$a^*b^*c^* \setminus \{a^n b^n c^n | n ≥ 0\}$ is not regular using pumping lemma?
Lets assume towards contradiction that L is regular.
Then, by closure properties, we have $\bar L$ is regular, and thus also $\bar L \bigcap (a^*b^*c^*)$ is regular. But calculating it, we find out $\bar L \bigcap (a^*b^*c^*)=\{a^nb^nc^n\}$ is not regular.