The OP wanted CCHHHHHHHCC, not CCHHHHHHHHC. Maybe that's a typo, or maybe he wants all the surrounded characters substituted simultaneously, as in a cellular automaton. In that case, 
s/(?<=(\w))(?!\1).(?=\1)/$1/g;

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In reply to Re^2: Why my Regex doesn't work by Anonymous Monk
in thread Why my Regex doesn't work by flappygoat

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