diff --git a/src/libutil/regex-combinators.hh b/src/libutil/regex-combinators.hh
index 8a7b2c2c01e..75ccd4e6cf0 100644
--- a/src/libutil/regex-combinators.hh
+++ b/src/libutil/regex-combinators.hh
@@ -3,6 +3,7 @@
 
 #include <string_view>
 #include <string>
+#include <sstream>
 
 namespace nix::regex {
 
@@ -11,22 +12,23 @@ namespace nix::regex {
 
 static inline std::string either(std::string_view a, std::string_view b)
 {
-    return std::string { a } + "|" + b;
+    std::stringstream ss;
+    ss << a << "|" << b;
+    return ss.str();
 }
 
 static inline std::string group(std::string_view a)
 {
-    return std::string { "(" } + a + ")";
-}
-
-static inline std::string many(std::string_view a)
-{
-    return std::string { "(?:" } + a + ")*";
+    std::stringstream ss;
+    ss << "(" << a << ")";
+    return ss.str();
 }
 
 static inline std::string list(std::string_view a)
 {
-    return std::string { a } + many(group("," + a));
+    std::stringstream ss;
+    ss << a << "(," << a << ")*";
+    return ss.str();
 }
 
 }