===== count_char.l =====

%{
#include <stdio.h>
int line = 0, word = 0, ch = 0;
%}
%%
[a-zA-Z0-9]+    { ch += yyleng; word++; }
[ \t]+          { /* skip spaces/tabs */ }
\n              { line++; }
.               { ch++; }
%%
int main(){
    yyin = fopen("input1.txt", "r");
    yylex();
    printf("line=%d\n", line);
    printf("word=%d\n", word);
    printf("character=%d\n", ch);
    return 0;
}

===== input1.txt =====
Hello, world!
Lex is fun.
1234 are numbers.

===== output.txt =====
line=3
word=8
character=36

===== run =====
nano count_char.l
lex count_char.l
gcc lex.yy.c -o count_char -ll
./count_char