be
          backslash-escaped (see Backslash Escapes).  There are several
          other differences between basic and extended regular
          expressions, but these are the most significant.

     regexp/extended
          Compile an extended regular expression rather than a basic
          regexp.  This is the default behavior; this flag will not
          usually be needed.  If a call to `make-regexp' includes both
          `regexp/basic' and `regexp/extended' flags, the one which
          comes last will override the earlier one.