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### Thread Safety

The Boost.Regex library is thread safe when Boost is: you can verify that Boost is in thread safe mode by checking to see if `BOOST_HAS_THREADS` is defined: this macro is set automatically by the config system when threading support is turned on in your compiler.

Class `basic_regex` and its typedefs regex and wregex are thread safe, in that compiled regular expressions can safely be shared between threads. The matching algorithms `regex_match`, `regex_search`, and `regex_replace` are all re-entrant and thread safe. Class `match_results` is now thread safe, in that the results of a match can be safely copied from one thread to another (for example one thread may find matches and push `match_results` instances onto a queue, while another thread pops them off the other end), otherwise use a separate instance of `match_results` per thread.

The POSIX API functions are all re-entrant and thread safe, regular expressions compiled with regcomp can also be shared between threads.

The class RegEx is only thread safe if each thread gets its own RegEx instance (apartment threading) - this is a consequence of RegEx handling both compiling and matching regular expressions.

Finally note that changing the global locale invalidates all compiled regular expressions, therefore calling `set_locale` from one thread while another uses regular expressions will produce unpredictable results.

There is also a requirement that there is only one thread executing prior to the start of main().