In short no there is no way to do this nicely using the robots.txt standard. Remember the Disallow specifies a path prefix. Wildcards and allows are non-standard.
So the following approach (a kludge!) will work.
User-agent: *
Disallow: /a
Disallow: /b
Disallow: /c
...
Disallow: /z
Disallow: /A
Disallow: /B
Disallow: /C
...
Disallow: /Z
Disallow: /0
Disallow: /1
Disallow: /2
...
Disallow: /9
