diff --git a/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L b/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L
index 5838ed309f..782349b50d 100644
--- a/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L
+++ b/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L
@@ -62,6 +62,9 @@ Description
 
 // Flex may use register, which is deprecated and incompatible with C++17
 #pragma clang diagnostic ignored "-Wdeprecated-register"
+// A 'nothing' define to effectively remove from code as well (issue #3337)
+#undef  register
+#define register
 
 using namespace Foam;
 
diff --git a/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L b/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L
index 7e0686d064..10738061c2 100644
--- a/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L
+++ b/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L
@@ -56,6 +56,9 @@ Description
 
 // Flex may use register, which is deprecated and incompatible with C++17
 #pragma clang diagnostic ignored "-Wdeprecated-register"
+// A 'nothing' define to effectively remove from code as well (issue #3337)
+#undef  register
+#define register
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
diff --git a/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L b/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L
index d74f4ab01c..1f79995445 100644
--- a/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L
+++ b/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L
@@ -63,6 +63,9 @@ Description
 
 // Flex may use register, which is deprecated and incompatible with C++17
 #pragma clang diagnostic ignored "-Wdeprecated-register"
+// A 'nothing' define to effectively remove from code as well (issue #3337)
+#undef  register
+#define register
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
diff --git a/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L b/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L
index c29bff53e5..0dbb0d5278 100644
--- a/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L
+++ b/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L
@@ -51,6 +51,9 @@ Description
 
 // Flex may use register, which is deprecated and incompatible with C++17
 #pragma clang diagnostic ignored "-Wdeprecated-register"
+// A 'nothing' define to effectively remove from code as well (issue #3337)
+#undef  register
+#define register
 
 using namespace Foam;
 
diff --git a/src/fileFormats/stl/STLAsciiParseFlex.L b/src/fileFormats/stl/STLAsciiParseFlex.L
index 0f66ea3eeb..2c93fdb7e9 100644
--- a/src/fileFormats/stl/STLAsciiParseFlex.L
+++ b/src/fileFormats/stl/STLAsciiParseFlex.L
@@ -44,6 +44,9 @@ Description
 
 // Flex may use register, which is deprecated and incompatible with C++17
 #pragma clang diagnostic ignored "-Wdeprecated-register"
+// A 'nothing' define to effectively remove from code as well (issue #3337)
+#undef  register
+#define register
 
 using namespace Foam;
 
diff --git a/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L b/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L
index 4c01e3ea67..a77953325d 100644
--- a/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L
+++ b/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L
@@ -39,6 +39,9 @@ License
 
 // Flex may use register, which is deprecated and incompatible with C++17
 #pragma clang diagnostic ignored "-Wdeprecated-register"
+// A 'nothing' define to effectively remove from code as well (issue #3337)
+#undef  register
+#define register
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //