request for isprint character class code in string.gsub patterns

[David Jones <drj@...>]

I'm writing a function to quote a string, that is, it takes a string 
and returns a string that when printed is acceptable as a lua literal 
(and the literal is equal to the passed string).

Has anyone already written such a code?  (And is it freely available)  
It seems like such a commonly needed function.

There's a bit in this function where it expands unprintable codes to 
the \nnn representation.  It currently looks a bit like this:

string.gsub("\7ab","[^ %p%w]", function(x)return 
'\\'..string.byte(x)end)

What I want to do is match all non-printable characters.  C provides 
this concept via isprint and isgraph, but in Lua I have to use the 
union of " ", "%p" (punctuation), and "%w" (alphanumerics).  Which 
relies on isprint being a union of space, isprint, and isalnum (which 
the C standard guarantees, but it seems tenuous).

Would it be possible to have another % code for the isprint character 
class? %t perhaps (for typeable or text)?  From a completeness aspect I 
suppose we should also have one for isgraph.  Or do The Implementors 
think that the current situation, synthesis via %w and %p, is ok?

David Jones