Let $M_n$ denote the set of strings consisting of just $1$'s whose length is a multiple of $n$; i.e.

Clearly, each $M_n$ is regular. Additionally, recall that if $A$ and $B$ are regular languages, then the intersection $A \cap B$ and the complement $\bar{A}$ are regular.

By the fundamental theorem of arithmetic, a number is a power of $2$ exactly when $2$ is the only prime number that divides it, so if there were finitely many primes $\{2, 3, \dots, p\}$, then the set

of strings of $1$'s whose length is a power of $2$ is regular as $A$ is equal to the intersection $M_2 \cap \overline{M_3} \cap \dots \cap \overline{M_p}$ of (finitely many) regular languages.

However, we can show $A$ is not regular by using the pumping lemma. For $k \geq 0$, then if we let $w = 1^{2^k}$, then $w \in A$ and $|w| = 2^k \geq k$. If $w=xyz$ for strings $x, y, z$ with $|xy| \leq k$ and $|y| \geq 1$, then as $|xy^2z| = |xyz| + |y|$, $|xyz| = 2^k$, and $|y| \geq 1$, then $|xy^2z| > 2^k$. Additionally, as $|xy| \leq k < 2^k$ and $|xy| + |z| = |xyz|$, then $|z| > 0$ so

Therefore, we have $2^k < |xy^2z| < 2^{k+1}$, so the length of $xy^2z$ cannot be a power of two and thus $xy^2z \notin A$. Thus, by the pumping lemma, $A$ is not regular so there must be infinitely many primes.

This post was adapted from u/JoshuaZ1's comment thread in r/math on different ways of proving the infinitude of primes.