Let S$=\{0,1,2,3,...,9,-,=\}$ and

SUB $=\{$x$-$y$=$z $|$ x$,$y$,$z are unsigned integers, and z is the difference of x and y$\}.$

For example, string $99-21=78$ is in SUB, while string $99-21=77$ is not in SUB.

Prove that SUB is not a regular language.