**Sipser Exercise 1.29**

**Theorem**: The language is not regular.

*Proof*:
Suppose were regular and let be the constant guaranteed by the pumping lemma.
Take , noting that and .
Then let such that and .

Because , and is a prefix of , it must be the case that and consist entirely of ’s. In particular, for some . But then , which is not in , contradicting the pumping lemma, so is not regular.