diff --git a/asio/include/asio/detail/regex_fwd.hpp b/asio/include/asio/detail/regex_fwd.hpp
index 11bfed96d3..fe1092231e 100644
--- a/asio/include/asio/detail/regex_fwd.hpp
+++ b/asio/include/asio/detail/regex_fwd.hpp
@@ -17,18 +17,6 @@
 
 #if defined(ASIO_HAS_BOOST_REGEX)
 
-#include <boost/regex_fwd.hpp>
-#include <boost/version.hpp>
-#if BOOST_VERSION >= 107600
-# if defined(BOOST_REGEX_CXX03)
-#  include <boost/regex/v4/match_flags.hpp>
-# else // defined(BOOST_REGEX_CXX03)
-#  include <boost/regex/v5/match_flags.hpp>
-# endif // defined(BOOST_REGEX_CXX03)
-#else // BOOST_VERSION >= 107600
-# include <boost/regex/v4/match_flags.hpp>
-#endif // BOOST_VERSION >= 107600
-
 namespace boost {
 
 template <class BidiIterator>
@@ -37,6 +25,9 @@ struct sub_match;
 template <class BidiIterator, class Allocator>
 class match_results;
 
+template <class CharT, class Traits>
+class basic_regex;
+
 } // namespace boost
 
 #endif // defined(ASIO_HAS_BOOST_REGEX)
diff --git a/asio/include/asio/impl/read_until.hpp b/asio/include/asio/impl/read_until.hpp
index c1d7dbcce8..f3f1fbb554 100644
--- a/asio/include/asio/impl/read_until.hpp
+++ b/asio/include/asio/impl/read_until.hpp
@@ -231,10 +231,22 @@ std::size_t read_until(SyncReadStream& s,
 #if !defined(ASIO_NO_EXTENSIONS)
 #if defined(ASIO_HAS_BOOST_REGEX)
 
-template <typename SyncReadStream, typename DynamicBuffer_v1>
-inline std::size_t read_until(SyncReadStream& s,
-    DynamicBuffer_v1&& buffers,
-    const boost::regex& expr,
+namespace detail {
+
+struct regex_match_flags
+{
+  template <typename T>
+  operator T() const
+  {
+    return T::match_default | T::match_partial;
+  }
+};
+
+} // namespace detail
+
+template <typename SyncReadStream, typename DynamicBuffer_v1, typename Traits>
+inline std::size_t read_until(SyncReadStream& s, DynamicBuffer_v1&& buffers,
+    const boost::basic_regex<char, Traits>& expr,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
@@ -249,10 +261,9 @@ inline std::size_t read_until(SyncReadStream& s,
   return bytes_transferred;
 }
 
-template <typename SyncReadStream, typename DynamicBuffer_v1>
-std::size_t read_until(SyncReadStream& s,
-    DynamicBuffer_v1&& buffers,
-    const boost::regex& expr, asio::error_code& ec,
+template <typename SyncReadStream, typename DynamicBuffer_v1, typename Traits>
+std::size_t read_until(SyncReadStream& s, DynamicBuffer_v1&& buffers,
+    const boost::basic_regex<char, Traits>& expr, asio::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
@@ -278,8 +289,8 @@ std::size_t read_until(SyncReadStream& s,
     boost::match_results<iterator,
       typename std::vector<boost::sub_match<iterator>>::allocator_type>
         match_results;
-    if (regex_search(start_pos, end, match_results, expr,
-          boost::match_default | boost::match_partial))
+    if (regex_search(start_pos, end, match_results,
+          expr, detail::regex_match_flags()))
     {
       if (match_results[0].matched)
       {
@@ -441,16 +452,18 @@ inline std::size_t read_until(SyncReadStream& s,
 
 #if defined(ASIO_HAS_BOOST_REGEX)
 
-template <typename SyncReadStream, typename Allocator>
+template <typename SyncReadStream, typename Allocator, typename Traits>
 inline std::size_t read_until(SyncReadStream& s,
-    asio::basic_streambuf<Allocator>& b, const boost::regex& expr)
+    asio::basic_streambuf<Allocator>& b,
+    const boost::basic_regex<char, Traits>& expr)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), expr);
 }
 
-template <typename SyncReadStream, typename Allocator>
+template <typename SyncReadStream, typename Allocator, typename Traits>
 inline std::size_t read_until(SyncReadStream& s,
-    asio::basic_streambuf<Allocator>& b, const boost::regex& expr,
+    asio::basic_streambuf<Allocator>& b,
+    const boost::basic_regex<char, Traits>& expr,
     asio::error_code& ec)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), expr, ec);
@@ -629,9 +642,9 @@ std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
 #if !defined(ASIO_NO_EXTENSIONS)
 #if defined(ASIO_HAS_BOOST_REGEX)
 
-template <typename SyncReadStream, typename DynamicBuffer_v2>
-inline std::size_t read_until(SyncReadStream& s,
-    DynamicBuffer_v2 buffers, const boost::regex& expr,
+template <typename SyncReadStream, typename DynamicBuffer_v2, typename Traits>
+inline std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
+    const boost::basic_regex<char, Traits>& expr,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
@@ -643,9 +656,9 @@ inline std::size_t read_until(SyncReadStream& s,
   return bytes_transferred;
 }
 
-template <typename SyncReadStream, typename DynamicBuffer_v2>
+template <typename SyncReadStream, typename DynamicBuffer_v2, typename Traits>
 std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
-    const boost::regex& expr, asio::error_code& ec,
+    const boost::basic_regex<char, Traits>& expr, asio::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
@@ -668,8 +681,8 @@ std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
     boost::match_results<iterator,
       typename std::vector<boost::sub_match<iterator>>::allocator_type>
         match_results;
-    if (regex_search(start_pos, end, match_results, expr,
-          boost::match_default | boost::match_partial))
+    if (regex_search(start_pos, end, match_results,
+          expr, detail::regex_match_flags()))
     {
       if (match_results[0].matched)
       {
@@ -1303,10 +1316,9 @@ namespace detail
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
-    template <typename BufferSequence>
-    read_until_expr_op_v1(AsyncReadStream& stream,
-        BufferSequence&& buffers,
-        const boost::regex& expr, ReadHandler& handler)
+    template <typename BufferSequence, typename Traits>
+    read_until_expr_op_v1(AsyncReadStream& stream, BufferSequence&& buffers,
+        const boost::basic_regex<char, Traits>& expr, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
@@ -1365,8 +1377,8 @@ namespace detail
             boost::match_results<iterator,
               typename std::vector<boost::sub_match<iterator>>::allocator_type>
                 match_results;
-            bool match = regex_search(start_pos, end, match_results, expr_,
-                boost::match_default | boost::match_partial);
+            bool match = regex_search(start_pos, end,
+                match_results, expr_, regex_match_flags());
             if (match && match_results[0].matched)
             {
               // Full match. We're done.
@@ -1525,11 +1537,11 @@ struct associator<Associator,
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
-template <typename AsyncReadStream, typename DynamicBuffer_v1,
+template <typename AsyncReadStream, typename DynamicBuffer_v1, typename Traits,
     ASIO_COMPLETION_TOKEN_FOR(void (asio::error_code,
       std::size_t)) ReadToken>
 inline auto async_read_until(AsyncReadStream& s, DynamicBuffer_v1&& buffers,
-    const boost::regex& expr, ReadToken&& token,
+    const boost::basic_regex<char, Traits>& expr, ReadToken&& token,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
@@ -1849,12 +1861,12 @@ inline auto async_read_until(AsyncReadStream& s,
 
 #if defined(ASIO_HAS_BOOST_REGEX)
 
-template <typename AsyncReadStream, typename Allocator,
+template <typename AsyncReadStream, typename Allocator, typename Traits,
     ASIO_COMPLETION_TOKEN_FOR(void (asio::error_code,
       std::size_t)) ReadToken>
 inline auto async_read_until(AsyncReadStream& s,
     asio::basic_streambuf<Allocator>& b,
-    const boost::regex& expr, ReadToken&& token)
+    const boost::basic_regex<char, Traits>& expr, ReadToken&& token)
   -> decltype(
     async_initiate<ReadToken,
       void (asio::error_code, std::size_t)>(
@@ -2407,10 +2419,9 @@ namespace detail
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
-    template <typename BufferSequence>
-    read_until_expr_op_v2(AsyncReadStream& stream,
-        BufferSequence&& buffers,
-        const boost::regex& expr, ReadHandler& handler)
+    template <typename BufferSequence, typename Traits>
+    read_until_expr_op_v2(AsyncReadStream& stream, BufferSequence&& buffers,
+        const boost::basic_regex<char, Traits>& expr, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
@@ -2474,8 +2485,8 @@ namespace detail
             boost::match_results<iterator,
               typename std::vector<boost::sub_match<iterator>>::allocator_type>
                 match_results;
-            bool match = regex_search(start_pos, end, match_results, expr_,
-                boost::match_default | boost::match_partial);
+            bool match = regex_search(start_pos, end,
+                match_results, expr_, regex_match_flags());
             if (match && match_results[0].matched)
             {
               // Full match. We're done.
@@ -2638,11 +2649,11 @@ struct associator<Associator,
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
-template <typename AsyncReadStream, typename DynamicBuffer_v2,
+template <typename AsyncReadStream, typename DynamicBuffer_v2, typename Traits,
     ASIO_COMPLETION_TOKEN_FOR(void (asio::error_code,
       std::size_t)) ReadToken>
 inline auto async_read_until(AsyncReadStream& s, DynamicBuffer_v2 buffers,
-    const boost::regex& expr, ReadToken&& token,
+    const boost::basic_regex<char, Traits>& expr, ReadToken&& token,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
diff --git a/asio/include/asio/read_until.hpp b/asio/include/asio/read_until.hpp
index 8e820f0fc0..7ce82d385a 100644
--- a/asio/include/asio/read_until.hpp
+++ b/asio/include/asio/read_until.hpp
@@ -365,10 +365,9 @@ std::size_t read_until(SyncReadStream& s,
  * This data may be the start of a new line, to be extracted by a subsequent
  * @c read_until operation.
  */
-template <typename SyncReadStream, typename DynamicBuffer_v1>
-std::size_t read_until(SyncReadStream& s,
-    DynamicBuffer_v1&& buffers,
-    const boost::regex& expr,
+template <typename SyncReadStream, typename DynamicBuffer_v1, typename Traits>
+std::size_t read_until(SyncReadStream& s, DynamicBuffer_v1&& buffers,
+    const boost::basic_regex<char, Traits>& expr,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     > = 0,
@@ -412,10 +411,9 @@ std::size_t read_until(SyncReadStream& s,
  * expression. An application will typically leave that data in the dynamic
  * buffer sequence for a subsequent read_until operation to examine.
  */
-template <typename SyncReadStream, typename DynamicBuffer_v1>
-std::size_t read_until(SyncReadStream& s,
-    DynamicBuffer_v1&& buffers,
-    const boost::regex& expr, asio::error_code& ec,
+template <typename SyncReadStream, typename DynamicBuffer_v1, typename Traits>
+std::size_t read_until(SyncReadStream& s, DynamicBuffer_v1&& buffers,
+    const boost::basic_regex<char, Traits>& expr, asio::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     > = 0,
@@ -841,9 +839,10 @@ std::size_t read_until(SyncReadStream& s,
  * This data may be the start of a new line, to be extracted by a subsequent
  * @c read_until operation.
  */
-template <typename SyncReadStream, typename Allocator>
+template <typename SyncReadStream, typename Allocator, typename Traits>
 std::size_t read_until(SyncReadStream& s,
-    asio::basic_streambuf<Allocator>& b, const boost::regex& expr);
+    asio::basic_streambuf<Allocator>& b,
+    const boost::basic_regex<char, Traits>& expr);
 
 /// Read data into a streambuf until some part of the data it contains matches
 /// a regular expression.
@@ -878,9 +877,10 @@ std::size_t read_until(SyncReadStream& s,
  * application will typically leave that data in the streambuf for a subsequent
  * read_until operation to examine.
  */
-template <typename SyncReadStream, typename Allocator>
+template <typename SyncReadStream, typename Allocator, typename Traits>
 std::size_t read_until(SyncReadStream& s,
-    asio::basic_streambuf<Allocator>& b, const boost::regex& expr,
+    asio::basic_streambuf<Allocator>& b,
+    const boost::basic_regex<char, Traits>& expr,
     asio::error_code& ec);
 
 #endif // defined(ASIO_HAS_BOOST_REGEX)
@@ -1309,9 +1309,9 @@ std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
  * This data may be the start of a new line, to be extracted by a subsequent
  * @c read_until operation.
  */
-template <typename SyncReadStream, typename DynamicBuffer_v2>
+template <typename SyncReadStream, typename DynamicBuffer_v2, typename Traits>
 std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
-    const boost::regex& expr,
+    const boost::basic_regex<char, Traits>& expr,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     > = 0);
@@ -1352,9 +1352,9 @@ std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
  * expression. An application will typically leave that data in the dynamic
  * buffer sequence for a subsequent read_until operation to examine.
  */
-template <typename SyncReadStream, typename DynamicBuffer_v2>
+template <typename SyncReadStream, typename DynamicBuffer_v2, typename Traits>
 std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
-    const boost::regex& expr, asio::error_code& ec,
+    const boost::basic_regex<char, Traits>& expr, asio::error_code& ec,
     constraint_t<
         is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     > = 0);
@@ -1897,12 +1897,12 @@ auto async_read_until(AsyncReadStream& s,
  * if they are also supported by the @c AsyncReadStream type's
  * @c async_read_some operation.
  */
-template <typename AsyncReadStream, typename DynamicBuffer_v1,
+template <typename AsyncReadStream, typename DynamicBuffer_v1, typename Traits,
     ASIO_COMPLETION_TOKEN_FOR(void (asio::error_code,
       std::size_t)) ReadToken = default_completion_token_t<
         typename AsyncReadStream::executor_type>>
-auto async_read_until(AsyncReadStream& s,
-    DynamicBuffer_v1&& buffers, const boost::regex& expr,
+auto async_read_until(AsyncReadStream& s, DynamicBuffer_v1&& buffers,
+    const boost::basic_regex<char, Traits>& expr,
     ReadToken&& token = default_completion_token_t<
       typename AsyncReadStream::executor_type>(),
     constraint_t<
@@ -2413,12 +2413,13 @@ auto async_read_until(AsyncReadStream& s,
  * if they are also supported by the @c AsyncReadStream type's
  * @c async_read_some operation.
  */
-template <typename AsyncReadStream, typename Allocator,
+template <typename AsyncReadStream, typename Allocator, typename Traits,
     ASIO_COMPLETION_TOKEN_FOR(void (asio::error_code,
       std::size_t)) ReadToken = default_completion_token_t<
         typename AsyncReadStream::executor_type>>
 auto async_read_until(AsyncReadStream& s,
-    asio::basic_streambuf<Allocator>& b, const boost::regex& expr,
+    asio::basic_streambuf<Allocator>& b,
+    const boost::basic_regex<char, Traits>& expr,
     ReadToken&& token = default_completion_token_t<
       typename AsyncReadStream::executor_type>())
   -> decltype(
@@ -2925,12 +2926,12 @@ auto async_read_until(AsyncReadStream& s, DynamicBuffer_v2 buffers,
  * if they are also supported by the @c AsyncReadStream type's
  * @c async_read_some operation.
  */
-template <typename AsyncReadStream, typename DynamicBuffer_v2,
+template <typename AsyncReadStream, typename DynamicBuffer_v2, typename Traits,
     ASIO_COMPLETION_TOKEN_FOR(void (asio::error_code,
       std::size_t)) ReadToken = default_completion_token_t<
         typename AsyncReadStream::executor_type>>
-auto async_read_until(AsyncReadStream& s,
-    DynamicBuffer_v2 buffers, const boost::regex& expr,
+auto async_read_until(AsyncReadStream& s, DynamicBuffer_v2 buffers,
+    const boost::basic_regex<char, Traits>& expr,
     ReadToken&& token = default_completion_token_t<
       typename AsyncReadStream::executor_type>(),
     constraint_t<