$data =~ s/
          <                 # First '<'
           (?!              # Not followed by (Everything in this list is allowed)
            (?:             # (with non-grouping parens)
             \/?br>         # A break tag
             |              # or
             \/?p>          # A paragraph tag
             |              # or
             \/?font[^>]*>  # A font tag
             |              # or
             \/?h[1-6]>     # A headline
            )               # Close non-grouping parens
           )                # End of negative lookahead
           (                # Capture to $1
            [^>]*           # Everything until the final '>'
           )                # End capture
          >                 # Final '>'
          /&lt;$1&gt;/gsix;