# Is there a subset of a non regular language that is regular

I am just curious is there any non regular language whose subset is regular?

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Sure, any finite subset of a language is regular. Or the set of palindromes in $\{a,b\}$ is irregular but the set of palindromes in one letter is regular, if you need an infinite example. – Thomas Andrews Nov 13 '12 at 18:23
@Thomas what do you mean by a set of palindromes in one letter. Can u give an example – user34790 Nov 13 '12 at 18:25
Palindromes in one letter are just any $a*$ or $b*$. Basically, any word with only one letter is a palindrome. – Thomas Andrews Nov 13 '12 at 18:25
Ouch! You've asked 21 questions, and accepted only a handful of answers. If you have received answers which have been particularly helpful, please take the time to show appreciation by accepting such answers. – amWhy Nov 13 '12 at 18:37
I can provide a cheat sheet from my "Theoretical Computer Science" course (in German). Just have a look at the top picture with all the boxes. From top to bottom (on thr right hand side) the descriptions are: all languages $\Sigma^*$, semi-decidable (Type 0), decidable, LOOP-decidable, NP, P, context free (Type 2), regular (Type 3), finite. Hope this might help :) – Christian Ivicevic Nov 13 '12 at 18:53

The empty language is a regular subset of any language at all.

More generally, every finite subset of any language is regular.

For example, let $L$ be the language $\{ a^n | n\text{ is prime} \} = \{ aa, aaa, aaaaa, a^7, \ldots\}$. Let $R$ be the subset of $L$ consisting just of $\{ aa, aaaaa, a^{11}, a^{109} \}$. Then $L$ is not regular, but $R$ is.

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Can u give an example? – user34790 Nov 13 '12 at 18:25
@user34790: Every finite language is regular because you can create a enumerate all elements extensionally. Furthermore there exists a regular expression of the form "word1 | word2 | ... | wordN" which implies the regularity of all finite languages. – Christian Ivicevic Nov 13 '12 at 18:56
@MJD How is R regular? – user34790 Nov 13 '12 at 19:57
Every finite set is regular. math.stackexchange.com/questions/216047/… – MJD Nov 13 '12 at 20:03

You are looking for regular "within" non-regular. Not an answer, but related, and more amusing is the following.

There operations on languages that (however complex the language) yield a regular language. As an example there is the operator "all subsequences" (sometimes called "sparse subwords"). Starting with $\{ a^n b^{n^2}\mid n\ge 1\}$ we get $a^* b^*$.

Shallit (in his book second course in formal language theory) starts with the languages of primes in ternary notation $\{ 2,10,12,21,102,111,122,201,212,1002,\dots\}$, and shows this gives the subsequence language $\Sigma^*2\Sigma^* \cup \Sigma^*1\Sigma^*0\Sigma^* \cup \Sigma^*1\Sigma^*1\Sigma^*1\Sigma^*$, $\Sigma = \{0,1,2\}$.

Nice, huh?

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For example {a^n b^n| n>=0} is a non-regular language, but it has infinite number of subsets that are regular languages: 1) the empty set, 2) {lambda} 3) {ab} 4) {a^2 b^2} . . . and any finite combinations of these languages.

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