Thank you very much! ccn, jbware
Your solution is great.
I've made a one-liner from it and it looks like
$text=~s{((?:([^\[\]\s])(?:(\[[^\]]*\])*)){3})(?!(?:(\[[^\]]*\])*)(?:\s|\Z))(?=[^\[\]]*(?:\[|\z))}{$1-}gx;
It's so nice! Can be used in some obfuscation code... Thanks again!