diff --git a/CMakeLists.txt b/CMakeLists.txt
index 25845d68..2f499fbf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -452,8 +452,8 @@ target_include_directories(OpenSTA
 target_compile_options(OpenSTA
   PRIVATE
   $<$<CXX_COMPILER_ID:GNU>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security>
-  $<$<CXX_COMPILER_ID:AppleClang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security>
-  $<$<CXX_COMPILER_ID:Clang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security>
+  $<$<CXX_COMPILER_ID:AppleClang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security -Wno-deprecated-register>
+  $<$<CXX_COMPILER_ID:Clang>:-Wall -Wextra -pedantic -Wcast-qual -Wredundant-decls -Wformat-security -Wno-deprecated-register>
   )
 
 # Disable compiler specific extensions like gnu++11.
diff --git a/liberty/LibertyExprLex.ll b/liberty/LibertyExprLex.ll
index ab588a24..9388620d 100644
--- a/liberty/LibertyExprLex.ll
+++ b/liberty/LibertyExprLex.ll
@@ -18,7 +18,6 @@
 
 // Liberty function expression lexical analyzer
 
-#include "util/FlexDisableRegister.hh"
 #include "Debug.hh"
 #include "StringUtil.hh"
 #include "liberty/LibertyExprPvt.hh"
diff --git a/liberty/LibertyLex.ll b/liberty/LibertyLex.ll
index 092aafb7..a2cc0373 100644
--- a/liberty/LibertyLex.ll
+++ b/liberty/LibertyLex.ll
@@ -18,7 +18,6 @@
 #include <ctype.h>
 #include <string.h>
 
-#include "util/FlexDisableRegister.hh"
 #include "liberty/LibertyParser.hh"
 #include "LibertyParse.hh"
 
diff --git a/parasitics/SpefLex.ll b/parasitics/SpefLex.ll
index 1bfbc115..3c64a848 100644
--- a/parasitics/SpefLex.ll
+++ b/parasitics/SpefLex.ll
@@ -19,7 +19,6 @@
 #include <string.h>
 #include <string>
 
-#include "util/FlexDisableRegister.hh"
 #include "StringUtil.hh"
 #include "parasitics/SpefReaderPvt.hh"
 #include "SpefParse.hh"
diff --git a/sdf/SdfLex.ll b/sdf/SdfLex.ll
index e893af61..14b9876a 100644
--- a/sdf/SdfLex.ll
+++ b/sdf/SdfLex.ll
@@ -16,7 +16,6 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-#include "util/FlexDisableRegister.hh"
 #include "sdf/Sdf.hh"
 #include "SdfParse.hh"
 
diff --git a/util/FlexDisableRegister.hh b/util/FlexDisableRegister.hh
deleted file mode 100644
index 5e823a5f..00000000
--- a/util/FlexDisableRegister.hh
+++ /dev/null
@@ -1,2 +0,0 @@
-// Remove register declaration (not allowed in c++17).
-#define register
diff --git a/verilog/VerilogLex.ll b/verilog/VerilogLex.ll
index c1491a0d..797feb2f 100644
--- a/verilog/VerilogLex.ll
+++ b/verilog/VerilogLex.ll
@@ -16,7 +16,6 @@
 // You should have received a copy of the GNU General Public License
 // along with this program.  If not, see <https://www.gnu.org/licenses/>.
 
-#include "util/FlexDisableRegister.hh"
 #include "Debug.hh"
 #include "VerilogNamespace.hh"
 #include "verilog/VerilogReaderPvt.hh"