shell$ perl -pi.bak -le 's/(?<=[^n])n|n(?=[^n])/^/ig' yourfile.txt
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will do the substitution and leave a backup file named yourfile.txt.bak

UPDATE: it will not work, because your file has newlines (somehow I missed it). Try this instead:

perl -pi.bak -0377 -e 's/n([n\n]*)n/^$1^/ig' yourfile.txt
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