e class="code">string</code> that can be
      empty, if the next input character does not match the range. The set of
      characters from <code class="code">c1</code> to <code class="code">c2</code> (inclusively) is denoted by <code class="code">c1-c2</code>.
      Hence, <code class="code">%[0-9]</code> returns a string representing a decimal number
      or an empty string if no decimal digit is found; similarly,
      <code class="code">%[0-9a-f]</code> returns a string of hexadecimal digits.
      If