This example gives the complete Yacc specification for a small desk calculator; the desk calculator has 26 registers, labeled ``a'' through ``z'', and accepts arithmetic expressions made up of the operators +, -, *, /, % (mod operator), & (bitwise and), | (bitwise or), and assignment. If an expression at the top level is an assignment, the value is not printed; otherwise it is. As in C, an integer that begins with 0 (zero) is assumed to be octal; otherwise, it is assumed to be decimal.

As an example of a Yacc specification, the desk calculator does a reasonable job of showing how precedences and ambiguities are used, and demonstrating simple error recovery. The major oversimplifications are that the lexical analysis phase is much simpler than for most applications, and the output is produced immediately, line by line. Note the way that decimal and octal integers are read in by the grammar rules; This job is probably better done by the lexical analyzer.

%{
# include <stdio.h>
# include <ctype.h>

int regs[26];
int base;

%}

%start list

%token DIGIT LETTER

%left \'|\'
%left \'&\'
%left \'+\' \'-\'
%left \'*\' \'/\' \'%\'
%left UMINUS /* supplies precedence for unary minus */

%% /* beginning of rules section */

list : /* empty */
 | list stat \'\en\'
 | list error \'\en\'
 { yyerrok; }
 ;

stat : expr
 { printf( "%d\en", $1 ); }
 | LETTER \'=\' expr
 { regs[$1] = $3; }
 ;

expr : \'(\' expr \')\'
 { $$ = $2; }
 | expr \'+\' expr
 { $$ = $1 + $3; }
 | expr \'-\' expr
 { $$ = $1 - $3; }
 | expr \'*\' expr
 { $$ = $1 * $3; }
 | expr \'/\' expr
 { $$ = $1 / $3; }
 | expr \'%\' expr
 { $$ = $1 % $3; }
 | expr \'&\' expr
 { $$ = $1 & $3; }
 | expr \'|\' expr
 { $$ = $1 | $3; }
 | \'-\' expr %prec UMINUS
 { $$ = - $2; }
 | LETTER
 { $$ = regs[$1]; }
 | number 
 ;

number : DIGIT
 { $$ = $1; base = ($1==0) ? 8 : 10; }
 | number DIGIT
 { $$ = base * $1 + $2; }
 ;

%% /* start of programs */

yylex() { /* lexical analysis routine */
 /* returns LETTER for a lower case letter, yylval = 0 through 25 */
 /* return DIGIT for a digit, yylval = 0 through 9 */
 /* all other characters are returned immediately */

 int c;

 while( (c=getchar()) == \' \' ) { /* skip blanks */ }

 /* c is now nonblank */

 if( islower( c ) ) { 
 yylval = c - \'a\';
 return ( LETTER );
 }
 if( isdigit( c ) ) { 
 yylval = c - \'0\';
 return( DIGIT );
 }
 return( c );
 }