>> What builtin quoted printable facilities?
>
> Look up the "M" format for Array.pack.

So here's the cheat solution:

class String
  def to_quoted_printable(*args)
    [self].pack("M").gsub(/\n/, "\r\n")
  end
  def from_quoted_printable
    self.gsub(/\r\n/, "\n").unpack("M").first
  end
end

(Just add my original if __FILE__ block to make it almost quiz-compatible)

And here's how it fares against my test suite:

Loaded suite TC_QuotedPrintable
Started
.............FF.FFFFFFF..
Finished in 0.39 seconds.

So it's 10 times the speed of my original one (against random binary data), but 
chops lines too early, ends up with 73- instead of 76-character lines. Of 
course, this one won't do XML.

Interestingly, if I use a gsub! instead of a loop with sub!s in my soft_break! 
method, I get a 5x speedup... and fail the same tests.

Cheers,
Dave