<firstterm>constraints</firstterm>, concatenated.
    It matches a match for the first, followed by a match for the second, etc.;
    an empty branch matches the empty string.
   </para>

   <para>
    A quantified atom is an <firstterm>atom</firstterm> possibly followed
    by a single <firstterm>quantifier</firstterm>.
    Without a quantifier, it matches a match for the atom.
    With a quantifier, it can match some number of matches of the atom.
    An <firstterm>atom</firstterm> can be any of the possibilities
    shown in <xref linkend="posix-atoms-table"/>.
    The possible quantifiers and their meanings are shown in
    <xref linkend="posix-quantifiers-table"/>.
   </para>

   <para>
    A <firstterm>constraint</firstterm> matches an empty string, but matches only when
    specific conditions are met.  A constraint can be used where an atom
    could be used, except it cannot be followed by a quantifier.
    The simple constraints are shown in
    <xref linkend="posix-constraints-table"/>;
    some more constraints are described later.
   </para>


   <table id="posix-atoms-table">
    <title>Regular Expression Atoms</title>

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

      <tbody>
       <row>
       <entry> <literal>(</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> (where <replaceable>re</replaceable> is any regular expression)
       matches a match for
       <replaceable>re</replaceable>, with the match noted for possible reporting </entry>
       </row>

       <row>
       <entry> <literal>(?:</literal><replaceable>re</replaceable><literal>)</literal> </entry>
       <entry> as above, but the match is not noted for reporting
       (a <quote>non-capturing</quote> set of parentheses)
       (AREs only) </entry>
       </row>

       <row>
       <entry> <literal>.</literal> </entry>
       <entry> matches any single character </entry>
       </row>

       <row>
       <entry> <literal>[</literal><replaceable>chars</replaceable><literal>]</literal> </entry>
       <entry> a <firstterm>bracket expression</firstterm>,
       matching any one of the <replaceable>chars</replaceable> (see
       <xref linkend="posix-bracket-expressions"/> for more detail) </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>k</replaceable> </entry>
       <entry> (where <replaceable>k</replaceable> is a non-alphanumeric character)
       matches that character taken as an ordinary character,
       e.g., <literal>\\</literal> matches a backslash character </entry>
       </row>

       <row>
       <entry> <literal>\</literal><replaceable>c</replaceable> </entry>
       <entry> where <replaceable>c</replaceable> is alphanumeric
       (possibly followed by other characters)
       is an <firstterm>escape</firstterm>, see <xref linkend="posix-escape-sequences"/>
       (AREs only; in EREs and BREs, this matches <replaceable>c</replaceable>) </entry>
       </row>

       <row>
       <entry> <literal>{</literal> </entry>
       <entry> when followed by a character other than a digit,
       matches the left-brace character <literal>{</literal>;
       when followed by a digit, it is the beginning of a
       <replaceable>bound</replaceable> (see below) </entry>
       </row>

       <row>
       <entry> <replaceable>x</replaceable> </entry>
       <entry> where <replaceable>x</replaceable> is a single character with no other
       significance, matches that character </entry>
       </row>
      </tbody>
     </tgroup>
    </table>

   <para>
    An RE cannot end with a backslash (<literal>\</literal>).
   </para>

   <note>
    <para>
     If you have <xref linkend="guc-standard-conforming-strings"/> turned off,
     any backslashes you write in literal string constants will need to be
     doubled.  See <xref linkend="sql-syntax-strings"/> for more information.
    </para>