rewrite HTML in response

Something like as follows right?

Taken from the creditcard scrubber.

when HTTP_RESPONSE {
   if { $scrub_content } {
      if { [HTTP::header exists "Content-Length"] } {
         set content_length [HTTP::header "Content-Length"]
         } else {
            set content_length 4294967295
         }
         if { $content_length > 0 } {
            HTTP::collect $content_length
         }
      }
}
when HTTP_RESPONSE_DATA {
    set old_link "www.somedomain.com/someuri"
    set new_link "www.somedomain1.com/somenewuri"
 
    if { [regsub -all {$old_link} [HTTP::payload] $new_link newdata] } {
       log "Replacing $old_link with $new_link"  
       HTTP::payload replace 0 [HTTP::payload length] $newdata  
    } else {
       log "replacing nothing"
   } 
} 

I think the issue is that I am not replacing the string correctly, the regsub seems like it should work, if it matches the old_link in the payload, replace etc. But I think I am off target.