1. Syntactic Analysis Tools
Natawut Nupairoj, Ph.D.
Department of Computer Engineering
Chulalongkorn University

2. Outline
Overview.
Yacc Specification Format.
Examples.

3. Parser Generator

4. Yacc Specification A Yacc source program has three parts: declarations%%
translation rules
supporting C-routines

5. Example: Calculator Program%{
#include <stdio.h> %}
%token DIGIT %%
line : expr '\n' { printf("%d\n", $1); } ;
expr : expr '+' term { $$ = $1 + $3; }
| term
term : term '*' factor { $$ = $1 * $3; }
| factor
factor : '(' expr ')' { $$ = $2; }
| DIGIT

6. Example: Calculator Program
yylex() {
int c;
c = getchar();
if(c >= '0' && c <= '9') {
yylval = c - '0';
return DIGIT; }
return c;
yyerror(char* errmsg)
fprintf(stderr, "%s\n", errmsg);
main(int argc, char** argv)
yyparse();
return 0;

7. How to use Yacc with Lex
yyparse calls yylex to get the next token automatically.
yylex returns:
token type or 0 (EOF).
yylval - token attribute.
Tokens are defined in yacc definition
Lex definition can get them through “y.tab.h”.

8. Example: Yacc with Lex (Yacc)%{
#include <stdio.h> %}
%token EOL NUMBER %%
line : expr EOL { printf("%d\n", $1); } ;
expr : expr '+' term { $$ = $1 + $3; }
| term
term : term '*' factor { $$ = $1 * $3; }
| factor
factor : '(' expr ')' { $$ = $2; }
| NUMBER

9. Example: Yacc with Lex (Yacc)
yyerror(char* errmsg) {
fprintf(stderr, "%s\n", errmsg); }
main(int argc, char** argv)
yyparse();
return 0;

10. Example: Yacc with Lex (Lex)%{
/* define constants for C program here */
#include <stdlib.h>
#include "y.tab.h"
extern int yylval; %}
/* regular definitions */
delim [ \t]
ws {delim}+
eol \n
number [0-9]+
symbol [\+\*\(\)] %%
{ws} {/* no action and no return */}
{eol} { return EOL; }
{number} { yylval = atoi(yytext); return(NUMBER); }
{symbol} { return yytext[0]; }

11. Compile Yacc and Lex byacc –d calc.y flex calc.l
gcc –o calc y.tab.c lex.yy.c -lfl