Question

I am using Lex and Yacc to design a parser and encounter some issue about comment.

I use the following Lex rule.

'#'[^('\r'|'\n')]*    { /* do nothing */ }

It works, but at the end of execution all the comments are printed to the standard output. Is there way to clear that? Thank you for the suggestion.

Answer 1

The characters ', |, (, and ) have no special meaning in [], so you’re only matching (and discarding) comments that don’t contain them. In addition, in most versions of lex ' has no special meaning at all — only " can be used to quote literal strings. What you probably want is:

"#"[^\r\n]*     { /* do nothing */ }

In addition, # has no special meaning either, so there’s no real need to quote it.

In general, if you’re using lex (or flex) as the input to a parser, you NEVER want the default echoing behavior, so its best to add a ‘catch-all’ rule at the very end:

.|\n            { fprintf(stderr, "Unexpected character '%c' in input\n", *yytext); }