Lex Program to recognize and display comments, numbers, identifiers, and strings in a given statement
Problem definition:
Write a lex program to recognize comments, numbers, identifiers, and strings in a given statement and display them on standard output.
Structure of LEX Program:
| %{ Definition section %} %% Rules section %% User Subroutine section |
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Source Code to recognize and display comments, numbers, identifiers, and strings
/*Definition Section*/
%{
#include<stdio.h>
%}
/* Rules Section */
%%
[\t ]+ ;
[0-9]+|[0-9]*\.[0-9]+ { printf("\n%s is NUMBER", yytext);}
#.* { printf("\n%s is COMMENT", yytext);}
[a-zA-Z][a-zA-Z0-9]+ { printf("\n%s is IDENTIFIER", yytext);}
\"[^ \"\n]*\" { printf("\n%s is STRING", yytext);}
\n { ECHO;}
%%
/* User Subroutine section */
int main()
{
while( yylex());
}
int yywrap( )
{
return 1;
}Alternate source code (display result in the main part of code)
/*Definition Section*/
%{
#include<stdio.h>
#define NUMBER 400
#define COMMENT 401
#define STRINGS 402
#define IDENTIFIER 403
%}
/* Rules Section */
%%
[\t ]+ ;
[0-9]+ | [0-9]*\.[0-9]+ { return NUMBER;}
#.* { return COMMENT;}
\"[^ \"\n]*\" { return STRINGS;}
[a-zA-Z][a-zA-Z0-9]+ { return IDENTIFIER;}
\n { return '\n';}
%%
/* User Subroutine section */
main( )
{
int val;
while( val = yylex( ))
printf("value is %d\n",val);
}
int yywrap( )
{
return 1;
}Output:
maheshgh@maheshgh:~/LexYacc$ lex demo.l
maheshgh@maheshgh:~/LexYacc$ gcc lex.yy.c
maheshgh@maheshgh:~/LexYacc$ ./a.out
Mahesh Huddar
(Note: After every input string press enter and ctrl+d)
Mahesh: is IDENTIFIER
Huddar: is IDENTIFIER
10 10.20
10: is NUMBER
10.20 is NUMBER
#Mahesh Huddar
Mahesh Huddar: is COMMENT
“Mahesh Huddar”
“Mahesh Huddar”: is STRING
Summary:
This article discusses how to write a lex program to find comments, numbers, identifiers, and strings in a given statement and display them on standard output.. If you like the article, do share it with your friends.