Name: Shrinivas patil
Section: A
Roll No: A4-67
Practical - 2
E1 : Write YACC specification to check syntax of a simple expression involving operators +,
-, *
and /. Also convert the arithmetic expression to postfix.
q1.l:
%{
#include "q1.tab.h"
%}
%%
[0-9]+ { yylval = atoi(yytext); return NUMBER; }
[a-zA-Z]+ { return ID; }
[\n] { return NL; }
[ \t] ; // ignore whitespace
. { return yytext[0]; }
%%
q1.y :
%{
#include <stdio.h>
#include <stdlib.h>
extern int yylex();
extern char *yytext;
void yyerror(const char *s);
%}
%token NUMBER ID NL
%left '+' '-'
%left '*' '/'
%%
stmt:
exp NL {
printf("\nThe Entered expression is valid.\n");
printf("\n");
exit(0);
| exp1 NL {
printf("\nThe Entered expression is valid.\n");
printf("But calculation cannot be performed on variables.\n");
printf("\n");
exit(0);
exp:
exp '+' exp {printf(" ");$$ = $1 + $3;printf("+"); }
| exp '-' exp {printf(" ");$$ = $1 - $3;printf("-"); }
| exp '*' exp {printf(" ");$$ = $1 * $3;printf("*"); }
| exp '/' exp {printf(" ");$$ = $1 / $3;printf("/"); }
| '(' exp ')' {$$ = $2; }
| NUMBER {$$ = $1;printf("%d", $1);}
exp1:
exp1 '+' exp1 {printf(" ");printf("+");}
| exp1 '-' exp1 {printf(" ");printf("-");}
| exp1 '*' exp1 {printf(" ");printf("*");}
| exp1 '/' exp1 {printf(" ");printf("/");}
| '(' exp1 ')' {}
| ID {printf("%s", yytext);}
%%
void yyerror(const char *s) {
printf("\nThe Entered expression is invalid.\n");
exit(0);
int main() {
printf("Enter any expression:\n");
yyparse();
return 0;
int yywrap() {
return 1;
}
Output:-
E2: Write YACC specification to recognize strings that can be accepted by following
language:
Batch 4: L={ 0 n 1 n | n>=0} U { 1 n 0 n | n>=0}
q2.l:
%{
#include "q2.tab.h"
%}
%%
0 { return ZERO; }
1 { return ONE; }
\n { return '\n'; }
. { return *yytext; }
%%
int yywrap() { return 1; }
q2.y :
%{
#include <stdio.h>
#include <stdlib.h>
int yylex();
void yyerror(const char *s);
%}
%token ZERO ONE
%%
start : pattern '\n' { printf("Valid string.\n"); return 0; }
pattern : seq01
| seq10
seq01 : zeros ones
seq10 : ones zeros
zeros : /* empty */
| ZERO zeros
;
ones : /* empty */
| ONE ones
%%
void yyerror(const char *s) {
printf("Invalid string.\n");
int main() {
printf("Enter string: ");
yyparse();
return 0;
Output:
E3: To validate syntax of following programing language construct:
Batch 4: do while
q3.l:
%{
#include "q3.tab.h"
%}
%%
"do" return DO;
"while" return WHILE;
"{" return LBRACE;
"}" return RBRACE;
"(" return LPAREN;
")" return RPAREN;
";" return SEMICOLON;
[a-zA-Z][a-zA-Z0-9]* return IDENTIFIER;
[0-9]+ return NUMBER;
[ \t\n] ; /* ignore whitespace */
. return yytext[0]; /* for other operators */
%%
int yywrap() {
return 1;
q3.y:
%{
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
extern int yylex();
void yyerror(const char *s);
%}
%token DO WHILE LPAREN RPAREN LBRACE RBRACE SEMICOLON
%token IDENTIFIER NUMBER
%%
program:
dowhile_stmt
| program dowhile_stmt
dowhile_stmt:
DO LBRACE statements RBRACE WHILE LPAREN expression RPAREN SEMICOLON
{ printf("Valid do-while loop\n"); }
statements:
statement
| statements statement
statement:
IDENTIFIER SEMICOLON
expression:
IDENTIFIER
| NUMBER
| expression '+' expression
| expression '-' expression
| expression '*' expression
| expression '/' expression
| expression '<' expression
| expression '>' expression
| expression "==" expression
| expression "!=" expression
%%
void yyerror(const char *s) {
fprintf(stderr, "Syntax error: %s\n", s);
exit(1);
int main() {
printf("Enter a do-while loop:\n");
yyparse();
return 0;
}
Output:
E4: Implementation Batchwise:
Batch 4: Write a YACC program that handles nested blocks using { } and validates proper
nesting.
q4.l:
%{
#include "q4.tab.h"
%}
%%
"{" return LBRACE;
"}" return RBRACE;
[ \t\n]+ ; /* Ignore whitespace */
. {
fprintf(stderr, "Invalid character: %s\n", yytext);
exit(1);
%%
q4.y:
%{
#include <stdio.h>
#include <stdlib.h>
extern int yylex();
void yyerror(const char *s);
%}
%token LBRACE RBRACE
%%
program:
/* An empty program is valid */
| block
block:
LBRACE RBRACE
| LBRACE block RBRACE
| LBRACE RBRACE block
| block block
%%
void yyerror(const char *s) {
fprintf(stderr, "Error: Mismatched braces.\n");
exit(1);
}
int main() {
printf("Enter a sequence of braces (use { and }):\n");
yyparse();
printf("Valid nesting.\n");
return 0;
int yywrap() {
return 1;
Output:
