Would it be correct to say that on a operation with a Non-regular language (L) with a Regular language will always return the language L?

I'm came across a property that when we intersect a non-regular language (say L) with Regular Language the resultant result will always remain in Language L.

$L \cap Reg = L$

Does this property extent to other operations too?

  • 1
    $\begingroup$ $L\cap \varnothing = \varnothing$. $\endgroup$ Jan 29, 2023 at 19:06

1 Answer 1


For most of the time one can't make sweeping statements about operations between any non-regular language and any regular language. The intuitive reasoning is that regular languages are a very broad collection of languages. Every non-regular language is the subset of a regular language, and every non-regular language has a regular subset (though for some, all regular subsets are finite). Given a regular language $A$ and a non-regular language $B$, you can't eg. know if any of the following are regular or not, without further information on the languages:

  • $A \cap B$ (intersection)
  • $A \cup B$ (union)
  • $A \cdot B$ or $B \cdot A$ (concatenation)
  • $A \setminus B$ or $B \setminus A$ (set difference)
  • Any of the above with either or both languages' complement
  • [countless other operations]

Symmetric difference, as pointed out by HendrikJan in the comments, is one of the few set operations that provably results in a non-regular language when applied between a regular $A$ and a non-regular $B$.

  • $\begingroup$ That's one reading of the question. On the other hand, if $\mathscr{F}$ is one of the four families in the Chomsky hierarchy and $R$ is a regular language, then $L\in\mathscr{F}\implies L\cap R\in \mathscr{F}$ (Also true for union, I believe.) $\endgroup$
    – rici
    Jan 30, 2023 at 8:10
  • $\begingroup$ @kviiri hey thanks for looking in to the question; my question was whether A op B = B [if A is reg & B is non-reg] or not, but from your answer I'm concluding that it is not really possible to find out whether that's the case $\endgroup$
    – h4kr
    Jan 30, 2023 at 8:55
  • $\begingroup$ @kviiri Symmetric difference seems an exception in your extensive list. First observe regular languages are closed under $\triangle$. Assume that $A$ is regular and $B$ nonregular. Then $A\triangle B$ must be nonregular. Otherwise $B = A\triangle (A\triangle B)$ would be regular. $\endgroup$ Jan 30, 2023 at 12:31
  • $\begingroup$ @HendrikJan Good addition. It was indeed my morning grogginess at play! $\endgroup$
    – kviiri
    Jan 30, 2023 at 13:38

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