parameter</primary>
     </indexterm>
    </term>
    <listitem>
     <para>
      <varname>isn.weak</varname> enables the weak input mode, which allows
      ISN input values to be accepted even when their check digit is wrong.
      The default is <literal>false</literal>, which rejects invalid check
      digits.
     </para>
    </listitem>
   </varlistentry>
  </variablelist>

  <para>
   Why would you want to use the weak mode? Well, it could be that
   you have a huge collection of ISBN numbers, and that there are so many of
   them that for weird reasons some have the wrong check digit (perhaps the
   numbers were scanned from a printed list and the OCR got the numbers wrong,
   perhaps the numbers were manually captured... who knows). Anyway, the point
   is you might want to clean the mess up, but you still want to be able to
   have all the numbers in your database and maybe use an external tool to
   locate the invalid numbers in the database so you can verify the
   information and validate it more easily; so for example you'd want to
   select all the invalid numbers in the table.
  </para>

  <para>
   When you insert invalid numbers in a table using the weak mode, the number
   will be inserted with the corrected check digit, but it will be displayed
   with an exclamation mark (<literal>!</literal>) at the end, for example
   <literal>0-11-000322-5!</literal>.  This invalid marker can be checked with
   the <function>is_valid</function> function and cleared with the
   <function>make_valid</function> function.
  </para>

  <para>
   You can also force the insertion of marked-as-invalid numbers even when not
   in the weak mode, by appending the <literal>!</literal> character at the
   end of the number.
  </para>

  <para>
   Another special feature is that during input, you can write
   <literal>?</literal> in place of the check digit, and the correct check digit
   will be inserted automatically.
  </para>
 </sect2>

 <sect2 id="isn-examples">
  <title>Examples</title>

<programlisting>
--Using the types directly:
SELECT isbn('978-0-393-04002-9');
SELECT isbn13('0901690546');
SELECT issn('1436-4522');

--Casting types:
-- note that you can only cast from ean13 to another type when the
-- number would be valid in the realm of the target type;
-- thus, the following will NOT work: select isbn(ean13('0220356483481'));
-- but these will:
SELECT upc(ean13('0220356483481'));
SELECT ean13(upc('220356483481'));