invalid regular expression: invalid backreference number

I've got a function that generates usernames and passwords on insert if
they haven't yet been set.  The code block is:

   -- create a new username for new people
   IF (LENGTH(COALESCE(new_pp_username, '')) = 0) THEN
     LOOP
       gen_pp_username := LOWER(SUBSTRING(new_pp_first_name from 1 for
2)) || LOWER(SUBSTRING(new_pp_last_name from 1 for 8)) ||
round(random()*100);
       gen_pp_username := regexp_replace(gen_pp_username, E'\\W', '', 'g');
       EXIT WHEN ((SELECT COUNT(*) FROM people WHERE pp_username =
gen_pp_username AND pp_provisional_p='f') = 0);
     END LOOP;
   ELSE
     gen_pp_username := new_pp_username;
   END IF;

   -- create a new password if there is none
   IF (LENGTH(COALESCE(new_pp_password, '')) = 0) THEN
     chars :=
'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ1234567890';
     FOR i in 1..8 LOOP
       gen_pp_password := gen_pp_password || SUBSTRING(chars,
ceil(random()*LENGTH(chars)), 1);2007-02-15 15:32:57.264727500
     END LOOP;
   ELSE
     gen_pp_password := new_pp_password;
   END IF;

This used to work before my upgrade to 8.2.1.

The error the function now throws is:

<jross%wykids>ERROR:  invalid regular expression: invalid backreference
number
2007-02-15 15:32:57.264729500 <jross%wykids>CONTEXT:  SQL function
"substring" statement 1
2007-02-15 15:32:57.264730500   PL/pgSQL function "set_people" line 58
at assignment

I've futzed around with the various ways I can call substring, but I
don't understand why this is throwing the error.

Any help would be greatly appreciated!

Jeff Ross