This substitution, s/([^^#])/$1$1/g, will do what you want. I'd expect it to be the fastest. It replaces any non-^ non-# character with itself twice.
Jeff japhy Pinyan, P.L., P.M., P.O.D, X.S.: Perl, regex, and perl hacker
How can we ever be the sold short or the cheated, we who for every service have long ago been overpaid? ~~ Meister Eckhart

In reply to Re: modifying a string in place by japhy
in thread modifying a string in place by davidj

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