title: Regular expression in PHP preg_replace() yields empty results
body: <p>I'm struggling with the regular expression syntax needed to simply transform this string:</p>
  
  <pre><code>posts/2012-03-16-23-07_an-awesome-post.md
  </code></pre>
  
  <p>into this string:</p>
  
  <pre><code>an-awesome-post
  </code></pre>
  
  <p>using the <code>preg_replace()</code> PHP function. Here's what I've got so far:</p>
  
  <pre><code>preg_replace('posts/[0-9-]*_([a-z-]*).md', '$1', 'posts/2012-03-16-23-07_an-awesome-post.md');
  </code></pre>
  
  <p>No dice. When I assign the result to a variable and <code>echo()</code> out that variable, I get nothing. My regex syntax is a bit rusty, but Googling around a bit makes me think that, at the very least, I'm on the right track.</p>