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LEXICAL ANALYZER
By : Kiran Acharya
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TOPICS COVERED Introduction to lexical Analyzer Input buffering Specifications of tokens Regular expressions
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REGULAR EXPRESSIONSThere are two basic rules in regular
expression.1. ɛ is a regular expression and L(ɛ), that
is the language whose sole number is an empty string.
2. If a is a symbol in Ʃ , then a is a regular expression with one string of length one , with a at its first position.
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INDUCTION (r)|(s) is a regular expression denoting
L(r)ỤL(s). (r)(s) is a regular expression means
L(r)L(s). (r)* means regular expression(L(r)).
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Unary operator * has the highest
precedence and its left associative. Concatenation has second and its left
associative. | has the lowest and left associative.
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Language that can be defined by regular
expression is called regular set if regular expression r and s are from
same set they are equivalent. (a|b)=(b|a)
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REGULAR DEFINITIONS It’s a sequence of definitions of the
form: d1→r1;
d2→r2; each d is a new symbol r is regular expression over alphabets.
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EXTENSION OF REGULAR EXPRESSION Kleene closure extended in 50’s One or more instances Zero or one instance. Character class.
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RECOGANITION OF TOKENS Taking the patterns from tokens and
build piece of code that examines the input find the prefix that is the lexeme matching one of the pattern.
Methods:1. Transition Diagrams2. Recognition of reverse words and
identifier3. Completion of running example.
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TRANSITION DIAGRAMS These are first flow charts. Conversion from patterns to transition
diagram. It has states. Edges input
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NOTE ON TD First and final state Accepting state Start state
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RECOGNITION OF REVERSE WORDS AND IDENTIFIER Finding keywords and identifiers are the
problem.Return(gettoken(),installid())
0 10 11start
letter
Letter or digit
other
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LEXICAL ANALYZER GENERATOR Tool lex Input is lex language tool itself is a lex
compiler. Input file is lex.l Compiler transform it into c program Lex.yy.c And later the file is compiled by c to
a.out
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STRUCTURE OF LEX PROGRAM Declarations: %% Translation rules %% Auxiliary functions
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CONFLICT RESOLUTION IN LEX Always prefer a longer prefix over the
shorter It longer matches the two or more
patterns then prefer the pattern listed first.
Look Ahead operator: / is inserted to know the end of the part of lexeme.
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FINITE AUTOMATE The heart of the transition of lex turning
input program to lexical analyzer is finite automata.
Finite automata are recognizers they just say yes or no.
Two types:1. Non deterministic2. Deterministic
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NON DETERMINISTIC No restrictions to the edges from the
same state. Finite state of state s Input alphabet Ʃ Transition function Start state Final state
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TRANSITION TABLEState a b ɛ
0 {0,1} {0} ф
1 ф {2} ф
2 ф {3} ф
3 ф ф Ф
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ACCEPTANCE OF INPUT STRING BY NFA aabb (a|b)* abb
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DFA There is exactly one edge form the input
to the next state.
