Programming Gems::flex

 flex -o code.c code.y
 gcc -o code code.c -ll
 echo "2+(2+2)))" | ./code

     int nm = 0, np = 0;
     int nErrors = 0;
     #define EOS " "
 %%
 -[0-9]+         { printf("%s"EOS, yytext); };
 [0-9]+          { printf("%s"EOS, yytext); }
 "+"             { printf("PLUS"EOS); }
 "-"             { printf("MINUS"EOS); }
 "("             { np = np + 1; printf("OPEN"EOS); }
 ")"             { nm = nm + 1; printf("CLOSE"EOS);
                   if (nm > np) {
                       nErrors = nErrors+1;
                   }
                 }
 "\r"            { printf("[RET]"EOS); }
 "\n"            { printf("[EOL]"EOS); }
 <<EOF>>         { printf("[EOF]"EOS); return 0; }
 %%
 int main () {
     yylex();
     if (nm != np) {
         printf("Non-completed: %d errors\n",(nErrors==0?1:nErrors));
     }
     else {
         printf("Completed\n");
     }
     return 0;
 }