Program: identifier [a-zA-Z][a-zA-Z0-9]* %% #.* {printf("\n%s This is a preprocessor directive",yytext);} int | float | char | double | while | for | struct | typedef | do | if | break | continue | void | switch | return | else | goto {printf("\n\t%s is a keyword",yytext);} [a-zA-Z][a-zA-Z0-9]* {printf("\n%s is an identifier", yytext);} = {printf("\n\t %s is an ASSIGNMENT OPERATOR",yytext);} \<= | \>= | \< | == | \> {printf("\n\t%s is a RELATIONAL OPERATOR",yytext);} %% int main(int argc, char **argv) { FILE *file; file=fopen("var.c","r"); if(!file) { printf("could not open the file"); exit(0); } yyin=file; yylex(); printf("\n"); return(0); } int yywrap() { return(1); } INPUT: //var.c #include<stdio.h> #include<conio.h> void main() { int a,b,c; a=1; b=2; c=a+b; printf("Sum:%d",c); } #include<stdio.h> This is a preprocessor directive #include<conio.h> This is a preprocessor directive void is a keyword main is an identifier int is a keyword a is an identifier b is an identifier c is an identifier a is an identifier = is an ASSIGNMENT OPERATOR b is an identifier = is an ASSIGNMENT OPERATOR c is an identifier = is an ASSIGNMENT OPERATOR a is an identifier b is an identifier printf is an identifier