#### async_read_until (7 of 12 overloads)

Start an asynchronous operation to read data into a streambuf until some part of its data matches a regular expression.

    template<
        typename AsyncReadStream,
        typename Allocator,
        typename Traits,
        typename ReadToken = default_completion_token_t<        typename AsyncReadStream::executor_type>>
    DEDUCED async_read_until(
        AsyncReadStream & s,
        asio::basic_streambuf< Allocator > & b,
        const boost::basic_regex< char, Traits > & expr,
        ReadToken && token = default_completion_token_t< typename AsyncReadStream::executor_type >());

This function is used to asynchronously read data into the specified streambuf until the streambuf's get area contains some data that matches a regular expression. It is an initiating function for an asynchronous operation, and always returns immediately. The asynchronous operation will continue until one of the following conditions is true:

- A substring of the streambuf's get area matches the regular expression.
- An error occurred.

This operation is implemented in terms of zero or more calls to the stream's async_read_some function, and is known as a composed operation. If the streambuf's get area already contains data that matches the regular expression, this asynchronous operation completes immediately. The program must ensure that the stream performs no other read operations (such as async_read, async_read_until, the stream's async_read_some function, or any other composed operations that perform reads) until this operation completes.

###### Parameters

s

  The stream from which the data is to be read. The type must support the AsyncReadStream concept.

b

  A streambuf object into which the data will be read. Ownership of the streambuf is retained by the caller, which must guarantee that it remains valid until the completion handler is called.

expr

  The regular expression.

token

  The completion token that will be used to produce a completion handler, which will be called when the read completes. Potential completion tokens include `use_future`, `use_awaitable`, `yield_context`, or a function object with the correct completion signature. The function signature of the completion handler must be:

      void handler(
        // Result of operation.
        const asio::error_code& error,

        // The number of bytes in the streambuf's get
        // area up to and including the substring
        // that matches the regular. expression.
        // 0 if an error occurred.
        std::size_t bytes_transferred
      );

  Regardless of whether the asynchronous operation completes immediately or not, the completion handler will not be invoked from within this function. On immediate completion, invocation of the handler will be performed in a manner equivalent to using `async_immediate`.

###### Completion Signature

    void(asio::error_code, std::size_t)

###### Remarks

After a successful async_read_until operation, the streambuf may contain additional data beyond that which matched the regular expression. An application will typically leave that data in the streambuf for a subsequent async_read_until operation to examine.

###### Example

To asynchronously read data into a streambuf until a CR-LF sequence is encountered:

    asio::streambuf b;
    ...
    void handler(const asio::error_code& e, std::size_t size)
    {
      if (!e)
      {
        std::istream is(&b);
        std::string line;
        std::getline(is, line);
        ...
      }
    }
    ...
    asio::async_read_until(s, b, boost::regex("\r\n"), handler);

After the `async_read_until` operation completes successfully, the buffer `b` contains the data which matched the regular expression:

    { 'a', 'b', ..., 'c', '\r', '\n', 'd', 'e', ... }

The call to `std::getline` then extracts the data up to and including the newline (which is discarded), so that the string `line` contains:

    { 'a', 'b', ..., 'c', '\r' }

The remaining data is left in the buffer `b` as follows:

    { 'd', 'e', ... }

This data may be the start of a new line, to be extracted by a subsequent `async_read_until` operation.

###### Per-Operation Cancellation

This asynchronous operation supports cancellation for the following `cancellation_type` values:

- `cancellation_type::terminal`
- `cancellation_type::partial`

if they are also supported by the `AsyncReadStream` type's `async_read_some` operation.