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Overview of LEX and YACC - Department of Computer Engineering

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This presentation is an Overview of LEX and YACC. Its presented by Professor Deptii Chaudhari from the department of Computer Engineering at International Institute of Information Technology, I²IT. The presentation includes topics such as what is Lex and Yacc, LEX Program Structure, Pattern Matching Primitives, Operation of yylex(), Lex Predefined Variables and Structure of YACC Program. – PowerPoint PPT presentation

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Date added: 2 September 2019
Updated: 2 September 2019
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Title: Overview of LEX and YACC - Department of Computer Engineering


1
Systems Programming Operating
Systems Unit III
International Institute of Information
Technology, Pune Department of Computer
Engineering
  • Case Study Overview of LEX and YACC
  • Prof. Deptii Chaudhari
  • Assistant Professor
  • Department of Computer Engineering

2
LEX YACC
  • What is Lex?
  • Lex is officially known as a "Lexical Analyser".
  • It's main job is to break up an input stream into
    more usable elements.
  • Or in, other words, to identify the "interesting
    bits" in a text file.
  • What is Yacc?
  • Yacc is officially known as a "parser".
  • In the course of it's normal work, the parser
    also verifies that the input is syntactically
    sound.
  • YACC stands for "Yet Another Compiler Compiler".
    This is because this kind of analysis of text
    files is normally associated with writing
    compilers.

Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
3
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
4
LEX Program Structure
C global variables, prototype, Comments
Definitions
Production Rules
------------------------------------
User Subroutine Section (Optional)
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
5
  • In the rules section, each rule is made up of two
    parts a pattern and an action separated by
    whitespace.
  • The lexer that lex generates will execute the
    action when it recognizes the pattern.
  • The user subroutine section, consists of any
    legal C code.
  • Lex copies it to the C file after the end of the
    lex generated code.
  • Lex translates the Lex specification into C
    source file called lex.yy.c which we compile and
    link with lex library ll.
  • Then we can execute the resulting program to
    check that it works as we expected.

Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
6
Example
  • include ltstdio.hgt
  • 0123456789 printf("NUMBER\n")
  • a-zA-Za-zA-Z0-9 printf("WORD\n")
  • Running the Program
  • lex example_lex.l
  • gcc lex.yy.c ll
  • ./a.out

Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
7
Pattern Matching Primitives
Metacharacter Matches
. any character except newline
\n newline
zero or more copies of the preceding expression
one or more copies of the preceding expression
? zero or one copy of the preceding expression
beginning of line
end of line
ab a or b
(ab) one or more copies of ab (grouping)
"ab" literal "ab" (C escapes still work)
character class
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
8
Pattern Matching Examples
Expression Matches
abc abc
abc ab abc abcc abccc ...
abc abc, abcc, abccc, abcccc, ...
a(bc) abc, abcbc, abcbcbc, ...
a(bc)? a, abc
abc one of a, b, c
a-z any letter, a through z
a\-z one of a, -, z
-az one of - a z
A-Za-z0-9 one or more alphanumeric characters
\t\n whitespace
ab anything except a, b
ab a, , b
ab a, , b
ab a, b
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
9
Operation of yylex()
  • When lex compiles the input specification, it
    generates the C file lex.yy.c that contains the
    routine int yylex(void).
  • This routine reads the input string trying to
    match it with any of the token patterns specified
    in the rules section.
  • On a match associated action is executed.
  • When we call yylex() function, it starts the
    process of pattern matching.
  • Lex keeps the matched string into the address
    pointed by pointer yytext.
  • Matched string's length is kept in yyleng while
    value of token is kept in variable yylval.

Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
10
int com0 "/"\n"/"
comfprintf(yyout, " ") int
main() printf("Write a C program\n") yyoutfope
n("output", "w") yylex() printf("Commentd\n",c
om) return 0
cc lex.yy.c -ll ./a.out Write a C
program includeltstdio.hgt int main() int a,
b /float c/ printf(Hi)
/printf(Hello)/ Comment2 cat
output includeltstdio.hgt int main() int a,
b printf(Hi)
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
11
Lex Predefined Variables
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
12
YACC
  • YACC is a parser generator that takes an input
    file with an attribute-enriched BNF (Backus
    Naur Form) grammar specification.
  • It generates the output C file y.tab.c containing
    the function int yyparse(void) that implements
    its parser.
  • This function automatically invokes yylex()
    everytime it needs a token to continue parsing.

Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
13
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
14
Structure of YACC Program
C global variables, prototype, Comments
Definitions
Context free grammar action for each production
------------------------------------
Subroutines/Functions
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
15
Arithmatic.l includeltstdio.hgt include
"y.tab.h" extern int yylval 0-9
          yylvalatoi(yytext)           return
NUMBER        \t \n return 0 . return
yytext0 int yywrap() return 1
How To Run yacc -d arithmatic.y lex
arithmatic.l gcc lex.yy.c y.tab.c ./a.out
Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
16
References
  • https//www.epaperpress.com/lexandyacc/
  • John. R. Levine, Tony Mason and Doug Brown - Lex
    and Yacc?, O'Reilly

Deptii Chaudhari, Dept of Computer Engineering,
Hope Foundations International Institute of
Information Technology, I²IT P-14,Rajiv Gandhi
Infotech Park MIDC Phase 1, Hinjawadi, Pune
411057 Tel - 91 20 22933441/2/3
www.isquareit.edu.in info_at_isquareit.edu.in
17
  • THANK YOU
  • For further details, please contact
  • Deptii Chaudhari
  • deptiic_at_isquareit.edu.in
  • Department of Computer Engineering
  • Hope Foundations
  • International Institute of Information
    Technology, I²IT
  • P-14,Rajiv Gandhi Infotech Park
  • MIDC Phase 1, Hinjawadi, Pune 411057
  • Tel - 91 20 22933441/2/3
  • www.isquareit.edu.in info_at_isquareit.edu.in
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