How do I make a balanced capture between two character types?

Lets say I want to make a balanced capture between the occurrence of an uppercase an a lowercase letter.

words = "I WANT 9()?!., CHOCOLATE milk!"

using the balanced capture, %bxy, how would I match a pattern to correspond to the the the first occurrence of an uppercase letter, followed by a lowercase letter and everything inbetween?

My ideal solution does not work

for match in string.gmatch(words, "%b%u%l") do
    print (match)
    wait(.1)
end