perl -e '
for $n(10..14) {
  $str = "x"x($n*2+1); 
  $t=time;
  $str =~ /(x+x+){$n}y/;
  print "\$str:$str n:$n time:",time-$t,"sec\n"
}'
$str:xxxxxxxxxxxxxxxxxxxxx n:10 time:0sec
$str:xxxxxxxxxxxxxxxxxxxxxxx n:11 time:3sec
$str:xxxxxxxxxxxxxxxxxxxxxxxxx n:12 time:9sec
$str:xxxxxxxxxxxxxxxxxxxxxxxxxxx n:13 time:39sec
$str:xxxxxxxxxxxxxxxxxxxxxxxxxxxxx n:14 time:156sec