boost::algorithm::join_if — 条件连接算法。
// In header: <boost/algorithm/string/regex.hpp> template<typename SequenceSequenceT, typename Range1T, typename CharT, typename RegexTraitsT> range_value< SequenceSequenceT >::type join_if(const SequenceSequenceT & Input, const Range1T & Separator, const basic_regex< CharT, RegexTraitsT > & Rx, match_flag_type Flags = match_default);