This a powerpoint presentation about the lexical analuser

1. By : Kiran Acharya
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2. Introduction to lexical Analyzer
Input buffering
Specifications of tokens
Regular expressions
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3. There 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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4. (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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5. 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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6. 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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7. 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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8. Kleene closure extended in 50’s
One or more instances
Zero or one instance.
Character class.
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9. 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 Diagrams
2. Recognition of reverse words and identifier
3. Completion of running example.
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10. These are first flow charts.
Conversion from patterns to transition
diagram.
It has states.
Edges
input
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11. First and final state
Accepting state
Start state
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12. Finding keywords and identifiers are the
problem.
Return(gettoken(),installid())
0 10 11
start
letter
Letter
or digit
othe
r
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13. 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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14. Declarations: %%
Translation rules %%
Auxiliary functions
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15. 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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16. 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 deterministic
2. Deterministic
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17. 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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18. State a b ɛ
0 {0,1} {0} ф
1 ф {2} ф
2 ф {3} ф
3 ф ф Ф
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19. aabb
(a|b)* abb
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20. There is exactly one edge form the input to
the next state.
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