Grammar parser in C++

Recently I stumbled upon implementing a simple parser in C++. The task is very classic, however I couldn't find any good resources on the web to help me out.
I tried different tools (including ANTLR), but finally the easiest way I found was bison + flex. It's unbelievable that this technology from 1989 is still actively developed. Latest stable release is from May 14, 2011. Moreover, many important projects make use of it. Among them are Ruby, PHP, Google Go, Bash shell.
So I decided to create a minimalistic example, which works from scratch.
I published the code on github Calculator, so you can check it out.
The whole example is 88 lines long and evaluates common expressions, like 2+2*2-13*(7+19/2).
Let's start with lexer. In flex, you need to define regular expressions, which produce tokens. Such tokens are later processed by a scanner. So we have to define calculator.lex, like this:

	%{
	#include "calculator.tab.h"
	using namespace std;
	%}
	%%
	[ \t] ;
	\+ { return PLUS; }
	\- { return MINUS; }
	\* { return MUL; }
	\/ { return DIV; }
	\( { return LPAREN; }
	\) { return RPAREN; }
	[0-9]+ { yylval.ival = atoi(yytext); return INT; }
	[a-zA-Z0-9]+ {
	yylval.sval = strdup(yytext);
	return IDENT;
	}
	\n return ENDL;
	. ;
	%%

view raw calculator.lex hosted with ❤ by GitHub

Flex will generate yylex() function, which we can call later to produce tokens.
Next, we need to create a scanner (calculator.y), which specifies a grammar. It's simple like that:

	%{
	#include <cstdio>
	#include <iostream>
	using namespace std;
	int yylex();
	extern "C" int yyparse();
	extern "C" FILE *yyin;
	void yyerror(char *s);
	%}
	%union {
	int ival;
	float fval;
	char *sval;
	}
	%token ENDL PLUS MINUS MUL DIV LPAREN RPAREN
	%token <ival> INT
	%token <sval> IDENT
	%type <ival> expression1
	%type <ival> expression
	%left PLUS MINUS
	%left MUL DIV
	%%
	start:
	expressions;
	expressions:
	expressions expression1 ENDL
	| expression1 ENDL;
	expression1:
	expression { cout<<$1<<endl; };
	expression:
	expression PLUS expression { $$ = $1 + $3; }
	|expression MINUS expression { $$ = $1 - $3; }
	|expression MUL expression { $$ = $1 * $3; }
	|expression DIV expression { $$ = $1 / $3; }
	|LPAREN expression RPAREN { $$ = $2; }
	| INT { $$ = $1; };
	%%
	int main(int argc, char *argv[]) {
	if (argc!= 2) {
	cout <<"Usage: <command> filename"<<endl;
	return 1;
	}
	FILE *myfile = fopen(argv[1], "r");
	if (!myfile) {
	cout << "I can't open "<<argv[1]<< endl;
	return -1;
	}
	yyin = myfile;
	do {
	yyparse();
	} while (!feof(yyin));
	}
	void yyerror(char *s) {
	cout << "Parse error: " << s << endl;
	exit(-1);
	}

view raw calculator.y hosted with ❤ by GitHub

Here, we specify types for all tokens using C/C++ union like structure. Variable $$ is used to store result of particular reductions.
Additionally, we need to specify %left precedence for +, -, *, / operators to resolve shift / reduce conflicts between them.
And that's basically it. We have a working expression parser.
I implemented it in a way, that executable takes a file name containing expressions as an argument.
So you can try ./calculator input.txt to see the result.