diff --git a/httplib.h b/httplib.h
index 96cb915..fd7f7b7 100644
--- a/httplib.h
+++ b/httplib.h
@@ -4986,7 +4986,7 @@ inline bool parse_www_authenticate(const Response &res,
         s = s.substr(pos + 1);
         auto beg = std::sregex_iterator(s.begin(), s.end(), re);
         for (auto i = beg; i != std::sregex_iterator(); ++i) {
-          auto m = *i;
+          const auto &m = *i;
           auto key = s.substr(static_cast<size_t>(m.position(1)),
                               static_cast<size_t>(m.length(1)));
           auto val = m.length(2) > 0