!
  </code></pre>
  
  <p>Is this possible in a <strong>one-pass</strong> regex replace? A hashtag may contain many words.</p>