he POSIX standard defines no escape sequences for POSIX-Extended regular
        expressions, except that:
      </p>
<div class="itemizedlist"><ul class="itemizedlist" style="list-style-type: disc; ">
<li class="listitem">
            Any special character preceded by an escape shall match itself.
          </li>
<li class="listitem">
            The effect of any ordinary character being preceded by an escape is undefined.
          </li>
<li class="listitem">
            An escape inside a character clas