I scan the entire thread. I figured it's done seldom enough that speed isn't crucial. It's not THAT slow, either, at least not with my data storage setup (keeping posts on separate single lines of a HTML file).

Here's the relevant code (@page contains the lines of the current thread page, $num is the argument to q, @posts is filled with a list of which posts to display, $replyrange_re is a regexp to match the >> syntax, in_range() checks if a certain post number is contained in a >> reference.

push @posts,$num;
OUTER: foreach my $post (1..$total)
{
	next if($post eq $num);
	while($page[$post+1]=~/>>($replyrange_re)/g)
	{
		if(in_range($num,$1)) { push @posts,$post; next OUTER; }
	}
}

Here's in_range():

sub in_range($$)
{

my ($num,$ranges)=@_;

foreach my $range (split /(,|,)/,$ranges)
{
	if($range=~/^([0-9]*)-([0-9]*)$/)
	{
		my $start=($1 or 1);
		my $end=($2 or 1000000); # arbitary large number

		($start,$end)=($end,$start) if($start>$end);

		return 1 if($num>=$start and $num<=$end);
	}
	elsif($range=~/^([0-9]+)$/)
	{
		return 1 if($num==$1);
	}
	#elsif($range=~/^l([0-9]+)$/i) {} # l ranges never match
	#elsif($range=~/^r([0-9]*)$/i) {} # r ranges never match
	#elsif($range=~/^q([0-9]+)$/i) {} # q ranges never match
}
return 0;

}