%%writefile program.y
 
%{
    #include<stdio.h>
    #include<stdlib.h>
%}
%token DIGIT LETTER UND NL
%%
stmt: variable NL {printf("Valid Identifier\n");exit(0);}
variable: LETTER alphanumeric;
alphanumeric: LETTER alphanumeric
            | DIGIT alphanumeric
            | UND alphanumeric
            | LETTER
            | DIGIT
            | UND;
%%
 
int yyerror(){
    printf("Invalid Identifier\n");
    exit(0);
}
 
void main(){
    printf("Enter the variable name: ");
    yyparse();
}

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