how to disable ECHO() for unmatched characters in lex

Question

I need to disable flex output for unmatched characters. I can't disable this by default ECHO(), can't disable yyout. Any variants?

Answer 1

The creators of lex have provided a much simpler, more maintainable method. Add the following matching rules at the end of all your other rules:

[ \t\n]+          /* do nothing */
.+                /* do nothing */

Answer 2

I've found the solution. When you compile with gcc/g++, you have to add the option '-DECHO'. So the 'ECHO' macro will be defined and the program will not print anything.