Lex&Yacc Introduction

1. Introduction
2. An example
3. Build&Run

1 Introduction

Lex is a lexical analyzer and is used to break up the input stream into useful elements, and Yacc(also stands for Yet Another Compiler Compiler), given a grammar, generates a parser for it. When both tools are properly used, these tools allow you to parse complex languages and configure file with ease, just as the following example.

2 An example

here we use lex and yacc to parse the following configure file, you can find that it is so convenient to parse a configure file with these tools.

2.1 Configure File

2.2 Lex source file

%{
#include <stdlib.h>
#include "y.tab.h"
void yyerror(char *);
%}

%%

\[[^\n]+\]	{
			yylval.string = yytext;
			return ATCMD;
		}

\"[^\n\"]+\"	{
			yylval.string = yytext;
			return RESULT_CODE;
		}

[\[\]]		{
			return *yytext;
		}

\#[^\n]*	;	/* ignore comment line */

[ \t\n]+        ;       /* ignore whitespace */

.               yyerror("Unknown character");
%%

int yywrap(void) {
    return 1;
}

2.3 Yacc source file

%{
#include <stdio.h>
#include <stdlib.h>
#include <stdarg.h>

/* prototypes */
int yylex(void);
void yyerror(char *s);

extern FILE *yyin;
%}

%union {
	int number;		/* integer value */
	char *string;		/* string value */
 }

%token <number> NUMBER
%token <string> STRING
%token ATCMD RESULT_CODE

%%

parameters:
        | parameters	parameter
        ;

parameter:
	atcmd
	|
	result_code
        ;

atcmd:
	ATCMD		{ printf("%s\n", yylval.string); }
        ;

result_code:
	RESULT_CODE	{printf("%s\n", yylval.string);}
	;

%%


void yyerror(char *s) {
	fprintf(stdout, "%s\n", s);
}

int main(int argc, char **argv) {

	if (argc > 1) {
		yyin = fopen(argv[1], "r");
	}
	else {
		fprintf(stdout, "Usage: config <file_path>\n");
		return -1;
	}
	if (yyin == NULL) {
		fprintf(stdout, "Error: file open error...\n");
		return -1;
	}
	
	yyparse();
	
	fclose(yyin);
	
	return 0;
}