diff --git a/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L b/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L
index 9915c946d6..c3d443d2fe 100644
--- a/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L
+++ b/applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L
@@ -48,8 +48,6 @@ Description
 #include "scalar.H"
 #include "StringStream.H"
 
-using namespace Foam;
-
 #include "argList.H"
 #include "Time.H"
 #include "polyMesh.H"
@@ -59,6 +57,11 @@ using namespace Foam;
 #include "SLList.H"
 #include "SLPtrList.H"
 
+// Flex may use register, which is deprecated and incompatible with C++17
+#pragma GCC diagnostic ignored "-Wdeprecated-register"
+
+using namespace Foam;
+
 SLList<point> slPoints;
 SLList<label> slPointMap;
 label maxNodei = 0;
diff --git a/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L b/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L
index 111aba3252..9e28c08886 100644
--- a/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L
+++ b/applications/utilities/mesh/conversion/fluent3DMeshToFoam/fluent3DMeshToFoam.L
@@ -52,6 +52,9 @@ Description
 #include "IFstream.H"
 #include "readHexLabel.H"
 
+// Flex may use register, which is deprecated and incompatible with C++17
+#pragma GCC diagnostic ignored "-Wdeprecated-register"
+
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
 using namespace Foam;
diff --git a/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L b/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L
index 0449dd3b81..96c7c84b1a 100644
--- a/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L
+++ b/applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L
@@ -58,6 +58,8 @@ Description
 #include "cellShapeRecognition.H"
 #include "repatchPolyTopoChanger.H"
 
+// Flex may use register, which is deprecated and incompatible with C++17
+#pragma GCC diagnostic ignored "-Wdeprecated-register"
 
 // * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
 
@@ -65,9 +67,6 @@ using namespace Foam;
 
 const scalar convertToMeters = 1.0;
 
-// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
-
-
 label dimensionOfGrid = 0;
 
 label nPoints = 0;
diff --git a/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L b/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L
index 0925ada004..280964986d 100644
--- a/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L
+++ b/applications/utilities/mesh/conversion/gambitToFoam/gambitToFoam.L
@@ -46,6 +46,9 @@ Description
 // For EOF only
 #include <cstdio>
 
+// Flex may use register, which is deprecated and incompatible with C++17
+#pragma GCC diagnostic ignored "-Wdeprecated-register"
+
 using namespace Foam;
 
 #include "argList.H"
diff --git a/src/fileFormats/stl/STLReaderASCII.L b/src/fileFormats/stl/STLReaderASCII.L
index d742ae34df..1915bad978 100644
--- a/src/fileFormats/stl/STLReaderASCII.L
+++ b/src/fileFormats/stl/STLReaderASCII.L
@@ -26,6 +26,7 @@ License
 %option prefix="yySTL"
 %option yyclass="yySTLFlexLexer"
 
+
 %{
 
  /* ------------------------------------------------------------------------ *\
@@ -35,6 +36,9 @@ License
 #include "STLReader.H"
 #include "OSspecific.H"
 
+// Flex may use register, which is deprecated and incompatible with C++17
+#pragma GCC diagnostic ignored "-Wdeprecated-register"
+
 using namespace Foam;
 // Dummy yyFlexLexer::yylex() to keep the linker happy. It is not called
 //! \cond dummy
diff --git a/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L b/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L
index 0909813395..38966b6d41 100644
--- a/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L
+++ b/src/thermophysicalModels/reactionThermo/chemistryReaders/chemkinReader/chemkinLexer.L
@@ -34,6 +34,11 @@ License
 // For EOF only
 #include <cstdio>
 
+// Flex may use register, which is deprecated and incompatible with C++17
+#pragma GCC diagnostic ignored "-Wdeprecated-register"
+
+// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //
+
 // flex input buffer size
 int Foam::chemkinReader::yyBufSize = YY_BUF_SIZE;