[Exim] ${sg operator in a delivery

I thought I'd use the sg operator to double check that some strings
being passed to other programs to do deliveries don't contain evil
characters.

My first test was with procmail to replace any non-alnum and a few
other characters with nothing;

command = "procmail \
           -a ${sg {${substr_1:${local_part_suffix}}}{[^a-zA-Z0-9+._\\$-]}{}} \
           -d ${local_part}"

Which gave me;

2000-07-05 18:23:09 139xZU-0006nG-00 == pir@??? T=procmail_pipe defer
(0): Expansion of "${sg" from command "procmail -a ${sg
{${substr_1:${local_part_suffix}}{[^a-zA-Z0-9+._\$-]}{}} -d ${local_part}"
in procmail_pipe transport failed: missing or misplaced { or }

A count of {} seems to be the same number and in the right places to
me. ANything obviously stupid I'm doing ?

P.

-- 
pir                  pir@???                    pir@???