Special case for <hr />.

%r{
      (                                             # $1
              \A\n?                                # Start of doc + optional \n
              |                                    # or
              .*\n\n                               # anything + blank line
      )
      (                                             # save in $2
              [ ]*                         # Any spaces
              <hr                                  # Tag open
              \b                                   # Word break
              ([^<>])*?                    # Attributes
              /?>                                  # Tag close
              $                                    # followed by a blank line or end of document
      )
}ix