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 the previous parenthesized subexpression specified
    by the number <replaceable>n</replaceable>
    (see <xref linkend="posix-constraint-backref-table"/>).  For example,
    <literal>([bc])\1</literal> matches <literal>bb</literal> or <literal>cc</literal>
    but not <literal>bc</literal> or <literal>cb</literal>.
    The subexpression must entirely precede the back reference in the RE.
    Subexpressions are numbered in the order of their leading parentheses.
    Non-capturing parentheses do not define subexpressions.
    The back reference considers only the string characters matched by the
    referenced subexpression, not any constraints contained in it.  For
    example, <literal>(^\d)\1</literal> will match <literal>22</literal>.
   </para>

   <table id="posix-character-entry-escapes-table">
    <title>Regular Expression Character-Entry Escapes</title>

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

      <tbody>
       <row>
       <entry> <literal>\a</literal> </entry>
       <entry> alert (bell) character, as in C </entry>
       </row>

       <row>
       <entry> <literal>\b</literal> </entry>
       <entry> backspace, as in C </entry>
       </row>

       <row>
       <entry> <literal>\B</literal> </entry>
       <entry> synonym for backslash (<literal>\</literal>) to help reduce the need for backslash
       doubling </entry>
       </row>

       <row>
       <entry> <literal>\c</literal><replaceable>X</replaceable> </entry>
       <entry> (where <replaceable>X</replaceable> is any character) the character whose
       low-order 5 bits are the same as those of
       <replaceable>X</replaceable>, and whose other bits are all zero </entry>
       </row>

       <row>
       <entry> <literal>\e</literal> </entry>
       <entry> the character whose collating-sequence name
       is <literal>ESC</literal>,
       or failing that, the character with octal value <literal>033</literal> </entry>
       </row>

       <row>
       <entry> <literal>\f</literal> </entry>
       <entry> form feed, as in C </entry>
       </row>

       <row>
       <entry> <literal>\n</literal> </entry>
       <entry> newline, as in C </entry>
       </row>

       <row>
       <entry> <literal>\r</literal> </entry>
       <entry> carriage return, as in C </entry>
       </row>

       <row>
       <entry> <literal>\t</literal> </entry>
       <entry> horizontal tab, as in C </entry>
       </row>

       <row>
       <entry> <literal>\u</literal><replaceable>wxyz</replaceable> </entry>
       <entry> (where <replaceable>wxyz</replaceable> is exactly four hexadecimal digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>wxyz</replaceable>
       </entry>
       </row>

       <row>
       <entry> <literal>\U</literal><replaceable>stuvwxyz</replaceable> </entry>
       <entry> (where <replaceable>stuvwxyz</replaceable> is exactly eight hexadecimal
       digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>stuvwxyz</replaceable>
       </entry>
       </row>

       <row>
       <entry> <literal>\v</literal> </entry>
       <entry> vertical tab, as in C </entry>
       </row>

       <row>
       <entry> <literal>\x</literal><replaceable>hhh</replaceable> </entry>
       <entry> (where <replaceable>hhh</replaceable> is any sequence of hexadecimal
       digits)
       the character whose hexadecimal value is
       <literal>0x</literal><replaceable>hhh</replaceable>
       (a single character no matter how many hexadecimal digits are used)
       </entry>
       </row>

       <row>
       <entry> <literal>\0</literal> </entry>
       <entry> the character whose value is <literal>0</literal> (the null byte)</entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>xy</replaceable> </entry>
       <entry> (where <replaceable>xy</replaceable>
Title: Regular Expression Character-Entry Escapes
Summary
This section describes regular expression back references, where `\n` matches the same string as a previous parenthesized subexpression. It specifies that subexpressions must precede the back reference and are numbered by leading parentheses. The table then describes character-entry escapes such as `\a` (alert), `\b` (backspace), `\f` (form feed), `\n` (newline), `\r` (carriage return), `\t` (horizontal tab), `\v` (vertical tab), `\0` (null byte), and others for specifying special characters in regular expressions.