title: Regex to only match specific characters preceded by a space or nothing (start of line)
body: <p>Consider the following tweets:</p>
  
  <pre><code>RT @username This is my tweet
  Check this! RT @username This is my tweet
  I have PART 2 downloaded
  </code></pre>
  
  <p>In a <code>preg_replace()</code> call, I am using regex to replace the <code>RT</code> (common <em>retweet</em> syntax) with <code>{RT}</code>. It almost works, however, it also matches <code>RT</code> in <code>PART</code> in the last tweet:</p>
  
  <ul>
  <li><code>I have PART 2 downloaded</code> becomes <code>I have PA{RT} 2 downloaded</code></li>
  </ul>
  
  <p>I want the regex to only allow <strong>nothing</strong> (beginning of string) or a <strong>space (U+0020)</strong> in front of <code>RT</code>.</p>
  
  <p>The current <code>preg_replace()</code> call:</p>
  
  <pre><code>echo preg_replace("(\RT(?=\s)/", '{RT}', $tweet);
  </code></pre>