s[(STARTPRESERVE.*?STOPPRESERVE)|(\s+)|(.)]
 [($1||'') . ($2?' ':'') . ($3||'')]eg;

##</code><code>##

s#(STARTPRESERVE.*?STOPPRESERVE)|(\s+)|(.)#$1?$1:$2?' ':$3#eg;

##</code><code>##

s#(STARTPRESERVE.*?STOPPRESERVE)|(\s+)#$1?$1:$2?' ':''#eg;