Allowable characters in TXT rdata?

[Darren Pilgrim]

I'm working on a tinydns-to-bind conversion script and I'm having
trouble finding information on what BIND will and will not allow in the
rdata field of a TXT record.  The only things I have found say that TXT
rdata is a "character-string", yet make no qualifications of what is
and isn't a valid character.  Starting with the US ASCII character set,
for example, what is the valid representation for characters whose 
ASCII values are <32 or >126?  How do I represent such characters in a
BIND zone file?  Are there any characters in the 32-126 range that also
require non-literal representation?