COMPILER END

 flex example.l

gcc lex.yy.c -o lexer -lfl

./lexer



example.l

example.y

bison -d example.y        # generates y.tab.c and y.tab.h

flex example.l            # generates lex.yy.c

gcc lex.yy.c y.tab.c -o parser -lfl

./parser









Q1. Design a LEX Code to count the number of lines, space, tab-meta character, and

rest of characters in each Input pattern.

Solution:

%{

#include<stdio.h>

int line = 0 , space = 0 , tab = 0 , meta = 0 , other = 0;

%}

%%

"\n" {line++;}

" " {space++;}

"\t" {tab++;}

[\^\$\%\*\+\-\/\&\(\)\{\}\[\]\;\"] {meta++;}

. {other++;}

%%

int main(){

yylex();

printf("Line:%d\n",line);

printf("Space:%d\n",space);

printf("Tab:%d\n",tab);

printf("Meta:%d\n",meta);

printf("Other:%d\n",other);

return 0;

}

int yywrap(){

return 1;

}

Output:

int main(){

cout << "hello";

}

Line:3

Space:3

Tab:1

Meta:7

Other:18







Q2. Design a LEX Code to identify and print valid Identifier of C/C++ in given Input

pattern.

Solution:

%{

#include<stdio.h>

%}

%%

int|float|return|void|if|else {}

[a-zA-Z_]+[a-zA-Z_0-9]* {printf("Identifier:%s\n",yytext);}

[0-9]+[a-zA-Z_]* {}

. {}

%%

int main(){

yylex();

return 0;

}

int yywrap(){

return 1;

}

Output:

var_43

Identifier:var_43

num

Identifier:num

temp

Identifier:temp






Q3. Design a LEX Code to identify and print integer and float value in given Input

pattern.

Solution:

%{

#include<stdio.h>

%}

%%

[-]?[0-9]+\.[0-9]+ {printf("Float:%s\n",yytext);}

[-]?[0-9]+ {printf("Int:%s\n",yytext);}

[0-9]*[a-zA-z]*[0-9]* {printf("Nothing");}

. {}

%%

int main(){

yylex();

return 0;

}

int yywrap(){

return 1;

}

Output:

1234

Int:1234

123.54

Float:123.54

45_frr

Nothing









Q4. Design a LEX Code for Tokenizing (Identify and print OPERATORS,

SEPERATORS, KEYWORDS, IDENTIFERS) in the C-fragment:

Solution:

%{

#include<stdio.h>

%}

KEYWORDS "int"|"float"|"return"|"if"|"else"|"for"|"while"

OPERATORS [+\-*/^<>&|!%]

SEPERATORS [;,\(\)\{\}\"]

%%

{KEYWORDS} {printf("Keyword: %s\n", yytext);}

{OPERATORS} {printf("Operators: %s\n", yytext);}

{SEPERATORS} {printf("Seperators: %s\n", yytext);}

[a-zA-Z_]+[a-zA-Z0-9_]* {printf("Id: %s\n", yytext);}

. {}

%%

int main(){

yylex();

return 0;

}

int yywrap(){

return 1;

}

Output:

int

Keyword: int

+

Operators: +

;

Seperators: ;

var_23

Id: var_23






Q5. Design a LEX Code to count and print the number of total characters, words, white spaces in

given ‘Input.txt’ file.

Solution:

%{

#include<stdio.h>

int word =0, char_len=0, space = 0;

%}

%%

[a-zA-Z0-9]+ {

word++;

char_len += yyleng;}

[ \n\t] {space++;}

%%

int main(){

FILE *file = fopen("z-5-6.txt", "r");

if(!file) {printf("no file"); return 1;}

yyin = file;

yylex();

printf("word:%d\n",word);

printf("char:%d\n",char_len);

printf("space:%d\n",space);

fclose(file);

return 0;

}

int yywrap() {

return 1;

}

Output:

word:7

char:46

space:48










Q6. Design a LEX Code to replace white spaces of ‘Input.txt’ file by a single blank character

into ‘Output.txt’ file

Solution:

%{

#include <stdio.h>

#include <stdlib.h>

FILE *out;

%}

%%

[ \t\n]+ %%

{ fprintf(out, " "); } // Replace multiple spaces, tabs, or newlines with a single space

. { fprintf(out, "%s", yytext); } // Write other characters as they are

int main() {

// Open input and output files

yyin = fopen("z-5-6.txt", "r");

if (!yyin) {

perror("Error opening input.txt");

return 1;

}

out = fopen("output.txt", "w");

if (!out) {

perror("Error creating output.txt");

fclose(yyin);

return 1;

}

yylex();

fclose(yyin);

fclose(out);

printf("Whitespace replaced successfully. Check output.txt\n");

return 0;

}

int yywrap(){

return 1;

}

Output:

input : hello my name is justin

jhdjhejdjjdejdjehd ehidhehdhd

output :

hello my name is justin jhdjhejdjjdejdjehd ehidhehdhd







Q7. Design a LEX Code to remove the comments from any C-Program given at run-time

and store into ‘out.c’ file.

Solution:

%{

#include <stdio.h>

#include <stdlib.h>

FILE *out;

%}

%%

"//".* { }

"/\\*([^*]|\\*+[^*/])*\\*+\\*/" { }

. { fprintf(out, "%s", yytext); }

\n { fprintf(out, "\n"); }

%%

int main() {

char input_file[100];

printf("Enter the C program filename (e.g., program.c): ");

scanf("%s", input_file);

yyin = fopen(input_file, "r");

if (!yyin) {

perror("Error opening input file");

return 1;

}

out = fopen("out.c", "w");

if (!out) {

perror("Error opening out.c");

fclose(yyin);

return 1;

}

yylex();

fclose(yyin);

fclose(out);

printf("Comments removed successfully. Check out.c\n");

return 0;

}

int yywrap(){

return 1;

}

Output:

input :

#include <stdio.h> // header file

int main()

{ // program start

printf("Hello"); // print stmt

} // exit

output :

#include <stdio.h>

int main()

{

printf("Hello");

}






Q8. Design a LEX Code to extract all html tags in the given HTML file at run time and

store into Text file given at run time.

Solution:

%{

#include <stdio.h>

#include <stdlib.h>

FILE *out;

%}

%%

\ !--([^-\n]|(-[^-])|(\n))*--> {}

\ <[a-zA-Z/!][^>]*> . {}

\n { fprintf(out, "\n"); }

%%

int main() {

char input_file[100], output_file[100];

{ fprintf(out, "%s\n", yytext); }

printf("Enter the HTML file name (e.g., index.html): ");

scanf("%s", input_file);

printf("Enter the output file name (e.g., tags.txt): ");

scanf("%s", output_file);

yyin = fopen(input_file, "r");

if (!yyin) {

perror("Error opening input file");

return 1;

}

out = fopen(output_file, "w");

if (!out) {

perror("Error opening output file");

fclose(yyin);

return 1;

}

yylex();

fclose(yyin);

fclose(out);

printf("HTML tags extracted successfully. Check %s\n", output_file);

return 0;

}

int yywrap(){

return 1;

}







Q9. Design a DFA in LEX Code which accepts string containing even number of ‘a’ and

even number of ‘b’ over input alphabet (a, b}.

Solution:

%{

#include <stdio.h>

#include <ctype.h>

int state = 0;

int invalid_input = 0;

%}

%%

a {

if (state == 0) state = 1;

else if (state == 1) state = 0;

else if (state == 2) state = 3;

else if (state == 3) state = 2;

}

b {

if (state == 0) state = 2;

else if (state == 1) state = 3;

else if (state == 2) state = 0;

else if (state == 3) state = 1;

}

[c-zC-Z0-9]+ {

invalid_input = 1;

}

\n {

if (invalid_input == 1) {

printf("Rejected (Invalid characters found)\n");

} else if (state == 0) {

printf("Accepted\n");

} else {

printf("Rejected\n");

}

state = 0;

invalid_input = 0;

}

%%

int main() {

char input[100];

while (1) {

printf("Enter a string of a's and b's (end with Enter, type 'exit' to quit): ");

fgets(input, sizeof(input), stdin);

if (input[0] == 'e' && input[1] == 'x' && input[2] == 'i' && input[3] == 't') {

break;

}

yylex();

}

return 0;

}

int yywrap(){

return 1;

}

Output:

Enter a string of a's and b's (end with Enter, type 'exit' to quit):

aabb

Accepted

abab

Accepted

bbaab

Rejected







Q10. Design a DFA in LEX Code which accepts string containing third last element ‘a’

over input alphabet (a, b}.

Solution:

%{

#include <stdio.h>

#include <string.h>

char window[4] = " ";

%}

%%

[a|b] {

window[0] = window[1];

window[1] = window[2];

window[2] = yytext[0];

}

\n {

if (strlen(window) >= 3 && window[0] == 'a') {

printf("Accepted\n");

} else {

printf("Rejected\n");

}

strcpy(window, " ");

}

. {}

%%

int main() {

char input[100];

while (1) {

printf("Enter a string of a's and b's (end with Enter, type 'exit' to quit): ");

fgets(input, sizeof(input), stdin);

if (strncmp(input, "exit", 4) == 0) {

break;

}

yylex();

}

return 0;

}

int yywrap(){

return 1;

}

Output:

aabb

Accepted

aaabbb

Rejected

aaaaab

Accepted






Q11. Design a DFA in LEX Code to Identify and print Integer &amp; Float Constants

and Identifier.

Solution:

%{

#include <stdio.h>

%}

%%

([0-9]+) {printf("Integer: %s\n", yytext);}

([0-9]+\.[0-9]+) | (\.[0-9]+) | ([0-9]+\.) {printf("Float: %s\n", yytext); }

[a-zA-Z_]+[a-zA-Z_0-9]* {printf("Identifier: %s\n", yytext); }

[\n\t ]+ { }

. {printf("Unrecognized token: %s\n", yytext);}

%%

int main() {

yylex();

return 0;

printf("Enter your input (type 'exit' to quit):\n");

}

int yywrap(){

return 1;

}

Output:

Enter your input (type 'exit' to quit):

1234567

Integer: 1234567

123456.34567

Float: 123456.34567

dfgh

Identifier: dfgh

23456.21

Float: 23456.21







Q12. Design YACC/LEX code to recognize valid arithmetic expression with operators

+,-, * and /.

Solution:

Yaac:

%{

#include <stdio.h>

#include <stdlib.h>

void yyerror(const char *msg);

int yylex();

%}

%token NUMBER

%token PLUS MINUS MUL DIV LPAREN RPAREN EOL

%%

input:

expression EOL | error EOL ;

expression:

expression PLUS term

| expression MINUS term

| term

;

term:

term MUL factor

| term DIV factor

| factor

;

factor:

NUMBER

| LPAREN expression RPAREN

;

{ printf("Valid Expression\n"); }

{ printf("Invalid Expression\n"); yyerrok; }

%%

void yyerror(const char *msg) {

fprintf(stderr, "Error: %s\n", msg);

}

int main() {

yyparse();

return 0;

printf("Enter an arithmetic expression (end with Enter):\n");

}

Lex:

%{

#include "12.tab.h"

#include <ctype.h>

%}

%%

[0-9]+ { yylval = atoi(yytext); return NUMBER; }

[ \t] ; // Ignore spaces and tabs

"+" { return PLUS; }

"-" { return MINUS; }

"*" { return MUL; }

"/" { return DIV; }

"(" { return LPAREN; }

")" { return RPAREN; }

\n { return EOL; }

. { return yytext[0]; } // Catch all for any other character

%%

int yywrap() {

return 1;

}



Q13. Design YACC/LEX code to evaluate arithmetic expression involving operators

+, -* and / without operator precedence grammar &amp; with operator precedence

grammar.

Solution:

Yaac:

%{

#include <stdio.h>

#include <stdlib.h>

int yylex();

void yyerror(const char *s) {

printf("Error: %s\n", s);

}

%}

%token NUMBER

%token PLUS MINUS MULTIPLY DIVIDE

%left PLUS MINUS

%left MULTIPLY DIVIDE

%%

expression:

expression PLUS expression { $$ = $1 + $3; }

| expression MINUS expression { $$ = $1 - $3; }

| expression MULTIPLY expression { $$ = $1 * $3; }

| expression DIVIDE expression { $$ = $1 / $3; }

| '(' expression ')' { $$ = $2; }

| NUMBER { $$ = $1; }

;

%%

int main() {

yyparse();

return 0;

printf("Enter an arithmetic expression (with precedence): ");

}

Lex:

%{

#include "13.tab.h"

%}

%%

[0-9]+ { yylval = atoi(yytext); return NUMBER; }

[\t\n ] { /* Ignore whitespace */ }

"+" { return PLUS; }

"-" { return MINUS; }

"*" { return MULTIPLY; }

"/" { return DIVIDE; }

. { return yytext[0]; }

%%

int yywrap() {

return 1;

}






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