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Postings from August 2010
On Fri, Aug 13, 2010 at 08:21:17PM +0100, Ben Morrow wrote:
> Quoth rjk@tamias.net (Ronald J Kimball):
> > Perhaps someone will write a regex generator that allows the user to
> > specify which options are on and which are off. Disallowing (?.-i: means
> > the program would have to keep track of which options were off by default.
> >
> > The program would also have to be updated each time a new option was added
> > to the language.
>
> *NO*.
>
> (Is this really that hard to understand, or am I just explaining it
> badly?)
I think you misunderstood my point. That is all one paragraph, discussing
a program that generates regular expressions.
Suppose I have a program that generates regular expressions, using the
(?. syntax, like this:
my @options_on = qw/ s a /; # where a and b are hypothetical options
my @options_off = qw/ i b /; # that are on by default
my $pattern = "whatever";
my $regex =
"(?." . join('', @options_on) . '-' . join('', @options_off) . ':' .
$pattern .
")";
That wouldn't work under your proposal where (?.-i: is disallowed, because
it produces (?.sa-ib:whatever)
Instead, it would have to be something like:
my %default_on = map { $_ => 1 } qw/ a b /;
my %default_off = map { $_ => 1 } qw/ i m s x /;
my @options_on = qw/ s a /; # where a and b are hypothetical options
my @options_off = qw/ i b /; # that are on by default
my $pattern = "whatever";
my $regex =
"(?." . join('', grep !$default_on{$_}, @options_on) .
'-' . join('', grep !$default_off{$_}, @options_off) . ':' .
$pattern .
")";
which produces (?.s-b:whatever)
Furthermore, this program would have to be updated each time a new option
is added to the language, including it in %default_on or %default_off as
appropriate.
> The whole point is that you *don't* have to keep track of which flags
> are off by default. Any flags you don't specify will be 'off', including
> flags you don't know about because they haven't been added to perl yet.
If you disallow disabling flags that are off by default, then you *do* have
to keep track of which flags are off by default, because you have to know
which flags you're allowed to disable and which you're not.
Ronald
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