<row>
       <entry> <literal>\d</literal> </entry>
       <entry> matches any digit, like
        <literal>[[:digit:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\s</literal> </entry>
       <entry> matches any whitespace character, like
        <literal>[[:space:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\w</literal> </entry>
       <entry> matches any word character, like
        <literal>[[:word:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\D</literal> </entry>
       <entry> matches any non-digit, like
        <literal>[^[:digit:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\S</literal> </entry>
       <entry> matches any non-whitespace character, like
        <literal>[^[:space:]]</literal> </entry>
       </row>

       <row>
       <entry> <literal>\W</literal> </entry>
       <entry> matches any non-word character, like
        <literal>[^[:word:]]</literal> </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    The class-shorthand escapes also work within bracket expressions,
    although the definitions shown above are not quite syntactically
    valid in that context.
    For example, <literal>[a-c\d]</literal> is equivalent to
    <literal>[a-c[:digit:]]</literal>.
   </para>

   <table id="posix-constraint-escapes-table">
    <title>Regular Expression Constraint Escapes</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\A</literal> </entry>
       <entry> matches only at the beginning of the string
       (see <xref linkend="posix-matching-rules"/> for how this differs from
       <literal>^</literal>) </entry>
       </row>

       <row>
       <entry> <literal>\m</literal> </entry>
       <entry> matches only at the beginning of a word </entry>
       </row>

       <row>
       <entry> <literal>\M</literal> </entry>
       <entry> matches only at the end of a word </entry>
       </row>

       <row>
       <entry> <literal>\y</literal> </entry>
       <entry> matches only at the beginning or end of a word </entry>
       </row>

       <row>
       <entry> <literal>\Y</literal> </entry>
       <entry> matches only at a point that is not the beginning or end of a
       word </entry>
       </row>

       <row>
       <entry> <literal>\Z</literal> </entry>
       <entry> matches only at the end of the string
       (see <xref linkend="posix-matching-rules"/> for how this differs from
       <literal>$</literal>) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    A word is defined as in the specification of
    <literal>[[:&lt;:]]</literal> and <literal>[[:&gt;:]]</literal> above.
    Constraint escapes are illegal within bracket expressions.
   </para>

   <table id="posix-constraint-backref-table">
    <title>Regular Expression Back References</title>

    <tgroup cols="2">
     <thead>
      <row>
       <entry>Escape</entry>
       <entry>Description</entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry> <literal>\</literal><replaceable>m</replaceable> </entry>
       <entry> (where <replaceable>m</replaceable> is a nonzero digit)
       a back reference to the <replaceable>m</replaceable>'th subexpression </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>mnn</replaceable> </entry>
       <entry> (where <replaceable>m</replaceable> is a nonzero digit, and
       <replaceable>nn</replaceable> is some more digits, and the decimal value
       <replaceable>mnn</replaceable> is not greater than the number of closing capturing
       parentheses seen so far)
       a back reference to the <replaceable>mnn</replaceable>'th subexpression </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <note>
    <para>
     There is an