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#### async_read_until (3 of 4 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 ReadHandler>
    void-or-deduced async_read_until(
        AsyncReadStream & s,
        boost::asio::basic_streambuf< Allocator > & b,
        const boost::regex & expr,
        ReadHandler handler);

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. The function call 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 handler is called.

expr

  The regular expression.

handler

  The handler to be called when the read operation completes. Copies will be made of the handler as required. The function signature of the handler must be:

      void handler(
        // Result of operation.
        const boost::system::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 handler will not be invoked from within this function. Invocation of the handler will be performed in a manner equivalent to using `boost::asio::io_service::post()`.

###### 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:

    boost::asio::streambuf b;
    ...
    void handler(const boost::system::error_code& e, std::size_t size)
    {
      if (!e)
      {
        std::istream is(&b);
        std::string line;
        std::getline(is, line);
        ...
      }
    }
    ...
    boost::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 match, so that the string `line` contains:

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

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.