examples, with <literal>#&quot;</literal> delimiting the return string:
<programlisting>
substring('foobar' similar '%#"o_b#"%' escape '#')   <lineannotation>oob</lineannotation>
substring('foobar' similar '#"o_b#"%' escape '#')    <lineannotation>NULL</lineannotation>
</programlisting>
   </para>
  </sect2>

  <sect2 id="functions-posix-regexp">
   <title><acronym>POSIX</acronym> Regular Expressions</title>

   <indexterm zone="functions-posix-regexp">
    <primary>regular expression</primary>
    <seealso>pattern matching</seealso>
   </indexterm>
   <indexterm>
    <primary>substring</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_count</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_instr</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_like</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_match</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_matches</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_replace</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_split_to_table</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_split_to_array</primary>
   </indexterm>
   <indexterm>
    <primary>regexp_substr</primary>
   </indexterm>

   <para>
    <xref linkend="functions-posix-table"/> lists the available
    operators for pattern matching using POSIX regular expressions.
   </para>

   <table id="functions-posix-table">
    <title>Regular Expression Match Operators</title>

    <tgroup cols="1">
     <thead>
      <row>
       <entry role="func_table_entry"><para role="func_signature">
        Operator
       </para>
       <para>
        Description
       </para>
       <para>
        Example(s)
       </para></entry>
      </row>
     </thead>

      <tbody>
       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>~</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String matches regular expression, case sensitively
       </para>
       <para>
        <literal>'thomas' ~ 't.*ma'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>~*</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String matches regular expression, case-insensitively
       </para>
       <para>
        <literal>'thomas' ~* 'T.*ma'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>!~</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String does not match regular expression, case sensitively
       </para>
       <para>
        <literal>'thomas' !~ 't.*max'</literal>
        <returnvalue>t</returnvalue>
       </para></entry>
       </row>

       <row>
       <entry role="func_table_entry"><para role="func_signature">
        <type>text</type> <literal>!~*</literal> <type>text</type>
        <returnvalue>boolean</returnvalue>
       </para>
       <para>
        String does not match regular expression, case-insensitively
       </para>
       <para>
        <literal>'thomas' !~* 'T.*ma'</literal>
        <returnvalue>f</returnvalue>
       </para></entry>
       </row>
      </tbody>
     </tgroup>
    </table>

    <para>
     <acronym>POSIX</acronym> regular expressions provide a more
     powerful means for pattern matching than the <function>LIKE</function> and
     <function>SIMILAR TO</function> operators.
     Many Unix tools such as <command>egrep</command>,
     <command>sed</command>, or <command>awk</command> use a pattern
     matching language that is similar to the one described here.
    </para>

    <para>
     A regular expression