[exim] Subject from the .vacation.msg

My users would like to customize their Subject line in their
.vacation.msg. I found this post which comes close to what I need:

http://www.exim.org/lurker/message/20080301.212100.fb8dc5ea.fi.html

But it won't work unless everyone adds the Subject line. I am looking
for an "if there's a Subject line in .vacation.msg use it, else use
generic Auto-Reply Re: $h_subject". I would like some feedback on this
vacation transport and see if anyone can help refine it. The regexes
seem to do what I expect in pcretest, but I am not very confident about
the if match syntax:

uservacation_transport:
   driver = autoreply
   user = ${local_part}
   mode = 700
   file = ${home}/.vacation.msg
   file_expand
   once = ${home}/.vacation.db
   log = ${home}/.vacation.log
   once_repeat = 7d
   from = $local_part@$domain
   to = $sender_address
   address_data = ${readfile{${home}/.vacation.msg}}
             # If there is a subject: line
   subject = ${if {match {$address_data}{^[Ss]ubject:.*\n.*}}\
             # True, so use it
             {${sg {$address_data}{^([Ss]ubject:.*)\n(.*)}{\$1}}}\
             # False, use generic subject
             {"Auto-Reply Re: $h_subject"}}
          # If there is a subject: line
   text = ${if {match {$address_data}{^[Ss]ubject:.*\n.*}}\
          # True, so use everything but that line
          ${sg {$address_data}{^([Ss]ubject:.*)\n(.*)}{\$2}}}\
          # False, use the raw .vacation.msg
          {$address_data}}

Thanks

Matt