With apologies to all concerned, and thanks to Aleksey Sanin for tracking down the problem, there is a const-correctness bug in Boost.Regex-1.33.x that strikes only in multithreaded programs on Unix platforms, if: * Multiple threads construct regexes concurrently, or * Multiple threads perform search and replace operations concurrently. There are two possible fixes: * Apply the attached patch (this has just gone into cvs along with an updated test case that detects the issue). Or, * define BOOST_REGEX_USE_C_LOCALE in boost/user.hpp and rebuild everything. Regards, John Maddock.