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 //
 // impl/read_until.hpp
 // ~~~~~~~~~~~~~~~~~~~
 //
 // Copyright (c) 2003-2024 Christopher M. Kohlhoff (chris at kohlhoff dot com)
 //
 // Distributed under the Boost Software License, Version 1.0. (See accompanying
 // file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
 //
 
 #ifndef BOOST_ASIO_IMPL_READ_UNTIL_HPP
 #define BOOST_ASIO_IMPL_READ_UNTIL_HPP
 
 #if defined(_MSC_VER) && (_MSC_VER >= 1200)
 # pragma once
 #endif // defined(_MSC_VER) && (_MSC_VER >= 1200)
 
 #include <algorithm>
 #include <string>
 #include <vector>
 #include <utility>
 #include <boost/asio/associator.hpp>
 #include <boost/asio/buffer.hpp>
 #include <boost/asio/buffers_iterator.hpp>
 #include <boost/asio/detail/base_from_cancellation_state.hpp>
 #include <boost/asio/detail/bind_handler.hpp>
 #include <boost/asio/detail/handler_cont_helpers.hpp>
 #include <boost/asio/detail/handler_tracking.hpp>
 #include <boost/asio/detail/handler_type_requirements.hpp>
 #include <boost/asio/detail/limits.hpp>
 #include <boost/asio/detail/non_const_lvalue.hpp>
 #include <boost/asio/detail/throw_error.hpp>
 
 #include <boost/asio/detail/push_options.hpp>
 
 namespace boost {
 namespace asio {
 
 namespace detail
 {
   // Algorithm that finds a subsequence of equal values in a sequence. Returns
   // (iterator,true) if a full match was found, in which case the iterator
   // points to the beginning of the match. Returns (iterator,false) if a
   // partial match was found at the end of the first sequence, in which case
   // the iterator points to the beginning of the partial match. Returns
   // (last1,false) if no full or partial match was found.
   template <typename Iterator1, typename Iterator2>
   std::pair<Iterator1, bool> partial_search(
       Iterator1 first1, Iterator1 last1, Iterator2 first2, Iterator2 last2)
   {
     for (Iterator1 iter1 = first1; iter1 != last1; ++iter1)
     {
       Iterator1 test_iter1 = iter1;
       Iterator2 test_iter2 = first2;
       for (;; ++test_iter1, ++test_iter2)
       {
         if (test_iter2 == last2)
           return std::make_pair(iter1, true);
         if (test_iter1 == last1)
         {
           if (test_iter2 != first2)
             return std::make_pair(iter1, false);
           else
             break;
         }
         if (*test_iter1 != *test_iter2)
           break;
       }
     }
     return std::make_pair(last1, false);
   }
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if defined(BOOST_ASIO_HAS_BOOST_REGEX)
   struct regex_match_flags
   {
     template <typename T>
     operator T() const
     {
       return T::match_default | T::match_partial;
     }
   };
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 #endif // defined(BOOST_ASIO_HAS_BOOST_REGEX)
 } // namespace detail
 
 #if !defined(BOOST_ASIO_NO_DYNAMIC_BUFFER_V1)
 
 template <typename SyncReadStream, typename DynamicBuffer_v1>
 inline std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v1&& buffers, char delim,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v1&&>(buffers), delim, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 template <typename SyncReadStream, typename DynamicBuffer_v1>
 std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v1&& buffers,
     char delim, boost::system::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   decay_t<DynamicBuffer_v1> b(
       static_cast<DynamicBuffer_v1&&>(buffers));
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v1::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers = b.data();
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     iterator iter = std::find(start_pos, end, delim);
     if (iter != end)
     {
       // Found a match. We're done.
       ec = boost::system::error_code();
       return iter - begin + 1;
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     b.commit(s.read_some(b.prepare(bytes_to_read), ec));
     if (ec)
       return 0;
   }
 }
 
 template <typename SyncReadStream, typename DynamicBuffer_v1>
 inline std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v1&& buffers,
     BOOST_ASIO_STRING_VIEW_PARAM delim,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v1&&>(buffers), delim, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 template <typename SyncReadStream, typename DynamicBuffer_v1>
 std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v1&& buffers,
     BOOST_ASIO_STRING_VIEW_PARAM delim, boost::system::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   decay_t<DynamicBuffer_v1> b(
       static_cast<DynamicBuffer_v1&&>(buffers));
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v1::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers = b.data();
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     std::pair<iterator, bool> result = detail::partial_search(
         start_pos, end, delim.begin(), delim.end());
     if (result.first != end)
     {
       if (result.second)
       {
         // Full match. We're done.
         ec = boost::system::error_code();
         return result.first - begin + delim.length();
       }
       else
       {
         // Partial match. Next search needs to start from beginning of match.
         search_position = result.first - begin;
       }
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     b.commit(s.read_some(b.prepare(bytes_to_read), ec));
     if (ec)
       return 0;
   }
 }
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 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
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v1&&>(buffers), expr, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 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, boost::system::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   decay_t<DynamicBuffer_v1> b(
       static_cast<DynamicBuffer_v1&&>(buffers));
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v1::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers = b.data();
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     boost::match_results<iterator,
       typename std::vector<boost::sub_match<iterator>>::allocator_type>
         match_results;
     if (regex_search(start_pos, end, match_results,
           expr, detail::regex_match_flags()))
     {
       if (match_results[0].matched)
       {
         // Full match. We're done.
         ec = boost::system::error_code();
         return match_results[0].second - begin;
       }
       else
       {
         // Partial match. Next search needs to start from beginning of match.
         search_position = match_results[0].first - begin;
       }
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     b.commit(s.read_some(b.prepare(bytes_to_read), ec));
     if (ec)
       return 0;
   }
 }
 
 #endif // defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 template <typename SyncReadStream,
     typename DynamicBuffer_v1, typename MatchCondition>
 inline std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v1&& buffers,
     MatchCondition match_condition,
     constraint_t<
       is_match_condition<MatchCondition>::value
     >,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v1&&>(buffers),
       match_condition, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 template <typename SyncReadStream,
     typename DynamicBuffer_v1, typename MatchCondition>
 std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v1&& buffers,
     MatchCondition match_condition, boost::system::error_code& ec,
     constraint_t<
       is_match_condition<MatchCondition>::value
     >,
     constraint_t<
       is_dynamic_buffer_v1<decay_t<DynamicBuffer_v1>>::value
     >,
     constraint_t<
       !is_dynamic_buffer_v2<decay_t<DynamicBuffer_v1>>::value
     >)
 {
   decay_t<DynamicBuffer_v1> b(
       static_cast<DynamicBuffer_v1&&>(buffers));
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v1::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers = b.data();
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     std::pair<iterator, bool> result = match_condition(start_pos, end);
     if (result.second)
     {
       // Full match. We're done.
       ec = boost::system::error_code();
       return result.first - begin;
     }
     else if (result.first != end)
     {
       // Partial match. Next search needs to start from beginning of match.
       search_position = result.first - begin;
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     b.commit(s.read_some(b.prepare(bytes_to_read), ec));
     if (ec)
       return 0;
   }
 }
 
 #if !defined(BOOST_ASIO_NO_IOSTREAM)
 
 template <typename SyncReadStream, typename Allocator>
 inline std::size_t read_until(SyncReadStream& s,
     boost::asio::basic_streambuf<Allocator>& b, char delim)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), delim);
 }
 
 template <typename SyncReadStream, typename Allocator>
 inline std::size_t read_until(SyncReadStream& s,
     boost::asio::basic_streambuf<Allocator>& b, char delim,
     boost::system::error_code& ec)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), delim, ec);
 }
 
 template <typename SyncReadStream, typename Allocator>
 inline std::size_t read_until(SyncReadStream& s,
     boost::asio::basic_streambuf<Allocator>& b,
     BOOST_ASIO_STRING_VIEW_PARAM delim)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), delim);
 }
 
 template <typename SyncReadStream, typename Allocator>
 inline std::size_t read_until(SyncReadStream& s,
     boost::asio::basic_streambuf<Allocator>& b,
     BOOST_ASIO_STRING_VIEW_PARAM delim, boost::system::error_code& ec)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), delim, ec);
 }
 
 #if defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 template <typename SyncReadStream, typename Allocator, typename Traits>
 inline std::size_t read_until(SyncReadStream& s,
     boost::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, typename Traits>
 inline std::size_t read_until(SyncReadStream& s,
     boost::asio::basic_streambuf<Allocator>& b,
     const boost::basic_regex<char, Traits>& expr,
     boost::system::error_code& ec)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), expr, ec);
 }
 
 #endif // defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 template <typename SyncReadStream, typename Allocator, typename MatchCondition>
 inline std::size_t read_until(SyncReadStream& s,
     boost::asio::basic_streambuf<Allocator>& b, MatchCondition match_condition,
     constraint_t<is_match_condition<MatchCondition>::value>)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), match_condition);
 }
 
 template <typename SyncReadStream, typename Allocator, typename MatchCondition>
 inline std::size_t read_until(SyncReadStream& s,
     boost::asio::basic_streambuf<Allocator>& b,
     MatchCondition match_condition, boost::system::error_code& ec,
     constraint_t<is_match_condition<MatchCondition>::value>)
 {
   return read_until(s, basic_streambuf_ref<Allocator>(b), match_condition, ec);
 }
 
 #endif // !defined(BOOST_ASIO_NO_IOSTREAM)
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 #endif // !defined(BOOST_ASIO_NO_DYNAMIC_BUFFER_V1)
 
 template <typename SyncReadStream, typename DynamicBuffer_v2>
 inline std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v2 buffers, char delim,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v2&&>(buffers), delim, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 template <typename SyncReadStream, typename DynamicBuffer_v2>
 std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
     char delim, boost::system::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
 {
   DynamicBuffer_v2& b = buffers;
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v2::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers =
       const_cast<const DynamicBuffer_v2&>(b).data(0, b.size());
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     iterator iter = std::find(start_pos, end, delim);
     if (iter != end)
     {
       // Found a match. We're done.
       ec = boost::system::error_code();
       return iter - begin + 1;
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     std::size_t pos = b.size();
     b.grow(bytes_to_read);
     std::size_t bytes_transferred = s.read_some(b.data(pos, bytes_to_read), ec);
     b.shrink(bytes_to_read - bytes_transferred);
     if (ec)
       return 0;
   }
 }
 
 template <typename SyncReadStream, typename DynamicBuffer_v2>
 inline std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v2 buffers, BOOST_ASIO_STRING_VIEW_PARAM delim,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v2&&>(buffers), delim, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 template <typename SyncReadStream, typename DynamicBuffer_v2>
 std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
     BOOST_ASIO_STRING_VIEW_PARAM delim, boost::system::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
 {
   DynamicBuffer_v2& b = buffers;
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v2::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers =
       const_cast<const DynamicBuffer_v2&>(b).data(0, b.size());
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     std::pair<iterator, bool> result = detail::partial_search(
         start_pos, end, delim.begin(), delim.end());
     if (result.first != end)
     {
       if (result.second)
       {
         // Full match. We're done.
         ec = boost::system::error_code();
         return result.first - begin + delim.length();
       }
       else
       {
         // Partial match. Next search needs to start from beginning of match.
         search_position = result.first - begin;
       }
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     std::size_t pos = b.size();
     b.grow(bytes_to_read);
     std::size_t bytes_transferred = s.read_some(b.data(pos, bytes_to_read), ec);
     b.shrink(bytes_to_read - bytes_transferred);
     if (ec)
       return 0;
   }
 }
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 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
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v2&&>(buffers), expr, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 template <typename SyncReadStream, typename DynamicBuffer_v2, typename Traits>
 std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
     const boost::basic_regex<char, Traits>& expr, boost::system::error_code& ec,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
 {
   DynamicBuffer_v2& b = buffers;
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v2::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers =
       const_cast<const DynamicBuffer_v2&>(b).data(0, b.size());
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     boost::match_results<iterator,
       typename std::vector<boost::sub_match<iterator>>::allocator_type>
         match_results;
     if (regex_search(start_pos, end, match_results,
           expr, detail::regex_match_flags()))
     {
       if (match_results[0].matched)
       {
         // Full match. We're done.
         ec = boost::system::error_code();
         return match_results[0].second - begin;
       }
       else
       {
         // Partial match. Next search needs to start from beginning of match.
         search_position = match_results[0].first - begin;
       }
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     std::size_t pos = b.size();
     b.grow(bytes_to_read);
     std::size_t bytes_transferred = s.read_some(b.data(pos, bytes_to_read), ec);
     b.shrink(bytes_to_read - bytes_transferred);
     if (ec)
       return 0;
   }
 }
 
 #endif // defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 template <typename SyncReadStream,
     typename DynamicBuffer_v2, typename MatchCondition>
 inline std::size_t read_until(SyncReadStream& s,
     DynamicBuffer_v2 buffers, MatchCondition match_condition,
     constraint_t<
       is_match_condition<MatchCondition>::value
     >,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
 {
   boost::system::error_code ec;
   std::size_t bytes_transferred = read_until(s,
       static_cast<DynamicBuffer_v2&&>(buffers),
       match_condition, ec);
   boost::asio::detail::throw_error(ec, "read_until");
   return bytes_transferred;
 }
 
 template <typename SyncReadStream,
     typename DynamicBuffer_v2, typename MatchCondition>
 std::size_t read_until(SyncReadStream& s, DynamicBuffer_v2 buffers,
     MatchCondition match_condition, boost::system::error_code& ec,
     constraint_t<
       is_match_condition<MatchCondition>::value
     >,
     constraint_t<
       is_dynamic_buffer_v2<DynamicBuffer_v2>::value
     >)
 {
   DynamicBuffer_v2& b = buffers;
 
   std::size_t search_position = 0;
   for (;;)
   {
     // Determine the range of the data to be searched.
     typedef typename DynamicBuffer_v2::const_buffers_type buffers_type;
     typedef buffers_iterator<buffers_type> iterator;
     buffers_type data_buffers =
       const_cast<const DynamicBuffer_v2&>(b).data(0, b.size());
     iterator begin = iterator::begin(data_buffers);
     iterator start_pos = begin + search_position;
     iterator end = iterator::end(data_buffers);
 
     // Look for a match.
     std::pair<iterator, bool> result = match_condition(start_pos, end);
     if (result.second)
     {
       // Full match. We're done.
       ec = boost::system::error_code();
       return result.first - begin;
     }
     else if (result.first != end)
     {
       // Partial match. Next search needs to start from beginning of match.
       search_position = result.first - begin;
     }
     else
     {
       // No match. Next search can start with the new data.
       search_position = end - begin;
     }
 
     // Check if buffer is full.
     if (b.size() == b.max_size())
     {
       ec = error::not_found;
       return 0;
     }
 
     // Need more data.
     std::size_t bytes_to_read = std::min<std::size_t>(
           std::max<std::size_t>(512, b.capacity() - b.size()),
           std::min<std::size_t>(65536, b.max_size() - b.size()));
     std::size_t pos = b.size();
     b.grow(bytes_to_read);
     std::size_t bytes_transferred = s.read_some(b.data(pos, bytes_to_read), ec);
     b.shrink(bytes_to_read - bytes_transferred);
     if (ec)
       return 0;
   }
 }
 
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 #if !defined(BOOST_ASIO_NO_DYNAMIC_BUFFER_V1)
 
 namespace detail
 {
   template <typename AsyncReadStream,
       typename DynamicBuffer_v1, typename ReadHandler>
   class read_until_delim_op_v1
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     template <typename BufferSequence>
     read_until_delim_op_v1(AsyncReadStream& stream,
         BufferSequence&& buffers,
         char delim, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         delim_(delim),
         start_(0),
         search_position_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_delim_op_v1(const read_until_delim_op_v1& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         delim_(other.delim_),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(other.handler_)
     {
     }
 
     read_until_delim_op_v1(read_until_delim_op_v1&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v1&&>(other.buffers_)),
         delim_(other.delim_),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t bytes_to_read;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v1::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers = buffers_.data();
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             iterator iter = std::find(start_pos, end, delim_);
             if (iter != end)
             {
               // Found a match. We're done.
               search_position_ = iter - begin + 1;
               bytes_to_read = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read = 0;
             }
 
             // Need to read some more data.
             else
             {
               // Next search can start with the new data.
               search_position_ = end - begin;
               bytes_to_read = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.prepare(bytes_to_read),
                 static_cast<read_until_delim_op_v1&&>(*this));
           }
           return; default:
           buffers_.commit(bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v1 buffers_;
     char delim_;
     int start_;
     std::size_t search_position_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream,
       typename DynamicBuffer_v1, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_delim_op_v1<AsyncReadStream,
         DynamicBuffer_v1, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_delim_v1
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_delim_v1(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler, typename DynamicBuffer_v1>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v1&& buffers,
         char delim) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_delim_op_v1<AsyncReadStream,
         decay_t<DynamicBuffer_v1>,
           decay_t<ReadHandler>>(
             stream_, static_cast<DynamicBuffer_v1&&>(buffers),
             delim, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v1,
     typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_delim_op_v1<AsyncReadStream,
       DynamicBuffer_v1, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_delim_op_v1<AsyncReadStream,
         DynamicBuffer_v1, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_delim_op_v1<AsyncReadStream,
         DynamicBuffer_v1, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 namespace detail
 {
   template <typename AsyncReadStream,
       typename DynamicBuffer_v1, typename ReadHandler>
   class read_until_delim_string_op_v1
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     template <typename BufferSequence>
     read_until_delim_string_op_v1(AsyncReadStream& stream,
         BufferSequence&& buffers,
         const std::string& delim, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         delim_(delim),
         start_(0),
         search_position_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_delim_string_op_v1(const read_until_delim_string_op_v1& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         delim_(other.delim_),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(other.handler_)
     {
     }
 
     read_until_delim_string_op_v1(read_until_delim_string_op_v1&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v1&&>(other.buffers_)),
         delim_(static_cast<std::string&&>(other.delim_)),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t bytes_to_read;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v1::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers = buffers_.data();
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             std::pair<iterator, bool> result = detail::partial_search(
                 start_pos, end, delim_.begin(), delim_.end());
             if (result.first != end && result.second)
             {
               // Full match. We're done.
               search_position_ = result.first - begin + delim_.length();
               bytes_to_read = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read = 0;
             }
 
             // Need to read some more data.
             else
             {
               if (result.first != end)
               {
                 // Partial match. Next search needs to start from beginning of
                 // match.
                 search_position_ = result.first - begin;
               }
               else
               {
                 // Next search can start with the new data.
                 search_position_ = end - begin;
               }
 
               bytes_to_read = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.prepare(bytes_to_read),
                 static_cast<read_until_delim_string_op_v1&&>(*this));
           }
           return; default:
           buffers_.commit(bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v1 buffers_;
     std::string delim_;
     int start_;
     std::size_t search_position_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream,
       typename DynamicBuffer_v1, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_delim_string_op_v1<AsyncReadStream,
         DynamicBuffer_v1, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_delim_string_v1
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_delim_string_v1(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler, typename DynamicBuffer_v1>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v1&& buffers,
         const std::string& delim) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_delim_string_op_v1<AsyncReadStream,
         decay_t<DynamicBuffer_v1>,
           decay_t<ReadHandler>>(
             stream_, static_cast<DynamicBuffer_v1&&>(buffers),
             delim, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v1,
     typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_delim_string_op_v1<AsyncReadStream,
       DynamicBuffer_v1, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_delim_string_op_v1<
         AsyncReadStream, DynamicBuffer_v1, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_delim_string_op_v1<
         AsyncReadStream, DynamicBuffer_v1, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 namespace detail
 {
   template <typename AsyncReadStream, typename DynamicBuffer_v1,
       typename RegEx, typename ReadHandler>
   class read_until_expr_op_v1
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     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),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         expr_(expr),
         start_(0),
         search_position_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_expr_op_v1(const read_until_expr_op_v1& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         expr_(other.expr_),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(other.handler_)
     {
     }
 
     read_until_expr_op_v1(read_until_expr_op_v1&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v1&&>(other.buffers_)),
         expr_(other.expr_),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t bytes_to_read;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v1::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers = buffers_.data();
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             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_, regex_match_flags());
             if (match && match_results[0].matched)
             {
               // Full match. We're done.
               search_position_ = match_results[0].second - begin;
               bytes_to_read = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read = 0;
             }
 
             // Need to read some more data.
             else
             {
               if (match)
               {
                 // Partial match. Next search needs to start from beginning of
                 // match.
                 search_position_ = match_results[0].first - begin;
               }
               else
               {
                 // Next search can start with the new data.
                 search_position_ = end - begin;
               }
 
               bytes_to_read = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.prepare(bytes_to_read),
                 static_cast<read_until_expr_op_v1&&>(*this));
           }
           return; default:
           buffers_.commit(bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v1 buffers_;
     RegEx expr_;
     int start_;
     std::size_t search_position_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream, typename DynamicBuffer_v1,
       typename RegEx, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_expr_op_v1<AsyncReadStream,
         DynamicBuffer_v1, RegEx, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_expr_v1
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_expr_v1(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler, typename DynamicBuffer_v1, typename RegEx>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v1&& buffers, const RegEx& expr) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_expr_op_v1<AsyncReadStream,
         decay_t<DynamicBuffer_v1>,
           RegEx, decay_t<ReadHandler>>(
             stream_, static_cast<DynamicBuffer_v1&&>(buffers),
             expr, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v1,
     typename RegEx, typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_expr_op_v1<AsyncReadStream,
       DynamicBuffer_v1, RegEx, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_expr_op_v1<AsyncReadStream,
         DynamicBuffer_v1, RegEx, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_expr_op_v1<AsyncReadStream,
         DynamicBuffer_v1, RegEx, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 #endif // defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 namespace detail
 {
   template <typename AsyncReadStream, typename DynamicBuffer_v1,
       typename MatchCondition, typename ReadHandler>
   class read_until_match_op_v1
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     template <typename BufferSequence>
     read_until_match_op_v1(AsyncReadStream& stream,
         BufferSequence&& buffers,
         MatchCondition match_condition, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         match_condition_(match_condition),
         start_(0),
         search_position_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_match_op_v1(const read_until_match_op_v1& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         match_condition_(other.match_condition_),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(other.handler_)
     {
     }
 
     read_until_match_op_v1(read_until_match_op_v1&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v1&&>(other.buffers_)),
         match_condition_(other.match_condition_),
         start_(other.start_),
         search_position_(other.search_position_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t bytes_to_read;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v1::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers = buffers_.data();
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             std::pair<iterator, bool> result = match_condition_(start_pos, end);
             if (result.second)
             {
               // Full match. We're done.
               search_position_ = result.first - begin;
               bytes_to_read = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read = 0;
             }
 
             // Need to read some more data.
             else
             {
               if (result.first != end)
               {
                 // Partial match. Next search needs to start from beginning of
                 // match.
                 search_position_ = result.first - begin;
               }
               else
               {
                 // Next search can start with the new data.
                 search_position_ = end - begin;
               }
 
               bytes_to_read = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.prepare(bytes_to_read),
                 static_cast<read_until_match_op_v1&&>(*this));
           }
           return; default:
           buffers_.commit(bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v1 buffers_;
     MatchCondition match_condition_;
     int start_;
     std::size_t search_position_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream, typename DynamicBuffer_v1,
       typename MatchCondition, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_match_op_v1<AsyncReadStream, DynamicBuffer_v1,
         MatchCondition, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_match_v1
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_match_v1(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler,
         typename DynamicBuffer_v1, typename MatchCondition>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v1&& buffers,
         MatchCondition match_condition) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_match_op_v1<AsyncReadStream,
         decay_t<DynamicBuffer_v1>,
           MatchCondition, decay_t<ReadHandler>>(
             stream_, static_cast<DynamicBuffer_v1&&>(buffers),
             match_condition, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v1,
     typename MatchCondition, typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_match_op_v1<AsyncReadStream,
       DynamicBuffer_v1, MatchCondition, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_match_op_v1<AsyncReadStream,
         DynamicBuffer_v1, MatchCondition, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_match_op_v1<AsyncReadStream,
         DynamicBuffer_v1, MatchCondition, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 #endif // !defined(BOOST_ASIO_NO_DYNAMIC_BUFFER_V1)
 
 namespace detail
 {
   template <typename AsyncReadStream,
       typename DynamicBuffer_v2, typename ReadHandler>
   class read_until_delim_op_v2
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     template <typename BufferSequence>
     read_until_delim_op_v2(AsyncReadStream& stream,
         BufferSequence&& buffers,
         char delim, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         delim_(delim),
         start_(0),
         search_position_(0),
         bytes_to_read_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_delim_op_v2(const read_until_delim_op_v2& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         delim_(other.delim_),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(other.handler_)
     {
     }
 
     read_until_delim_op_v2(read_until_delim_op_v2&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v2&&>(other.buffers_)),
         delim_(other.delim_),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t pos;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v2::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers =
               const_cast<const DynamicBuffer_v2&>(buffers_).data(
                   0, buffers_.size());
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             iterator iter = std::find(start_pos, end, delim_);
             if (iter != end)
             {
               // Found a match. We're done.
               search_position_ = iter - begin + 1;
               bytes_to_read_ = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read_ = 0;
             }
 
             // Need to read some more data.
             else
             {
               // Next search can start with the new data.
               search_position_ = end - begin;
               bytes_to_read_ = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read_ == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           pos = buffers_.size();
           buffers_.grow(bytes_to_read_);
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.data(pos, bytes_to_read_),
                 static_cast<read_until_delim_op_v2&&>(*this));
           }
           return; default:
           buffers_.shrink(bytes_to_read_ - bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v2 buffers_;
     char delim_;
     int start_;
     std::size_t search_position_;
     std::size_t bytes_to_read_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream,
       typename DynamicBuffer_v2, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_delim_op_v2<AsyncReadStream,
         DynamicBuffer_v2, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_delim_v2
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_delim_v2(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler, typename DynamicBuffer_v2>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v2&& buffers, char delim) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_delim_op_v2<AsyncReadStream,
         decay_t<DynamicBuffer_v2>,
           decay_t<ReadHandler>>(
             stream_, static_cast<DynamicBuffer_v2&&>(buffers),
             delim, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v2,
     typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_delim_op_v2<AsyncReadStream,
       DynamicBuffer_v2, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_delim_op_v2<AsyncReadStream,
         DynamicBuffer_v2, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_delim_op_v2<AsyncReadStream,
         DynamicBuffer_v2, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 namespace detail
 {
   template <typename AsyncReadStream,
       typename DynamicBuffer_v2, typename ReadHandler>
   class read_until_delim_string_op_v2
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     template <typename BufferSequence>
     read_until_delim_string_op_v2(AsyncReadStream& stream,
         BufferSequence&& buffers,
         const std::string& delim, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         delim_(delim),
         start_(0),
         search_position_(0),
         bytes_to_read_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_delim_string_op_v2(const read_until_delim_string_op_v2& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         delim_(other.delim_),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(other.handler_)
     {
     }
 
     read_until_delim_string_op_v2(read_until_delim_string_op_v2&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v2&&>(other.buffers_)),
         delim_(static_cast<std::string&&>(other.delim_)),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t pos;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v2::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers =
               const_cast<const DynamicBuffer_v2&>(buffers_).data(
                   0, buffers_.size());
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             std::pair<iterator, bool> result = detail::partial_search(
                 start_pos, end, delim_.begin(), delim_.end());
             if (result.first != end && result.second)
             {
               // Full match. We're done.
               search_position_ = result.first - begin + delim_.length();
               bytes_to_read_ = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read_ = 0;
             }
 
             // Need to read some more data.
             else
             {
               if (result.first != end)
               {
                 // Partial match. Next search needs to start from beginning of
                 // match.
                 search_position_ = result.first - begin;
               }
               else
               {
                 // Next search can start with the new data.
                 search_position_ = end - begin;
               }
 
               bytes_to_read_ = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read_ == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           pos = buffers_.size();
           buffers_.grow(bytes_to_read_);
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.data(pos, bytes_to_read_),
                 static_cast<read_until_delim_string_op_v2&&>(*this));
           }
           return; default:
           buffers_.shrink(bytes_to_read_ - bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v2 buffers_;
     std::string delim_;
     int start_;
     std::size_t search_position_;
     std::size_t bytes_to_read_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream,
       typename DynamicBuffer_v2, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_delim_string_op_v2<AsyncReadStream,
         DynamicBuffer_v2, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_delim_string_v2
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_delim_string_v2(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler, typename DynamicBuffer_v2>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v2&& buffers,
         const std::string& delim) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_delim_string_op_v2<AsyncReadStream,
         decay_t<DynamicBuffer_v2>,
           decay_t<ReadHandler>>(
             stream_, static_cast<DynamicBuffer_v2&&>(buffers),
             delim, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v2,
     typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_delim_string_op_v2<AsyncReadStream,
       DynamicBuffer_v2, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_delim_string_op_v2<
         AsyncReadStream, DynamicBuffer_v2, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_delim_string_op_v2<
         AsyncReadStream, DynamicBuffer_v2, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 #if !defined(BOOST_ASIO_NO_EXTENSIONS)
 #if defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 namespace detail
 {
   template <typename AsyncReadStream, typename DynamicBuffer_v2,
       typename RegEx, typename ReadHandler>
   class read_until_expr_op_v2
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     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),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         expr_(expr),
         start_(0),
         search_position_(0),
         bytes_to_read_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_expr_op_v2(const read_until_expr_op_v2& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         expr_(other.expr_),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(other.handler_)
     {
     }
 
     read_until_expr_op_v2(read_until_expr_op_v2&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v2&&>(other.buffers_)),
         expr_(other.expr_),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t pos;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v2::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers =
               const_cast<const DynamicBuffer_v2&>(buffers_).data(
                   0, buffers_.size());
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             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_, regex_match_flags());
             if (match && match_results[0].matched)
             {
               // Full match. We're done.
               search_position_ = match_results[0].second - begin;
               bytes_to_read_ = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read_ = 0;
             }
 
             // Need to read some more data.
             else
             {
               if (match)
               {
                 // Partial match. Next search needs to start from beginning of
                 // match.
                 search_position_ = match_results[0].first - begin;
               }
               else
               {
                 // Next search can start with the new data.
                 search_position_ = end - begin;
               }
 
               bytes_to_read_ = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read_ == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           pos = buffers_.size();
           buffers_.grow(bytes_to_read_);
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.data(pos, bytes_to_read_),
                 static_cast<read_until_expr_op_v2&&>(*this));
           }
           return; default:
           buffers_.shrink(bytes_to_read_ - bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v2 buffers_;
     RegEx expr_;
     int start_;
     std::size_t search_position_;
     std::size_t bytes_to_read_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream, typename DynamicBuffer_v2,
       typename RegEx, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_expr_op_v2<AsyncReadStream,
         DynamicBuffer_v2, RegEx, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_expr_v2
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_expr_v2(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler, typename DynamicBuffer_v2, typename RegEx>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v2&& buffers,
         const RegEx& expr) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_expr_op_v2<AsyncReadStream,
         decay_t<DynamicBuffer_v2>,
           RegEx, decay_t<ReadHandler>>(
             stream_, static_cast<DynamicBuffer_v2&&>(buffers),
             expr, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v2,
     typename RegEx, typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_expr_op_v2<AsyncReadStream,
       DynamicBuffer_v2, RegEx, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_expr_op_v2<AsyncReadStream,
         DynamicBuffer_v2, RegEx, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_expr_op_v2<AsyncReadStream,
         DynamicBuffer_v2, RegEx, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 #endif // defined(BOOST_ASIO_HAS_BOOST_REGEX)
 
 namespace detail
 {
   template <typename AsyncReadStream, typename DynamicBuffer_v2,
       typename MatchCondition, typename ReadHandler>
   class read_until_match_op_v2
     : public base_from_cancellation_state<ReadHandler>
   {
   public:
     template <typename BufferSequence>
     read_until_match_op_v2(AsyncReadStream& stream,
         BufferSequence&& buffers,
         MatchCondition match_condition, ReadHandler& handler)
       : base_from_cancellation_state<ReadHandler>(
           handler, enable_partial_cancellation()),
         stream_(stream),
         buffers_(static_cast<BufferSequence&&>(buffers)),
         match_condition_(match_condition),
         start_(0),
         search_position_(0),
         bytes_to_read_(0),
         handler_(static_cast<ReadHandler&&>(handler))
     {
     }
 
     read_until_match_op_v2(const read_until_match_op_v2& other)
       : base_from_cancellation_state<ReadHandler>(other),
         stream_(other.stream_),
         buffers_(other.buffers_),
         match_condition_(other.match_condition_),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(other.handler_)
     {
     }
 
     read_until_match_op_v2(read_until_match_op_v2&& other)
       : base_from_cancellation_state<ReadHandler>(
           static_cast<base_from_cancellation_state<ReadHandler>&&>(other)),
         stream_(other.stream_),
         buffers_(static_cast<DynamicBuffer_v2&&>(other.buffers_)),
         match_condition_(other.match_condition_),
         start_(other.start_),
         search_position_(other.search_position_),
         bytes_to_read_(other.bytes_to_read_),
         handler_(static_cast<ReadHandler&&>(other.handler_))
     {
     }
 
     void operator()(boost::system::error_code ec,
         std::size_t bytes_transferred, int start = 0)
     {
       const std::size_t not_found = (std::numeric_limits<std::size_t>::max)();
       std::size_t pos;
       switch (start_ = start)
       {
       case 1:
         for (;;)
         {
           {
             // Determine the range of the data to be searched.
             typedef typename DynamicBuffer_v2::const_buffers_type
               buffers_type;
             typedef buffers_iterator<buffers_type> iterator;
             buffers_type data_buffers =
               const_cast<const DynamicBuffer_v2&>(buffers_).data(
                   0, buffers_.size());
             iterator begin = iterator::begin(data_buffers);
             iterator start_pos = begin + search_position_;
             iterator end = iterator::end(data_buffers);
 
             // Look for a match.
             std::pair<iterator, bool> result = match_condition_(start_pos, end);
             if (result.second)
             {
               // Full match. We're done.
               search_position_ = result.first - begin;
               bytes_to_read_ = 0;
             }
 
             // No match yet. Check if buffer is full.
             else if (buffers_.size() == buffers_.max_size())
             {
               search_position_ = not_found;
               bytes_to_read_ = 0;
             }
 
             // Need to read some more data.
             else
             {
               if (result.first != end)
               {
                 // Partial match. Next search needs to start from beginning of
                 // match.
                 search_position_ = result.first - begin;
               }
               else
               {
                 // Next search can start with the new data.
                 search_position_ = end - begin;
               }
 
               bytes_to_read_ = std::min<std::size_t>(
                     std::max<std::size_t>(512,
                       buffers_.capacity() - buffers_.size()),
                     std::min<std::size_t>(65536,
                       buffers_.max_size() - buffers_.size()));
             }
           }
 
           // Check if we're done.
           if (!start && bytes_to_read_ == 0)
             break;
 
           // Start a new asynchronous read operation to obtain more data.
           pos = buffers_.size();
           buffers_.grow(bytes_to_read_);
           {
             BOOST_ASIO_HANDLER_LOCATION((
                   __FILE__, __LINE__, "async_read_until"));
             stream_.async_read_some(buffers_.data(pos, bytes_to_read_),
                 static_cast<read_until_match_op_v2&&>(*this));
           }
           return; default:
           buffers_.shrink(bytes_to_read_ - bytes_transferred);
           if (ec || bytes_transferred == 0)
             break;
           if (this->cancelled() != cancellation_type::none)
           {
             ec = error::operation_aborted;
             break;
           }
         }
 
         const boost::system::error_code result_ec =
           (search_position_ == not_found)
           ? error::not_found : ec;
 
         const std::size_t result_n =
           (ec || search_position_ == not_found)
           ? 0 : search_position_;
 
         static_cast<ReadHandler&&>(handler_)(result_ec, result_n);
       }
     }
 
   //private:
     AsyncReadStream& stream_;
     DynamicBuffer_v2 buffers_;
     MatchCondition match_condition_;
     int start_;
     std::size_t search_position_;
     std::size_t bytes_to_read_;
     ReadHandler handler_;
   };
 
   template <typename AsyncReadStream, typename DynamicBuffer_v2,
       typename MatchCondition, typename ReadHandler>
   inline bool asio_handler_is_continuation(
       read_until_match_op_v2<AsyncReadStream, DynamicBuffer_v2,
         MatchCondition, ReadHandler>* this_handler)
   {
     return this_handler->start_ == 0 ? true
       : boost_asio_handler_cont_helpers::is_continuation(
           this_handler->handler_);
   }
 
   template <typename AsyncReadStream>
   class initiate_async_read_until_match_v2
   {
   public:
     typedef typename AsyncReadStream::executor_type executor_type;
 
     explicit initiate_async_read_until_match_v2(AsyncReadStream& stream)
       : stream_(stream)
     {
     }
 
     executor_type get_executor() const noexcept
     {
       return stream_.get_executor();
     }
 
     template <typename ReadHandler,
         typename DynamicBuffer_v2, typename MatchCondition>
     void operator()(ReadHandler&& handler,
         DynamicBuffer_v2&& buffers,
         MatchCondition match_condition) const
     {
       // If you get an error on the following line it means that your handler
       // does not meet the documented type requirements for a ReadHandler.
       BOOST_ASIO_READ_HANDLER_CHECK(ReadHandler, handler) type_check;
 
       non_const_lvalue<ReadHandler> handler2(handler);
       read_until_match_op_v2<AsyncReadStream, decay_t<DynamicBuffer_v2>,
         MatchCondition, decay_t<ReadHandler>>(
           stream_, static_cast<DynamicBuffer_v2&&>(buffers),
           match_condition, handler2.value)(boost::system::error_code(), 0, 1);
     }
 
   private:
     AsyncReadStream& stream_;
   };
 } // namespace detail
 
 #if !defined(GENERATING_DOCUMENTATION)
 
 template <template <typename, typename> class Associator,
     typename AsyncReadStream, typename DynamicBuffer_v2,
     typename MatchCondition, typename ReadHandler, typename DefaultCandidate>
 struct associator<Associator,
     detail::read_until_match_op_v2<AsyncReadStream,
       DynamicBuffer_v2, MatchCondition, ReadHandler>,
     DefaultCandidate>
   : Associator<ReadHandler, DefaultCandidate>
 {
   static typename Associator<ReadHandler, DefaultCandidate>::type get(
       const detail::read_until_match_op_v2<AsyncReadStream,
         DynamicBuffer_v2, MatchCondition, ReadHandler>& h) noexcept
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_);
   }
 
   static auto get(
       const detail::read_until_match_op_v2<AsyncReadStream,
         DynamicBuffer_v2, MatchCondition, ReadHandler>& h,
       const DefaultCandidate& c) noexcept
     -> decltype(Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c))
   {
     return Associator<ReadHandler, DefaultCandidate>::get(h.handler_, c);
   }
 };
 
 #endif // !defined(GENERATING_DOCUMENTATION)
 
 #endif // !defined(BOOST_ASIO_NO_EXTENSIONS)
 
 } // namespace asio
 } // namespace boost
 
 #include <boost/asio/detail/pop_options.hpp>
 
 #endif // BOOST_ASIO_IMPL_READ_UNTIL_HPP