title: What does the $1 argument in this RewriteCond do?
body: <p>I'm setting up Apache rewrite rules to tidy up my CodeIgniter URLs.</p>
  
  <p><a href="https://stackoverflow.com/q/7182292/612253">This question</a> (and lots of forum posts etc that I've found around the place) document the use of the following rule (or something very similar):</p>
  
  <pre><code>RewriteEngine on
  RewriteCond $1 !^(index\.php|phpinfo\.php|images|robots\.txt|sitemap\.xml\.gz|sitemap\.xml|assets)
  RewriteRule ^(.*)$ /index.php/$1 [L]
  </code></pre>
  
  <p>I know the $1 after the RewriteRule refers to the captured string from (.*), but <strong>what does the first $1 (straight after the RewriteCond) represent?</strong> A lot of examples I've seen use something like %{REQUEST_URI} as the first argument for RewriteCond.</p>