1. Regular Language closed under union.
If L1 = Regular Language and L2 = Regular Language then L1 U L2 = Regular Language.
Example:
L1 = , L2 = and L3 = then
L= L1 U L2 U L3 = it is also regular language
2. Regular Language closed under intersection.
If L1= Regular Language and L2 = Regular Language then L1∩ L2 = Regular Language.
Example:
L1 = Mod 2 and L2 = Mod 4 then
L = L1 ∩ L2 = Mod 2 ∩ Mod 4 = Mod 4 = Regular Language
3. Regular Language closed under concatenation.
If L1 = Regular Language and L2 = Regular Language then L1 . L2 = Regular Language.
Example:
L1 = and L2 = then L = L1. L2 = . = Regular Language
4. Regular Language closed under Complement.
If L = Regular Language then = Regular Language
Example:
L = {At least three a i.e |a| ≥ 3 } then,
= = {At most two a i.e. |a| < 3 or |a| ≤ 2}
{At most two a i.e. |a| < 3 or |a| ≤ 2} = Regular Language
5. Regular Language closed under Kleene star operator.
If L = Regular Language then L*= Regular Language
Example:
L = then L* = = Regular Language
6. Regular Language closed under Set Difference.
If L1 = Regular Language and L2 = Regular Language then,
= Regular Language.
Example:
L1 = Mod 2 = {2, 4, 6, 8, 10, 12, 14, 16, 20…} and L2 = Mod 4 = {4, 8, 12, 16, 20….}
Now L = L1 - L2 = {2, 6, 14, ….} = Regular Language
Regular Expression Closed Under:
Union (U), Intersection (∩), Concatenation (.), Complement, Kleene star operator (*), Set Difference, Symmetric Difference (⊕), NOR, XNOR, Reversal, Homomorphism, Inverse, Homomorphism.