# Lecture 09

## Regular Languages

Built from

• finite language
• union
• concatenation
• repetition

#### Union

$$L_1 \cup L_2 = \{ x | x \in L_1 \lor x \in L_2\}$$

#### Concatenation

$$L_1\cdot L_2=\{xy | x \in L_1 \land y \in L_2\}$$

#### Repetition

$$L^* = \{\epsilon\} \cup \{ xy | x \in L^* \land y \in L\}$$

$$= \{\epsilon\} \cup L \cup L^2 \cup L^3 ...$$

0 or more occurrences of a word L

e.g. Show: $$\{a^{2n} b | b\geq 0 \}$$ is regular

$$(\{aa\})^*\cdot\{b\}$$

## Regular Expressions

Short hand for regular languages

Language Regular Expressions
$$\{\}$$ $$\varnothing$$ empty language
$$\{\epsilon\}$$ $$\epsilon$$ language consisting of the empty word
$$\{aaa\}$$ $$aaa$$ singleton language
$$L_1\cup L_2$$ $L_1 L_2$
$$L_1\cdot L_2$$ L_1L_2 concatenation
$$L^*$$ $$L^*$$ repetition

#### Is C regular?

A C program is a sequence of token, each of which comes from a regular language.

$$C\in \{$$Valid C Tokens$$\}^*$$

How can we recognize an arbitrary regular expression automatically?

e.g. $$\{a^{2n} b | b\geq 0 \}=(aa)^*b$$

e.g. MIPS labels

## Deterministic Finite Automaton (DFA)

• always start at start state
• for each char in the input, follow the corresponding arc to the next state
• if on an accepting state when input is exhausted, accept, else reject.

#### What if there is no transition?

• Fall of the machine = reject.
• More formally, an implicit "error" state all transitions go there.

No escape from error state, non-accepting.

Example: String over $$\{a,b\}$$ with an even number of $$a$$s and an odd number of $$b$$s

## Formal Definition of DFA

A DFA is a 5-tuple $$(\Sigma, Q, q, A, \delta)$$ where

• $$\Sigma$$ is a finite non-empty set (alphabet)
• $$Q$$ is a finite non-empty set (states)
• $$q_0 \in Q$$ (start state)
• $$A \in Q$$ (accepting state)
• $$\delta: Q \times \Sigma \rightarrow Q$$ (transition function: state + input char -> state)

$$\delta$$ consume one character

• can extend $$\delta$$ to a function that consumes an entire word

#### Definition

$$\delta ^*(q,\epsilon)=q$$
$$\delta ^*(q, cw)=\delta ^*(\delta(q,c), w)$$

We say a DFA $$(\Sigma, Q, q_0, A, \delta)$$ accepts a word $$w$$ if $$\delta ^*(q_0, w)\in A$$

If $$M$$ is a DFA, we denote by $$L(M)$$ ("the language of M") the set of all strings accepted by $$M$$: $$L(M)=\{w | M\enspace accepts\enspace w\}$$

### Kleene Theorem

$$L$$ is regular iff $$L=L(M)$$ for some DFA $$M$$. (The regular languages are the languagges accepted by DFAs)