The following MSDN article treats the issue of invalid characters in filenames: http://support.microsoft.com/kb/177506/EN-US/ Another article has a similar text (but misses the asterisk in the list): http://www.microsoft.com/technet/prodtechnol/windows2000serv/reskit/w2000Msg... A simple cpp program also throws on this simple issue. For example, with a question mark: #include "windows.h" int main(int argc, char* argv[]) { HANDLE hf = ::CreateFile("c:\\hi?.c", GENERIC_WRITE, 0, NULL, OPEN_ALWAYS, 0, 0); if (hf == INVALID_HANDLE_VALUE) { std::cout << GetLastError() << std::endl; } else { ::CloseHandle(hf); std::cout << "Success" << std::endl; } return 0; } The value returned is ERROR_INVALID_NAME == 123L from winerror.h However, boost::filesystem::path::windows_name does not have a problem with a question mark or asterisk. Also, the implementation seems to attempt to validate any name unless it has these characters whereas it could use a "valid list" like the unix implementation does. A missing character from the valid list will probably be an inconvenience but not as much of a problem as a missing character from the invalid list. My guess is that an implementation that wishes to support Unicode characters will need to use the GetStringTypeW or IsCharAlphaNumeric functions to determine if a letter is an alphanumeric character. Perhaps it is best to use an implementation that runs "CreateFile(filename, GENERIC_READ, 0, NULL, OPEN_EXISTING, 0, 0)" and checks the error code for ERROR_INVALID_NAME to decide if it is a valid filename or not. Yitzhak Sapir