Implement the Lexical Analyzer Using Lex Tool.

/* program name is lexp.l */
%{
/* program to recognize a c program */ int COMMENT=0;
%}
identifier [a-zA-Z][a-zA-Z0-9]*
%%
#.* { printf("\n%s is a PREPROCESSOR DIRECTIVE",yytext);} int |
float | char | double | while | for |
do | if |
break | continue | void | switch | case | long | struct | const | typedef | return | else |
goto {printf("\n\t%s is a KEYWORD",yytext);} "/*" {COMMENT = 1;}
/*{printf("\n\n\t%s is a COMMENT\n",yytext);}*/

"*/" {COMMENT = 0;}
/* printf("\n\n\t%s is a COMMENT\n",yytext);}*/
{identifier}\( {if(!COMMENT)printf("\n\nFUNCTION\n\t%s",yytext);}
\{ {if(!COMMENT) printf("\n BLOCK BEGINS");}
\} {if(!COMMENT) printf("\n BLOCK ENDS");}
{identifier}(\[[0-9]*\])? {if(!COMMENT) printf("\n %s IDENTIFIER",yytext);}
\".*\" {if(!COMMENT) printf("\n\t%s is a STRING",yytext);}
[0-9]+ {if(!COMMENT) printf("\n\t%s is a NUMBER",yytext);}
\)(\;)? {if(!COMMENT) printf("\n\t");ECHO;printf("\n");}
\( ECHO;
= {if(!COMMENT)printf("\n\t%s is an ASSIGNMENT OPERATOR",yytext);}
\<= |
\>= |
\< |
== |
\> {if(!COMMENT) printf("\n\t%s is a RELATIONAL OPERATOR",yytext);}
%%
int main(int argc,char **argv)
{
if (argc > 1)
{
FILE *file;
file = fopen(argv[1],"r"); if(!file)
{
printf("could not open %s \n",argv[1]); exit(0);
}
yyin = file;
}
yylex(); printf("\n\n"); return 0;
} int yywrap()
{
return 0;
}

Input:
$vi var.c #include<stdio.h> main()
{
int a,b;
}

Output:
$lex lex.l
$cc lex.yy.c
$./a.out var.c
#include<stdio.h> is a PREPROCESSOR DIRECTIVE FUNCTION
main (
)
BLOCK BEGINS
int is a KEYWORD a IDENTIFIER
b IDENTIFIER BLOCK ENDS

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