$foo = "xxx.x.....x......xxx...xx...x...xxx";

sub bar {
 my $n = shift;
 # do regex stuff on $foo ... 
 return $1 # or some such
}

# expected results for 
# n =
# 1 - x
# 2 - xx
# 3 - xxx
# 4 - xxx.x
# 5 - xxx.x.....x
# 6 - xxx.x.....x......x
# ... etc.