I have a case where I am compiling a dynamic regex based on client code input, and it works except for when I run my multi-threaded unit test 100-1000 times. When I run it with 1 thread, it never fails. When I run it with 2 or 4 threads it fails at this rare frequency. Except on my build server, where it seems to fail most of the time, which is how I learned about the issue to begin with. It's not crashing though, it's just very rarely returning the wrong results, and not always in the same place in the unit test. So I'm trying to understand Xpressive's thread safety. I understand it as far as not using a regex object in multiple threads until it's finished compiling and not sharing a match_results across threads, but what about compiling different regex objects for each thread: if two threads are compiling two different regexes (or even the same const regex string but from different threads, which is possible with my test code), is that ok? I'm seeing this failure on MSVC 12 but I think I also got it with VC10, just not as reliably on the build server (but the build server had to be upgraded to Windows 2012 to install VC12, so maybe that OS change had an effect as well). Thanks, -Matt