n = 
1 - x
2 - xx
3 - xxx.
4 - xxx.x....
5 - xxx.x.....x......

##</code><code>##

my $regex = '\A' . ('[^x]*x' x $n) .  '[^x]*';
$foo =~ m{ ($regex) }xms;    # PBP orthodoxy :)

##</code><code>##

my $regex = '\A' . ('[^x]*?x' x $n);