Bison and Flex Example

Bison and Flex Example
(May 2006)

Bison is a replacement for Yacc.
Flex is a replacement for Lex.
These are used for syntactic and grammatical parsing.

Example with blocks and identifiers

Example of the file we want to parse:

sample.txt

procedure Validate {
        begin {
                DoThis
                Dothat
        }
        end {
                CleanUp
        }
}

Parser.y

%{
#include <stdio.h>
%}

// Symbols.
%union
{
	char	*sval;
};
%token <sval> IDENTIFIER
%token PROCEDURE
%token BLOCK
%token ENDBLOCK

%start Procedure
%%

Procedure:
	PROCEDURE IDENTIFIER BLOCK { printf("Procedure : %s\n", $2); }
	Parts
	ENDBLOCK
	;

Parts:
	/* empty */
	| Parts Part
	;

Part:
	IDENTIFIER BLOCK { printf("\tPart : %s\n", $1); }
		Keywords
	ENDBLOCK
	;

Keywords:
	/* empty */
	| Keywords Keyword
	;

Keyword:
	IDENTIFIER { printf("\t\tKeyword : %s\n", $1); }
	;
%%

int yyerror(char *s) {
  printf("yyerror : %s\n",s);
}

int main(void) {
  yyparse();
}

Parser.lex

%{
#include "Parser.h"
%}

blanks          [ \t\n]+
identifier		[_a-zA-Z0-9]+

%%

{blanks}        { /* ignore */ }

"procedure"		return(PROCEDURE);
"{"				return(BLOCK);
"}"				return(ENDBLOCK);

{identifier}	{
				yylval.sval = malloc(strlen(yytext));
				strncpy(yylval.sval, yytext, strlen(yytext));
				return(IDENTIFIER);
}

Compilation commands:

bison -d Parser.y
mv Parser.tab.h Parser.h
mv Parser.tab.c Parser.y.c
flex Parser.lex
mv lex.yy.c Parser.lex.c
gcc -g -c Parser.lex.c -o Parser.lex.o
gcc -g -c Parser.y.c -o Parser.y.o
gcc -g -o Parser Parser.lex.o Parser.y.o -lfl

Note: on some Unix computers, one needs to use -ll instead of -lfl.
Execution of the parser:

Parser < sample.txt

Result:

Procedure : Validate
	Part : begin
		Keyword : DoThis
		Keyword : Dothat
	Part : end
		Keyword : CleanUp

Bison and Flex Example(May 2006)

Example with blocks and identifiers

Bison and Flex Example
(May 2006)