Proposed resolution:

1. In the class template sub_match synopsis [re.submatch]/1 change as indicated:

   template <class BidirectionalIterator>
   class sub_match : public std::pair<BidirectionalIterator, BidirectionalIterator> {
   public:
     typedef typename iterator_traits<BidirectionalIterator>::value_type value_type;
     typedef typename iterator_traits<BidirectionalIterator>::difference_type difference_type;
     typedef BidirectionalIterator iterator;
     typedef basic_string<value_type> string_type;
   
     bool matched;
   
     difference_type length() const;
     operator basic_string<value_type>string_type() const;
     basic_string<value_type>string_type str() const;
     int compare(const sub_match& s) const;
     int compare(const basic_string<value_type>string_type& s) const;
     int compare(const value_type* s) const;
   };
   

2. In [re.submatch.members]/2 change as indicated:

   operator basic_string<value_type>string_type() const;
   

   Returns: matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type().

3. In [re.submatch.members]/3 change as indicated:

   basic_string<value_type>string_type str() const;
   

   Returns: matched ? basic_string<value_type> string_type(first, second) : basic_string<value_type> string_type().

4. In [re.submatch.members]/5 change as indicated:

   int compare(const basic_string<value_type>string_type& s) const;
   

[ 2009-11-15 Moved to Tentatively Ready after 5 positive votes on c++std-lib. ]

The definition of class template sub_match is strongly dependent on the type basic_string<value_type>, both in interface and effects, but does not provide a corresponding typedef string_type, as e.g. class match_results does, which looks like an oversight to me that should be fixed.