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Context-Free Grammars

FormalismDerivations

Backus-Naur FormLeft- and Rightmost Derivations

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Informal Comments

A context-free grammar is a notationfor describing languages.It is more powerful than finiteautomata or REs, but still cannot defineall possible languages.

Useful for nested structures, e.g.,parentheses in programming languages.

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Informal Comments (2)

Basic idea is to use variables to standfor sets of strings (i.e., languages).These variables are defined recursively,in terms of one another.Recursive rules (productions) involveonly concatenation. Alternative rules for a variable allowunion.

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Example : CFG for { 0 n1n | n > 1}

Productions:S -> 01S -> 0S1

Basis : 01 is in the language.Induction : if w is in the language, thenso is 0w1.

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CFG Formalism

Terminals = symbols of the alphabetof the language being defined.Variables = nonterminals = a finiteset of other symbols, each of whichrepresents a language.

Start symbol = the variable whoselanguage is the one being defined.

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Productions

A production has the form variable ->string of variables and terminals .

Convention : A, B, C, are variables. a, b, c, are terminals.

, X, Y, Z are either terminals or variables. , w, x, y, z are strings of terminals only.

, , , are strings of terminals and/orvariables.

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Example : Formal CFG

Here is a formal CFG for { 0 n1n | n > 1}.Terminals = {0, 1}.

Variables = {S}.Start symbol = S.

Productions =S -> 01S -> 0S1

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Derivations Intuition

We derive strings in the language of aCFG by starting with the start symbol,and repeatedly replacing some variable

A by the right side of one of itsproductions.

That is, the productions for A are thosethat have A on the left side of the ->.

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Derivations Formalism

We say A => if A -> is aproduction.Example : S -> 01; S -> 0S1.S => 0S1 => 00S11 => 000111.

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Iterated Derivation

=>* means zero or more derivationsteps. Basis : =>* for any string .Induction : if =>* and => , then

=>* .

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Example : Iterated Derivation

S -> 01; S -> 0S1.S => 0S1 => 00S11 => 000111.So S =>* S; S =>* 0S1; S =>* 00S11;S =>* 000111.

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Sentential Forms

Any string of variables and/or terminalsderived from the start symbol is called asentential form .Formally, is a sentential form iff S =>* .

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Language of a Grammar

If G is a CFG, then L(G), the language of G , is {w | S =>* w}.

Note : w must be a terminal string, S is thestart symbol.

Example : G has productions S -> and

S -> 0S1.L(G) = {0 n1n | n > 0}. Note : is a legitimate

right side.

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Context-Free Languages

A language that is defined by someCFG is called a context-free language .There are CFLs that are not regularlanguages, such as the example justgiven.

But not all languages are CFLs. Intuitively : CFLs can count two things,not three.

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BNF NotationGrammars for programming languagesare often written in BNF ( Backus-Naur

Form ). Variables are words in ; Example :.

Terminals are often multicharacterstrings indicated by boldface orunderline; Example : while or WHILE.

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BNF Notation (2)

Symbol ::= is often used for ->.Symbol | is used for or.

A shorthand for a list of productions withthe same left side.

Example : S -> 0S1 | 01 is shorthandfor S -> 0S1 and S -> 01.

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BNF Notation Kleene Closure

Symbol is used for one or more. Example : ::= 0|1|2|3|4|5|6|7|8|9

::= Note: thats not exactly the * of REs.

Translation : Replace with a newvariable A and productions A -> A | .

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Example : Kleene Closure

Grammar for unsigned integers can bereplaced by:

U -> UD | DD -> 0|1|2|3|4|5|6|7|8|9

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BNF Notation: Optional Elements

Surround one or more symbols by []to make them optional.

Example : ::= if then [; else ]

Translation : replace [ ] by a newvariable A with productions A -> | .

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Example : Optional Elements

Grammar for if-then-else can bereplaced by:

S -> iCtSA A -> ;eS |

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BNF Notation Grouping

Use {} to surround a sequence of symbols that need to be treated as a

unit.Typically, they are followed by a for

one or more.

Example : ::= [{;}]

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Translation : Grouping

You may, if you wish, create a newvariable A for { }.

One production for A: A -> .Use A in place of { }.

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Example : Grouping

L -> S [{;S}] Replace by L - > S [A] A -> ;S

A stands for {;S}.Then by L -> SB B - > A | A -> ;S

B stands for [A] (zero or more As). Finally by L -> SB B -> C | C -> AC | A A -> ;S

C stands for A .

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Leftmost and RightmostDerivations

Derivations allow us to replace any of the variables in a string.

Leads to many different derivations of the same string.By forcing the leftmost variable (oralternatively, the rightmost variable) tobe replaced, we avoid these

distinctions without a difference.

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Leftmost Derivations

Say wA => lm w if w is a string of terminals only and A -> is a

production. Also, =>* lm if becomes by asequence of 0 or more => lm steps.

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Example : Leftmost Derivations

Balanced-parentheses grammmar:S -> SS | (S) | ()

S => lm SS => lm (S)S => lm (())S => lm (())()Thus, S =>* lm (())()S => SS => S() => (S)() => (())() is aderivation, but not a leftmost derivation.

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Rightmost Derivations

Say Aw => rm w if w is a string of terminals only and A -> is a

production. Also, =>* rm if becomes by asequence of 0 or more => rm steps.

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Example : Rightmost Derivations

Balanced-parentheses grammmar:S -> SS | (S) | ()

S => rm SS => rm S() => rm (S)() => rm (())()Thus, S =>* rm (())()S => SS => SSS => S()S => ()()S =>()()() is neither a rightmost nor aleftmost derivation.