basic_regex class constructor stack overflow

[Yang Dingning <yangdingning@…>]

Compile and run the following code:

#include <boost/regex.hpp>

int main()
{
	boost::regex reg("((?1)|a)");
	return 0;
}

Recursive calls to basic_regex_creator<charT, traits>::create_startmap() would consume all available stack space and crash the program, causing denial of service.

This problem has been verified to exist in Boost 1.46.1, and also in trunk code as of Jun. 9.

[John Maddock]

(In [72612]) Fix infinite recursion in bad recursive expressions. Fix bug that allows invalid regex to go unnoticed and crash later. Fixes #5613. Fixes #5612.