How would I prove that the regular expression rejects string? Prove that the regular expression $\Sigma\Sigma(ab\cup ba)^∗a$ rejects the string $aabaabba$.
Would this be because the union of $ab$ and $ba$ would not accept the string $aabaabba$? I'm just confused on how to explain it.
Any word accepted by this regular expression has to be either in length 3 or to end with $aba$ or $baa$.
Try to prove this fact, and use it to show that $aabaabba$ must be rejected.