# Lecture 11 Context-Free Grammar Lecture 11 Context-Free Grammar Definition A context-free grammar (CFG) G is a quadruple (V, , R, S) where V: a set of non-terminal symbols : a set of terminals (V = ) R: a set of rules (R: V (V U )*) S: a start symbol. Example V = {q, f,}

= {0, 1} R = {q 11q, q 00f, f 11f, f } S=q (R= {q 11q | 00f, f 11f | }) How do we use rules? If A B, then xAy xAy derivates xBy. xBy and we say that

If s t, then we write s * t. A string x in * is generated by G=(V,,R,S) if S * x. L(G) = { x in * | S * x}. Example G = ({S}, {0,1}. {S 0S1 | }, S) in L(G) because S .

01 in L(G) because S 0S1 01. 0011 in L(G) because S 0S1 00S11 0011. n n n n 0 1 in L(G) because S * 0 1 . n n L(G) = {0 1 | n > 0} Context-Free Language (CFL) A language L is context-free if there exists a CFG G such that L = L(G).

Theorem For every regular set, there exists a CFG G such that L=L(G). Proof. Let L=L(M) for a DFA M=(Q, , , s, F). Construct a CFG G=(V, , R, S) as follows. V = Q, = , R = { q ap | (q,a) = p } U { f | f in F}, S = s. x1

s xn q1 S f=qn x q x x q x1xnf x1xn 1 1

1 2 2 x in L(M) There is a path associated with x from initial state to a final state. S

*x Therefore, L(M) = L(G). Corollary Every regular language is a CFL. The class of regular languages is a proper subclass of CFLs. CFL Regular Why, proper?

Regular Grammar Regular grammar is a CFG (V, , R, S) such that every rule is in form V *(V+) Example G = ({S, A}, {0, 1}, {S 1A, A 00}, S) Remark: Every regular language can be generated by a regular grammar. Theorem Every regular grammar generates a regular language. Proof.

Consider a regular grammar G=(V, , R, S). Construct a string-labeled digraph with vertex set V U {f} as follows: For each rule A xB, x in * and B in V, x draw an edge A B. x For each rule A x, x in *, draw an edge A f Example 0

S G = ({S,A}, {0,1}, {S0S | 10A, A00}, S) 10 A 00 f This string-labeled digraph with initial state S and a final state f is a state diagram of an NFA M.

S * x in * There is a path associated with x from S to f in M. Therefore, L(G) = L(M). Corollary A language L is regular if and only if L can be generated by a regular grammar.

Right-Linear and Left-Linear The regular grammar is also called a rightlinear grammar. A grammar G=(V, , R, S) is left-linear if every rule is in form V (V+)*. (e.g., ({S,A}, {0, 1}, {SA01, A10}, S) Remark: Every language generated from a left-linear grammar is regular. Why? Why? For left-linear grammar G = (V, , R, S), R

R construct G = (V, , R , S) where R R R = {AW | AW in R}. R R G is right-linear. Hence, L(G ) is regular. R R Therefore, L(G) = L(G ) is regular. Example 1 G = ({S,A}, {0, 1}, {SA01, A10}, S)

R G =({S,A}, {0, 1}, {S 10A, A 01}, S) R NFA accepts L(G ) S R L(G )={1001} L(G)={1001} 10 A

01 Example 2 L(G) = 0*1 R L(G ) = 10* NFA accepts 10* S R 1

A 0 G = ({S,A}, {0,1}, {S 1A, A 0A|}, S) G = ({S,A}, {0,1}, {S A1, A A0|}, S)