In this proof, we will show that regularity is not only possible, but inevitable.
1. Let R be a regular language.
2. Then R is a subset of the language { w ∈ Σ* | w is in R ∪ {a}}
3. By definition, R = {a} ∪ (R \ {a})
4. By induction, R = {a, a, a, ...}
5. Therefore, R is regular.
By the previous proof, all languages are regular.