[exim] Headers

I am trying to remove the word spam from a subject line this is what I

tried.
headers_add = "X-Filter: $primary_hostname   \n\
                           new-subject: ${sg{\[SPAM\]}{h_subject:}} \n\
                            Subject: $h_new-subject:"
I get
 router failed to expand "X-Filter: $primary_hostname \nnew-subject: 
${sg{[SPAM]}{$h_subject}} \nSubject: $h_new-subject:" missing or 
misplaced { or }
does anyone have experience with this that works and  would like to 
share how it does?