tained by applying the mapping <code class="code">f</code> to the variable name. Inside the
   string pattern, a variable name immediately follows a non-escaped
   <code class="code">$</code> character and is one of the following:</p>
<ul>
<li>a non empty sequence of alphanumeric or <code class="code">_</code> characters,</li>
<li>an arbitrary sequence of characters enclosed by a pair of
   matching parentheses or curly brackets.
   An escaped <code class="code">$</code> character is a <code class="code">$<