template <class BiIter, class ST, class SA>
  bool operator==(
    const basic_string<
      typename iterator_traits<BiIter>::value_type, ST, SA>& lhs,
    const sub_match<BiIter>& rhs);

is specified as:

Returns: rhs.compare(lhs.c_str()) == 0.

This is odd because sub_match::compare(basic_string) is defined to honor embedded '\0' characters. This could allow a sub_match to == or != a std::string unexpectedly.