sed snippet?

[JT Williams]

Oh venerable sed studs,

I seek a sleek sed cameo to fill in the missing digits
of a page number, e.g., transform the page-number string

11331-8

into the string

11331--11338

i.e., fill in the missing digits for the second page number
and TeXify the separator from - to --.  (I've managed to do
it with a horrible table lookup algorithm, but there must be
a better way.)

I expect you get lots of requests for snippets, so please do
not feel obligated to reply.

jt williams