diff --git a/include/boost/regex/concepts.hpp b/include/boost/regex/concepts.hpp
index 2dd9a4de..7a891343 100644
--- a/include/boost/regex/concepts.hpp
+++ b/include/boost/regex/concepts.hpp
@@ -27,6 +27,7 @@
 #include <boost/regex.hpp>
 #include <bitset>
 #include <vector>
+#include <iostream>
 
 namespace boost{