title: Regex in sed to convert ##XXX## to ${XXX}
body: <p>I need to use sed to convert all occurences of <code>##XXX##</code> to <code>${XXX}</code>. X could be any alphabetic character or '_'. I know that I need to use something like:</p>
  <pre><code>'s/##/\${/g'
  </code></pre>
  <p>But of course that won't work properly, as it will convert <code>##FOO##</code> to <code>${FOO${</code></p>