Regular Languages are closed under intersection, i.e., if L1 and L2 are regular then L1 ∩ L2 is also regular. L1 and L2 are regular • L1 ∪ L2 is regular • Hence, L1 ∩ L2 = L1 ∪ L2 is regular.
What's more, we've seen that regular languages are closed under union, concatenation and Kleene star. This means every regular expression defines a regular language.
Closure under Union For any regular languages L and M, then L ∪ M is regular. Proof: Since L and M are regular, they have regular expressions, say: Let L = L(E) and M = L(F). Then L ∪ M = L(E + F) by the definition of the + operator.
The set of regular languages is closed under complementation. The complement of language L, written L, is all strings not in L but with the same alphabet. The statement says that if L is a regular lan- guage, then so is L. To see this fact, take deterministic FA for L and interchange the accept and reject states.
What are closure properties of regular languages? Regular languages are closed under complement, union, intersection, concatenation, Kleene star, reversal, homomorphism, and substitution.
A regular language is one which has an FA or an RE. Regular languages are closed under union, concatenation, star, and complementation.
Regular Languages are closed under complementation, i.e., if L is regular then L = Σ∗ \ L is also regular. Proof.
Formal definition If A is a regular language, A (Kleene star) is a regular language. Due to this, the empty string language {ε} is also regular. If A and B are regular languages, then A ∪ B (union) and A • B (concatenation) are regular languages. No other languages over Σ are regular.
Concatenation. If L1 and L2 are languages, then the concatenation of the two languages, L = L1 · L2, is the set of all strings of the form x1x2 where x1 ∈ L1 and x2 ∈ L2. Theorem If L1 and L2 are regular languages, then the new language L = L1 · L2 is regular.