<literal>(?&lt;=</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>positive lookbehind</firstterm> matches at any point
       where a substring matching <replaceable>re</replaceable> ends
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>(?&lt;!</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> <firstterm>negative lookbehind</firstterm> matches at any point
       where no substring matching <replaceable>re</replaceable> ends
       (AREs only) </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    Lookahead and lookbehind constraints cannot contain <firstterm>back
    references</firstterm> (see <xref linkend="posix-escape-sequences"/>),
    and all parentheses within them are considered non-capturing.
   </para>
   </sect3>

   <sect3 id="posix-bracket-expressions">
    <title>Bracket Expressions</title>

   <para>
    A <firstterm>bracket expression</firstterm> is a list of
    characters enclosed in <literal>[]</literal>.  It normally matches
    any single character from the list (but see below).  If the list
    begins with <literal>^</literal>, it matches any single character
    <emphasis>not</emphasis> from the rest of the list.
    If two characters
    in the list are separated by <literal>-</literal>, this is
    shorthand for the full range of characters between those two
    (inclusive) in the collating sequence,
    e.g., <literal>[0-9]</literal> in <acronym>ASCII</acronym> matches
    any decimal digit.  It is illegal for two ranges to share an
    endpoint, e.g.,  <literal>a-c-e</literal>.  Ranges are very
    collating-sequence-dependent, so portable programs should avoid
    relying on them.
   </para>

   <para>
    To include a literal <literal>]</literal> in the list, make it the
    first character (after <literal>^</literal>, if that is used).  To
    include a literal <literal>-</literal>, make it the first or last
    character, or the second endpoint of a range.  To use a literal
    <literal>-</literal> as the first endpoint of a range, enclose it
    in <literal>[.</literal> and <literal>.]</literal> to make it a
    collating element (see below).  With the exception of these characters,
    some combinations using <literal>[</literal>
    (see next paragraphs), and escapes (AREs only), all other special
    characters lose their special significance within a bracket expression.
    In particular, <literal>\</literal> is not special when following
    ERE or BRE rules, though it is special (as introducing an escape)
    in AREs.
   </para>

   <para>
    Within a bracket expression, a collating element (a character, a
    multiple-character sequence that collates as if it were a single
    character, or a collating-sequence name for either) enclosed in
    <literal>[.</literal> and <literal>.]</literal> stands for the
    sequence of characters of that collating element.  The sequence is
    treated as a single element of the bracket expression's list.  This
    allows a bracket
    expression containing a multiple-character collating element to
    match more than one character, e.g., if the collating sequence
    includes a <literal>ch</literal> collating element, then the RE
    <literal>[[.ch.]]*c</literal> matches the first five characters of
    <literal>chchcc</literal>.
   </para>

   <note>
    <para>
     <productname>PostgreSQL</productname> currently does not support multi-character collating
     elements. This information describes possible future behavior.
    </para>
   </note>

   <para>
    Within a bracket expression, a collating element enclosed in
    <literal>[=</literal> and <literal>=]</literal> is an <firstterm>equivalence
    class</firstterm>, standing for the sequences of characters of all collating
    elements equivalent to that one, including itself.  (If there are
    no other equivalent collating elements, the treatment is as if the