# Proving that a certain language is regular using pumping lemma

Let $$\Sigma = \{a,b,c,\ldots,x,y,z\}$$ be the Latin alphabet, consisting of 26 letters. Consider the language $$L$$ of all words $$\alpha$$ over $$\Sigma$$ satisfying the following constraints:

• If $$\alpha$$ contains $$a$$ then it contains exactly $$4$$ many $$a$$s.
• If $$\alpha$$ contains $$b$$ then it contains exactly $$8$$ many $$b$$s.
• ...
• If $$\alpha$$ contains $$`$$ then it contains exactly $$2^{27}$$ many $$z$$s.

How do I prove that $$L$$ is regular?

Note: I did research and I have found that we can use the pumping lemma.

• It seems unlikely that you can show that the language is regular by using the pumping lemma since it is a necessary but not sufficient condition for a language to be regular. Usually the pumping lemma is shown not to hold for some language $L$, thus proving that $L$ cannot be regular. Try to write your language as a finite union of regular languages. Jan 12, 2021 at 23:23

You cannot use the pumping lemma to show that a language is regular. The pumping lemma gives a property of regular languages: if $$L$$ is regular then $$L$$ can be "pumped". You can use this to show that $$L$$ is not regular: if $$L$$ cannot be pumped, then it is not regular. But you cannot use it to show that $$L$$ is regular: if $$L$$ can be pumped, it doesn't follow that $$L$$ is regular; moreover, there are examples of pumpable languages which are not regular. There are extensions of the pumping lemma which can be used to show that a language is regular, but they are not so useful in practice.
In your case, every word in $$L$$ contain $$0$$ or $$4$$ many $$a$$s, $$0$$ or $$8$$ many $$b$$s, and so on. In particular, it contains at most $$4$$ many $$a$$s, at most $$8$$ many $$b$$s, and so on. Therefore every word in $$L$$ has length at most $$4+8+\cdots+2^{27}$$. Consequently, $$L$$ is finite. Every finite language is regular.