A simple multithreaded application using 100 threads where each thread executes the following loop results in exceptions thrown by the boost::regex() constructor: for (int i=0;i<1000;++i) boost::regex("[a-z]{1}"); The exceptions thrown are either "Invalid content of repeat range" or "Unmatched { or \{". Since the regular expression used is valid and does not throw when executed in a single threaded environment I expect some static variable to be used by BoostRegex internally. Should BoostRegex be threadsafe? We use Red Hat Enterprise Linux 3 (gcc 3.2.3) as our development platform. Remark: The problem disappears when the curly braces are escaped by \\ or when using a "std::string" instead of a "const char*" argument. Best regards, Peter.