diff --git a/include/boost/regex/v4/basic_regex.hpp b/include/boost/regex/v4/basic_regex.hpp
index 6fa67348..e0feacfd 100644
--- a/include/boost/regex/v4/basic_regex.hpp
+++ b/include/boost/regex/v4/basic_regex.hpp
@@ -89,6 +89,12 @@ public:
       set_expression(ptr, f | regex_constants::use_except);
       return *this;
    }
+   reg_expression& assign(const charT* ptr, size_type len, flag_type f)
+   {
+      std::basic_string<charT> s(ptr, len);
+      set_expression(s.c_str(), f | regex_constants::use_except);
+      return *this;
+   }
 
    reg_expression& assign(const charT* first,
                           const charT* last,
@@ -172,9 +178,11 @@ public:
    locale_type BOOST_REGEX_CALL imbue(locale_type l){ return traits_inst.imbue(l); }
    locale_type BOOST_REGEX_CALL getloc()const{ return traits_inst.getloc(); }
    //
-   // flags:
+   // getflags:
+   // retained for backwards compatibility only, the base class has "flags"
+   // member which is now the prefered name:
    flag_type BOOST_REGEX_CALL getflags()const
-   { return flags(); }
+   { return this->flags(); }
    //
    // str:
    std::basic_string<charT> BOOST_REGEX_CALL str()const
diff --git a/include/boost/regex/v4/match_results.hpp b/include/boost/regex/v4/match_results.hpp
index a896cf42..8e82c1d3 100644
--- a/include/boost/regex/v4/match_results.hpp
+++ b/include/boost/regex/v4/match_results.hpp
@@ -38,7 +38,11 @@ private:
 #endif
 public: 
    typedef          sub_match<RandomAccessIterator>                         value_type;
+#ifndef BOOST_NO_STD_ALLOCATOR
+   typedef typename Allocator::const_reference                              const_reference;
+#else
    typedef          const value_type&                                       const_reference;
+#endif
    typedef          const_reference                                         reference;
    typedef typename vector_type::const_iterator                             const_iterator;
    typedef          const_iterator                                          iterator;
@@ -116,9 +120,7 @@ public:
       {
          return m_subs[sub];
       }
-      assert(0);
-      // we should never get here:
-      return m_subs[0];
+      return m_null;
    }
 
    const_reference prefix() const
@@ -181,6 +183,9 @@ public:
       m_subs[2].matched = true;
       m_subs[0].first = i;
       m_subs[0].matched = (m_subs[0].first != m_subs[0].second);
+      m_null.first = i;
+      m_null.second = i;
+      m_null.matched = false;
    }
 
    void BOOST_REGEX_CALL set_second(RandomAccessIterator i, size_type pos, bool m = true)
@@ -193,6 +198,9 @@ public:
       {
          m_subs[0].first = i;
          m_subs[0].matched = (m_subs[0].first != m_subs[0].second);
+         m_null.first = i;
+         m_null.second = i;
+         m_null.matched = false;
       }
    }
    void BOOST_REGEX_CALL set_size(size_type n, RandomAccessIterator i, RandomAccessIterator j)
@@ -244,6 +252,7 @@ public:
 private:
    vector_type            m_subs; // subexpressions
    RandomAccessIterator   m_base; // where the search started from
+   sub_match<RandomAccessIterator> m_null; // a null match
 };
 
 template <class RandomAccessIterator, class Allocator>
diff --git a/include/boost/regex/v4/regex_iterator.hpp b/include/boost/regex/v4/regex_iterator.hpp
index 82773005..9ef1dbab 100644
--- a/include/boost/regex/v4/regex_iterator.hpp
+++ b/include/boost/regex/v4/regex_iterator.hpp
@@ -107,17 +107,17 @@ public:
       pdata = that.pdata;
       return *this;
    }
-   bool operator==(const regex_iterator& that)
+   bool operator==(const regex_iterator& that)const
    { 
       if((pdata.get() == 0) || (that.pdata.get() == 0))
          return pdata.get() == that.pdata.get();
       return pdata->compare(*(that.pdata.get())); 
    }
-   bool operator!=(const regex_iterator& that)
+   bool operator!=(const regex_iterator& that)const
    { return !(*this == that); }
-   const value_type& operator*()
+   const value_type& operator*()const
    { return pdata->get(); }
-   const value_type* operator->()
+   const value_type* operator->()const
    { return &(pdata->get()); }
    regex_iterator& operator++()
    {
diff --git a/include/boost/regex/v4/regex_token_iterator.hpp b/include/boost/regex/v4/regex_token_iterator.hpp
index 94a34bd0..90fa7c21 100644
--- a/include/boost/regex/v4/regex_token_iterator.hpp
+++ b/include/boost/regex/v4/regex_token_iterator.hpp
@@ -211,17 +211,17 @@ public:
       pdata = that.pdata;
       return *this;
    }
-   bool operator==(const regex_token_iterator& that)
+   bool operator==(const regex_token_iterator& that)const
    { 
       if((pdata.get() == 0) || (that.pdata.get() == 0))
          return pdata.get() == that.pdata.get();
       return pdata->compare(*(that.pdata.get())); 
    }
-   bool operator!=(const regex_token_iterator& that)
+   bool operator!=(const regex_token_iterator& that)const
    { return !(*this == that); }
-   const value_type& operator*()
+   const value_type& operator*()const
    { return pdata->get(); }
-   const value_type* operator->()
+   const value_type* operator->()const
    { return &(pdata->get()); }
    regex_token_iterator& operator++()
    {
diff --git a/include/boost/regex/v4/sub_match.hpp b/include/boost/regex/v4/sub_match.hpp
index 80045a75..7e6712ac 100644
--- a/include/boost/regex/v4/sub_match.hpp
+++ b/include/boost/regex/v4/sub_match.hpp
@@ -102,156 +102,220 @@ struct sub_match : public std::pair<BidiIterator, BidiIterator>
 
 // comparison to std::basic_string<> part 1:
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator == (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
+inline bool operator == (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
                   const sub_match<RandomAccessIterator>& m)
 { return s.compare(m.str()) == 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator != (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
+inline bool operator != (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
                   const sub_match<RandomAccessIterator>& m)
 { return s.compare(m.str()) != 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator < (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
+inline bool operator < (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
                  const sub_match<RandomAccessIterator>& m)
 { return s.compare(m.str()) < 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator <= (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
+inline bool operator <= (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
                   const sub_match<RandomAccessIterator>& m)
 { return s.compare(m.str()) <= 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator >= (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
+inline bool operator >= (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
                   const sub_match<RandomAccessIterator>& m)
 { return s.compare(m.str()) >= 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator > (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
+inline bool operator > (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
                  const sub_match<RandomAccessIterator>& m)
 { return s.compare(m.str()) > 0; }
 // comparison to std::basic_string<> part 2:
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator == (const sub_match<RandomAccessIterator>& m,
+inline bool operator == (const sub_match<RandomAccessIterator>& m,
                   const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s)
 { return m.str().compare(s) == 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator != (const sub_match<RandomAccessIterator>& m,
+inline bool operator != (const sub_match<RandomAccessIterator>& m,
                   const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s)
 { return m.str().compare(s) != 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator < (const sub_match<RandomAccessIterator>& m,
+inline bool operator < (const sub_match<RandomAccessIterator>& m,
                   const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s)
 { return m.str().compare(s) < 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator > (const sub_match<RandomAccessIterator>& m,
+inline bool operator > (const sub_match<RandomAccessIterator>& m,
                   const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s)
 { return m.str().compare(s) > 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator <= (const sub_match<RandomAccessIterator>& m,
+inline bool operator <= (const sub_match<RandomAccessIterator>& m,
                   const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s)
 { return m.str().compare(s) <= 0; }
 template <class RandomAccessIterator, class traits, class Allocator>
-bool operator >= (const sub_match<RandomAccessIterator>& m,
+inline bool operator >= (const sub_match<RandomAccessIterator>& m,
                   const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s)
 { return m.str().compare(s) >= 0; }
 // comparison to const charT* part 1:
 template <class RandomAccessIterator>
-bool operator == (const sub_match<RandomAccessIterator>& m,
+inline bool operator == (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s)
 { return m.str().compare(s) == 0; }
 template <class RandomAccessIterator>
-bool operator != (const sub_match<RandomAccessIterator>& m,
+inline bool operator != (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s)
 { return m.str().compare(s) != 0; }
 template <class RandomAccessIterator>
-bool operator > (const sub_match<RandomAccessIterator>& m,
+inline bool operator > (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s)
 { return m.str().compare(s) > 0; }
 template <class RandomAccessIterator>
-bool operator < (const sub_match<RandomAccessIterator>& m,
+inline bool operator < (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s)
 { return m.str().compare(s) < 0; }
 template <class RandomAccessIterator>
-bool operator >= (const sub_match<RandomAccessIterator>& m,
+inline bool operator >= (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s)
 { return m.str().compare(s) >= 0; }
 template <class RandomAccessIterator>
-bool operator <= (const sub_match<RandomAccessIterator>& m,
+inline bool operator <= (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s)
 { return m.str().compare(s) <= 0; }
 // comparison to const charT* part 2:
 template <class RandomAccessIterator>
-bool operator == (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
+inline bool operator == (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(s) == 0; }
 template <class RandomAccessIterator>
-bool operator != (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
+inline bool operator != (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(s) != 0; }
 template <class RandomAccessIterator>
-bool operator < (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
+inline bool operator < (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(s) > 0; }
 template <class RandomAccessIterator>
-bool operator > (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
+inline bool operator > (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(s) < 0; }
 template <class RandomAccessIterator>
-bool operator <= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
+inline bool operator <= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(s) >= 0; }
 template <class RandomAccessIterator>
-bool operator >= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
+inline bool operator >= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(s) <= 0; }
 
 // comparison to const charT& part 1:
 template <class RandomAccessIterator>
-bool operator == (const sub_match<RandomAccessIterator>& m,
+inline bool operator == (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s)
 { return m.str().compare(0, m.length(), &s, 1) == 0; }
 template <class RandomAccessIterator>
-bool operator != (const sub_match<RandomAccessIterator>& m,
+inline bool operator != (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s)
 { return m.str().compare(0, m.length(), &s, 1) != 0; }
 template <class RandomAccessIterator>
-bool operator > (const sub_match<RandomAccessIterator>& m,
+inline bool operator > (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s)
 { return m.str().compare(0, m.length(), &s, 1) > 0; }
 template <class RandomAccessIterator>
-bool operator < (const sub_match<RandomAccessIterator>& m,
+inline bool operator < (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s)
 { return m.str().compare(0, m.length(), &s, 1) < 0; }
 template <class RandomAccessIterator>
-bool operator >= (const sub_match<RandomAccessIterator>& m,
+inline bool operator >= (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s)
 { return m.str().compare(0, m.length(), &s, 1) >= 0; }
 template <class RandomAccessIterator>
-bool operator <= (const sub_match<RandomAccessIterator>& m,
+inline bool operator <= (const sub_match<RandomAccessIterator>& m,
                   typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s)
 { return m.str().compare(0, m.length(), &s, 1) <= 0; }
 // comparison to const charT* part 2:
 template <class RandomAccessIterator>
-bool operator == (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
+inline bool operator == (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(0, m.length(), &s, 1) == 0; }
 template <class RandomAccessIterator>
-bool operator != (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
+inline bool operator != (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(0, m.length(), &s, 1) != 0; }
 template <class RandomAccessIterator>
-bool operator < (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
+inline bool operator < (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(0, m.length(), &s, 1) > 0; }
 template <class RandomAccessIterator>
-bool operator > (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
+inline bool operator > (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(0, m.length(), &s, 1) < 0; }
 template <class RandomAccessIterator>
-bool operator <= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
+inline bool operator <= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(0, m.length(), &s, 1) >= 0; }
 template <class RandomAccessIterator>
-bool operator >= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
+inline bool operator >= (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
                   const sub_match<RandomAccessIterator>& m)
 { return m.str().compare(0, m.length(), &s, 1) <= 0; }
 
+// addition operators:
+template <class RandomAccessIterator, class traits, class Allocator>
+inline std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator> 
+operator + (const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s,
+                  const sub_match<RandomAccessIterator>& m)
+{
+   std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator> result;
+   result.reserve(s.size() + m.length() + 1);
+   return result.append(s).append(m.first, m.second);
+}
+template <class RandomAccessIterator, class traits, class Allocator>
+inline std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator> 
+operator + (const sub_match<RandomAccessIterator>& m,
+            const std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator>& s)
+{
+   std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type, traits, Allocator> result;
+   result.reserve(s.size() + m.length() + 1);
+   return result.append(m.first, m.second).append(s);
+}
+template <class RandomAccessIterator>
+inline std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> 
+operator + (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const* s,
+                  const sub_match<RandomAccessIterator>& m)
+{
+   std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> result;
+   result.reserve(std::char_traits<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type>::length(s) + m.length() + 1);
+   return result.append(s).append(m.first, m.second);
+}
+template <class RandomAccessIterator>
+inline std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> 
+operator + (const sub_match<RandomAccessIterator>& m,
+            typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const * s)
+{
+   std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> result;
+   result.reserve(std::char_traits<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type>::length(s) + m.length() + 1);
+   return result.append(m.first, m.second).append(s);
+}
+template <class RandomAccessIterator>
+inline std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> 
+operator + (typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s,
+                  const sub_match<RandomAccessIterator>& m)
+{
+   std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> result;
+   result.reserve(m.length() + 2);
+   return result.append(1, s).append(m.first, m.second);
+}
+template <class RandomAccessIterator>
+inline std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> 
+operator + (const sub_match<RandomAccessIterator>& m,
+            typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type const& s)
+{
+   std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> result;
+   result.reserve(m.length() + 2);
+   return result.append(m.first, m.second).append(1, s);
+}
+template <class RandomAccessIterator>
+inline std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> 
+operator + (const sub_match<RandomAccessIterator>& m1,
+            const sub_match<RandomAccessIterator>& m2)
+{
+   std::basic_string<typename re_detail::regex_iterator_traits<RandomAccessIterator>::value_type> result;
+   result.reserve(m1.length() + m2.length() + 1);
+   return result.append(m1.first, m1.second).append(m2.first, m2.second);
+}
 #ifndef BOOST_NO_STD_LOCALE
 template <class charT, class traits, class RandomAccessIterator>
 std::basic_ostream<charT, traits>&