<literal>.</literal> and <literal>.*</literal> in POSIX regular
    expressions).
   </para>

   <para>
    In addition to these facilities borrowed from <function>LIKE</function>,
    <function>SIMILAR TO</function> supports these pattern-matching
    metacharacters borrowed from POSIX regular expressions:

   <itemizedlist>
    <listitem>
     <para>
      <literal>|</literal> denotes alternation (either of two alternatives).
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>*</literal> denotes repetition of the previous item zero
      or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>+</literal> denotes repetition of the previous item one
      or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>?</literal> denotes repetition of the previous item zero
      or one time.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>}</literal> denotes repetition
      of the previous item exactly <replaceable>m</replaceable> times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>,}</literal> denotes repetition
      of the previous item <replaceable>m</replaceable> or more times.
     </para>
    </listitem>
    <listitem>
     <para>
      <literal>{</literal><replaceable>m</replaceable><literal>,</literal><replaceable>n</replaceable><literal>}</literal>
      denotes repetition of the previous item at least <replaceable>m</replaceable> and
      not more than <replaceable>n</replaceable> times.
     </para>
    </listitem>
    <listitem>
     <para>
      Parentheses <literal>()</literal> can be used to group items into
      a single logical item.
     </para>
    </listitem>
    <listitem>
     <para>
      A bracket expression <literal>[...]</literal> specifies a character
      class, just as in POSIX regular expressions.
     </para>
    </listitem>
   </itemizedlist>

    Notice that the period (<literal>.</literal>) is not a metacharacter
    for <function>SIMILAR TO</function>.
   </para>

   <para>
    As with <function>LIKE</function>, a backslash disables the special
    meaning of any of these metacharacters.  A different escape character
    can be specified with <literal>ESCAPE</literal>, or the escape
    capability can be disabled by writing <literal>ESCAPE ''</literal>.
   </para>

   <para>
    According to the SQL standard, omitting <literal>ESCAPE</literal>
    means there is no escape character (rather than defaulting to a
    backslash), and a zero-length <literal>ESCAPE</literal> value is
    disallowed.  <productname>PostgreSQL</productname>'s behavior in
    this regard is therefore slightly nonstandard.
   </para>

   <para>
    Another nonstandard extension is that following the escape character
    with a letter or digit provides access to the escape sequences
    defined for POSIX regular expressions; see
    <xref linkend="posix-character-entry-escapes-table"/>,
    <xref linkend="posix-class-shorthand-escapes-table"/>, and
    <xref linkend="posix-constraint-escapes-table"/> below.
   </para>

   <para>
    Some examples:
<programlisting>
'abc' SIMILAR TO 'abc'          <lineannotation>true</lineannotation>
'abc' SIMILAR TO 'a'            <lineannotation>false</lineannotation>
'abc' SIMILAR TO '%(b|d)%'      <lineannotation>true</lineannotation>
'abc' SIMILAR TO '(b|c)%'       <lineannotation>false</lineannotation>
'-abc-' SIMILAR TO '%\mabc\M%'  <lineannotation>true</lineannotation>
'xabcy' SIMILAR TO '%\mabc\M%'  <lineannotation>false</lineannotation>
</programlisting>
   </para>

   <para>
    The <function>substring</function> function with three parameters
    provides extraction of a substring that matches an SQL
    regular expression pattern.  The function can be written according
    to standard SQL syntax:
<synopsis>
substring(<replaceable>string</replaceable> similar