Okay, I think this one looks good too: 
s[(STARTPRESERVE.*?STOPPRESERVE)|(\s+)|(.)]
 [($1||'') . ($2?' ':'') . ($3||'')]eg;

[download]
The ||'' is to avoid the warnings.

Update: And this is of course wrong, since I'm testing with || and should do defined instead.

Update2: This is a correct and shorter version: 

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

[download]
because $1 and $2 can both be tested for truth.

Update3 (ouch): 

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

[download]
Now this looks alot like tye's suggestion except for elimination of defined :-(

In reply to Re: RegEx to filter \s not between labels by abstracts
in thread RegEx to filter \s not between labels by gryphon

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