===== yacc_ident.l =====

%{
#include "y.tab.h"
%}
%%
[a-zA-Z_][a-zA-Z0-9_]*    { return IDENTIFIER; }
[0-9][a-zA-Z0-9_]*        { return INVALID; }
[ \t]+                   { /* skip whitespace */ }
[\n]                     { return 0; }
.                        { return INVALID; }
%%
int yywrap() {
    return 1;
}\

===== yacc_ident.y =====
%{
#include <stdio.h>
#include <stdlib.h>
%}
%token IDENTIFIER
%token INVALID
%%
input:
    IDENTIFIER { printf("Valid identifier\n"); }
    | INVALID  { printf("Invalid identifier\n"); } ;
%%
int main() {
    printf("Enter an identifier: ");
    yyparse();
    return 0;
}
int yyerror(const char* s) {
    return 0;
}

===== output.txt =====
Enter an identifier: 123
Invalid identifier

===== run =====
nano yacc_ident.l
nano yacc_ident.y
yacc -d yacc_ident.y
lex yacc_ident.l
gcc y.tab.c lex.yy.c -o yacc_ident -ll -ly
./yacc_ident