5. [20 points] Not regular. Let the alphabet & be {0,...,9,--=}. Define SUBTRACTION = {w/w is a string 2 - y = 2 with r, y, z decimal numbers such that the equation is correct}. Leading O's are allowed. E.g., 13-007 = 6 E SUBTRACTION, 17 = 17 SUBTRACTION. Prove that SUBTRACTION is not regular