my $re = qr{ ^           # match at the beginning.
             (?!         # Look-ahead assertion: Fail if the next matches
               .* first  # match "first" everywhere except at the beginning of the string
             )           # end of look-ahead assertion
             This .* \.  # Match "This" (at the beginning), followed by anything and a dot
             $           # Match the end of a string.
         }x;             # Allow these comments