<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 is a character sequence that is an
     abbreviated definition of a set of strings (a <firstterm>regular
     set</firstterm>).  A string is said to match a regular expression
     if it is a member of the regular set described by the regular
     expression.  As with <function>LIKE</function>, pattern characters
     match string characters exactly unless they are special characters
     in the regular expression language &mdash; but regular expressions use
     different special characters than <function>LIKE</function> does.
     Unlike <function>LIKE</function> patterns, a
     regular expression is allowed to match anywhere within a string, unless
     the regular expression is explicitly anchored to the beginning or
     end of the string.
    </para>

    <para>
     Some examples:
<programlisting>
'abcd' ~ 'bc'     <lineannotation>true</lineannotation>
'abcd' ~ 'a.c'    <lineannotation>true &mdash; dot matches any character</lineannotation>
'abcd' ~ 'a.*d'   <lineannotation>true &mdash; <literal>*</literal> repeats the preceding pattern item</lineannotation>
'abcd' ~ '(b|x)'  <lineannotation>true &mdash; <literal>|</literal> means OR, parentheses group</lineannotation>
'abcd' ~ '^a'     <lineannotation>true &mdash; <literal>^</literal> anchors to start of string</lineannotation>
'abcd' ~ '^(b|c)' <lineannotation>false &mdash; would match except for anchoring</lineannotation>
</programlisting>
    </para>

    <para>
     The <acronym>POSIX</acronym> pattern language is described in much
     greater detail below.
    </para>

    <para>
     The <function>substring</function> function with two parameters,
     <function>substring(<replaceable>string</replaceable> from
     <replaceable>pattern</replaceable>)</function>, provides extraction of a
     substring
     that matches a POSIX regular expression pattern.  It returns null if
     there is no match, otherwise the first portion of the text that matched the
     pattern.  But if the pattern contains any parentheses, the portion
     of the text that matched the first parenthesized subexpression (the
     one whose left parenthesis comes first) is
     returned.  You can put parentheses around the whole expression
     if you want to use parentheses within it without triggering this
     exception.  If you need parentheses in the pattern before the
     subexpression you want to extract, see the non-capturing parentheses
     described below.
    </para>

   <para>
    Some examples:
<programlisting>
substring('foobar' from 'o.b')     <lineannotation>oob</lineannotation>
substring('foobar' from 'o(.)b')   <lineannotation>o</lineannotation>
</programlisting>
   </para>

    <para>
     The <function>regexp_count</function> function counts the number of
     places where a POSIX regular expression pattern matches a string.
     It has the syntax
     <function>regexp_count</function>(<replaceable>string</replaceable>,
     <replaceable>pattern</replaceable>
     <optional>, <replaceable>start</replaceable>