diff --git a/src/V3String.cpp b/src/V3String.cpp
index 3ce3d3e57..d79a6ff58 100644
--- a/src/V3String.cpp
+++ b/src/V3String.cpp
@@ -79,13 +79,13 @@ string VString::dot(const string& a, const string& dot, const string& b) {
     return a + dot + b;
 }
 
-string VString::downcase(const string& str) {
+string VString::downcase(const string& str) VL_PURE {
     string result = str;
     for (char& cr : result) cr = std::tolower(cr);
     return result;
 }
 
-string VString::upcase(const string& str) {
+string VString::upcase(const string& str) VL_PURE {
     string result = str;
     for (char& cr : result) cr = std::toupper(cr);
     return result;
diff --git a/src/V3String.h b/src/V3String.h
index 5c2f007de..a69036cc9 100644
--- a/src/V3String.h
+++ b/src/V3String.h
@@ -90,9 +90,9 @@ public:
     // Return {a}{dot}{b}, omitting dot if a or b are empty
     static string dot(const string& a, const string& dot, const string& b);
     // Convert string to lowercase (tolower)
-    static string downcase(const string& str);
+    static string downcase(const string& str) VL_PURE;
     // Convert string to upper case (toupper)
-    static string upcase(const string& str);
+    static string upcase(const string& str) VL_PURE;
     // Insert esc just before tgt
     static string quoteAny(const string& str, char tgt, char esc);
     // Replace any \'s with \\  (two consecutive backslashes)