#include <iostream>
#include <memory>
#include <boost/regex.hpp>

int main(int argc, char* argv[])
{
	std::istream& inputstream = std::cin;
	boost::regex regex(argv[1]);

	std::size_t buffersize = 4; // for demonstration, normally use something like 4096
	std::size_t used_buffer = 0;
	// use unique pointer instead of "plain" pointer so that there is no memory leak in case of exception
	std::unique_ptr<char[]> buffer(new char[buffersize]);
	do
	{
		inputstream.read(buffer.get() + used_buffer, buffersize - used_buffer);
		char const* const buffer_end = buffer.get() + used_buffer + inputstream.gcount();
		char const* buffer_handled_until = buffer.get();

		// the whole following thing could be much easier by using a bidirectional input iterator here, but:
		// do not do this because input "file" could be a named pipe, stream or similar (no backwards iterating would be possible!)
		// so you have to manage the buffer (and release parts of it) yourself
		// for a more efficient solution (hopefully in the near future) see <https://svn.boost.org/trac/boost/ticket/11776>
		boost::cregex_iterator curr_match(buffer.get(), buffer_end, regex, boost::match_default | boost::match_partial);
		// add element to set when ...
		while (curr_match != boost::cregex_iterator() &&
			curr_match->begin()->matched && // ... match is a full match and ...
			(!inputstream || // ... when file is at end or ...
			// (see next line) when match does not touch end of buffer (otherwise element could be longer, e. g. partial match)
			!boost::regex_match(curr_match->begin()->first, buffer_end, regex, boost::match_default | boost::match_partial)))
		{
			std::cout << curr_match->str() << "\n";
			++curr_match;
		}

/*		while (curr_match != boost::cregex_iterator())
		{
			std::cout << curr_match->str() << " " << curr_match->size() << " " << (*curr_match)[1].matched << " " << ((*curr_match)[1].second - (*curr_match)[1].first) << "\n";
			++curr_match;
		}
*/

		// the last match is always a partial match except full match touches buffer end (or buffer is empty)
		// so mark begin of last match as new begin of buffer when filling it up in next round of do-while-loop
		buffer_handled_until = (curr_match != boost::cregex_iterator() ? curr_match->begin()->first : buffer_end);

		used_buffer = buffer_end - buffer_handled_until;
		if (buffer_handled_until == buffer.get())
		{
			// if current element fills the whole buffer, buffer is too small and thus doubled
			buffersize *= 2;
			std::unique_ptr<char[]> new_buffer(new char[buffersize]);
			std::memmove(new_buffer.get(), buffer_handled_until, used_buffer);
			buffer = std::move(new_buffer);
		}
		else
		{
			// move the rest of new element (buffer_handled_until) to beginning of buffer and mark it as used
			std::memmove(buffer.get(), buffer_handled_until, used_buffer);
		}
	} while (inputstream);
}