.html#boost_regex.posix_extended_syntax">POSIX-Extended
        features</a> the escape character is special inside a character class
        declaration.
      </p>
<p>
        In addition, some escape sequences that are not defined as part of POSIX-Extended
        specification are required to be supported - however Boost.Regex supports
        these by default anyway.
      </p>
<h4>
<a name="boost_regex.syntax.basic_extended.h31"></a>
        <span class="phrase"><a name="boost_regex.syntax.basic_exten