BUG #3435: problem with substring function

The following bug has been logged online:

Bug reference: 3435
Logged by: Carlos Quintero
Email address: carlos(dot)quintero(dot)sspa(at)juntadeandalucia(dot)es
PostgreSQL version: 8.2.4
Operating system: Linux RedHat Enterprise 3
Description: problem with substring function
Details:

Hi,

I like to use a numerical expression as parameter for the substring
function, but it works always as i use regular expressions. Please, try
this:

select substring('1234' from to_number('3', '999999') for 3);

What returns a empty string:

substring
-----------

(1 fila)

But the right result must be this:

select substring('1234' from 3 for 3);

substring
-----------
34
(1 fila)

If i use other numbers, i got even some errors:

select substring('1234' from to_number('33', '999999') for 3);

ERROR: la expresin regular no es vlida: invalid backreference number
CONTEXTO: funcin SQL substring en la sentencia 1

This is a sample, really i like to use an expression using date_part() as
the first parameter for substring.

Best Regards,
Carlos