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From: Kaz Kylheku <847-115-0292@kylheku.com>
Newsgroups: comp.compilers
Subject: Re: How do I match empty lines with Flex?
Date: Fri, 4 Oct 2019 16:55:25 +0000 (UTC)
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On 2019-10-04, Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> wrote:
> Dear comp.compilers,
>
> The following program matches lines in a text file, but ignores empty
> lines.  Is there any way I can alter it so it returns something on empty
> lines?

How about:

.+\n { num_lines++; return NONEMPTY_LINE; }
\n   { num_lines++; return EMPTY_LINE; }
.+   { num_lines++; return MISSING_LAST_NEWLINE; }

> ..* { yylval = yytext; return 1; }
> \n { yylineno += 1; }

I'd recommend not to rely on yylineno being defined, and certainly
don't increment it yourself.  It's not described by POSIX. I think some
implementations of Lex have it.

GNU Flex will generate this varaible and update its value if you use
%option yylineno, or --yylineno on the command line.

If you do your own line counting, invent some variable that doesn't
intrude into the reserved yacc yy* namespace.

><<EOF>> { return 0; }

<<EOF>> might be a GNU Flex extension; if you rely on that, you might
as well use %option yylineno.
[Either that or something with start states. -John]