It is not so hard to transform it to use each character *at most once*. The only slight difficulty comes from the constraint that we cannot "declare and then use" variables, because then the code will contain the variable name twice. This restriction is worked around by the $. global variable, the best friend of Ruby golfers.
The relatively interesting part is to use all the charcters *at least once*. Of course, this is easily accomplished by putting everything into a comment (i.e., #unused...) or to a string literal (%(unused...), note that normal string literals are forbidden since they use quotation marks twice). Hey, but that's not fun at all! I tried to minimize the escapeway.
* "%r{\"}mosx". Regex literal, with %-syntax. I don't even know what each m,o,s,x means...
* "?'" Symbol literal. The quote characters (' " \`) are the first obstacle to this trial because they have to be used in pair usually. These are escaped as \" and ?' and :\`.
* "4>6" "3\_0-~$.+=9/2^5" "18\*7". I had to consume many arithmetic operators +-\*/^~<>, but I only have ten literals 0 to 9 and $. as operands. Besides I have to express the print loop. This is an interesting puzzle.
