[Ned Freed]:
  > while this is easy to explain and understand, my original
  > suggestion was
  >
  >    ${1} - String before the last a
  >    ${2} - "a"
  >    ${3} - String after the last a.
  >
  > which I now realise may have merit when matching two lists
  > against eachother or when using the anyof test.
  
  Hmm. OK, I can see some utility here. Although I have to say this
  is getting awfully tricky and I'd rather see such stuff done with
  two separate tests.
you're right, the added expressiveness is not worth the complication.
if N tests is deemed unacceptable by the user, he will probably find
that :regex is available in most implementations anyway.
-- 
Kjetil T.