yacc program to check the validity of the identifier

 id.lex

********************************

%{

#include "id.tab.h"

%}

%option noyywrap

%%

[a-zA-Z_][a-zA-Z_0-9]*  return LETTER;

[0-9]     return DIGIT;

[ \t\n]+                { /* Ignore whitespace */ }

.                       { return yytext[0]; }

%%

id.y

*****************************

%{

#include <stdio.h>

#include <ctype.h>

void yyerror();

int valid=1;

%}

%token LETTER DIGIT

%%

input:

    | LETTER  line

    ;

line:

    LETTER line

    |DIGIT line

    |

    ;

%%

int main() {

    printf("Enter identifiers to check their validity:\n");

    yyparse();

   if (valid)

    printf("It is a valid identifier \n");

    return 0;

}

void yyerror() {

    fprintf(stderr, "Invalid Identifier:\n");

    valid=0;

}

Execution

$ bison -d id.y

$ lex id.lex

$ gcc lex.yy.c id.tab.c

$./a.out

Enter identifiers to check their validity:

dkj23

It is a valid identifier

$ ./a.out

Enter identifiers to check their validity:

12dd

Invalid Identifier:

$ ./a.out

Enter identifiers to check their validity:

_dhd23

It is a valid identifier