Tom - the code you wrote works great:

if rng = "" or _
( isnumeric(left(rng,1)) and _
lcase(mid(rng,2,1)) < ucase(mid(rng(2,1))) then

I did find a typo in the last expression - (the one to the right of the
inequality):

ucase(mid(rng(2,1)) should be written as ucase(mid(rng,2,1))

Thanks again for your help.

Paul