The class of regular languages is closed under union, intersection, complement, concatenation, Kleene star, reversal, and homomorphism. These closure properties mean that operations on regular languages always yield regular languages, which is crucial for language composition and algorithm design.
From your study of regular languages, you know they are recognized by finite automata and described by regular expressions. Closure properties answer a natural follow-up question: if you take two regular languages and combine them using standard set or string operations, is the result still regular? The answer is yes for a remarkably wide range of operations, and this fact is one of the most powerful tools in formal language theory.
Start with the most intuitive operation: union. If L₁ and L₂ are both regular languages, then L₁ ∪ L₂ — the set of all strings in either language — is also regular. You can prove this constructively by building an NFA that nondeterministically chooses to simulate either the machine for L₁ or the machine for L₂. Concatenation (L₁ · L₂, all strings formed by appending a string from L₂ to a string from L₁) is also closed: connect the accepting states of the first machine to the start state of the second via ε-transitions. Kleene star (L*, zero or more concatenations of strings from L) works similarly — loop the accepting states back to the start. These three constructions mirror the three operators in regular expressions, which is no coincidence: the closure properties are essentially the algebraic reason regular expressions work.
Complement closure is more surprising and more useful. If L is regular, then the set of all strings not in L is also regular. The proof is elegant: take the DFA for L and simply swap accepting and non-accepting states. This works because a DFA processes every string to exactly one state — there is no ambiguity. Note this argument requires a DFA, not an NFA; for an NFA, swapping accept states does not compute the complement because a string might have both accepting and non-accepting paths. Intersection then follows immediately from complement and union via De Morgan's law: L₁ ∩ L₂ = complement(complement(L₁) ∪ complement(L₂)). Alternatively, you can build a product automaton that simulates both DFAs simultaneously and accepts only when both accept.
Why do these properties matter practically? They let you build complex language specifications from simple components. If you can recognize identifiers and you can recognize keywords, closure under set difference (complement of intersection) lets you recognize "identifiers that are not keywords" — and the result is guaranteed to still be regular, still recognizable by a finite automaton, still efficient. Closure properties also serve as a proof technique: to show a language is not regular, you can assume it is, combine it with a known regular language using a closed operation, and derive a contradiction (often via the pumping lemma). The closure properties thus form both a construction toolkit and a reasoning framework for the entire theory of regular languages.