But if I'm allowed to execute code, than the constraint "do it in a single regexp" isn't a constraint at all.
So here we go with 3 regexps and no code execution (exept for initializing and printing): 

$_="x" x 30;
s:xx:$' $`$&\n:g;
s:(?<=\n).*:$&$`$`:s;
s/xx(?=(x*)(\1) )/ /g;
print;

[download]
s$$([},&%#}/&/]+}%&{})*;#$&&s&&$^X.($'^"%]=\&(|?*{%
+.+=%;.#_}\&"^"-+%*).}%:##%}={~=~:.")&e&&s""`$''`"e

In reply to Re^2: Resetting pos() within a RegEx? by Skeeve
in thread Resetting pos() within a RegEx? by LanX

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