Simple lex example to count lines and chars.

counter.lex

%{
#include <stdio.h>
int num_lines = 0, num_chars = 0;
%} 
%%
\n      ++num_lines; ++num_chars;
.       ++num_chars;
 
%%
main() {
    yylex();
    printf("# of lines = %d, # of chars = %d\n", num_lines, num_chars);
}

Compile with:

lex counter.lex
gcc lex.yy.c -ll -o counter

and run with:

./counter