diff --git a/clang-tools-extra/clang-tidy/google/CMakeLists.txt b/clang-tools-extra/clang-tidy/google/CMakeLists.txt
--- a/clang-tools-extra/clang-tidy/google/CMakeLists.txt
+++ b/clang-tools-extra/clang-tidy/google/CMakeLists.txt
@@ -14,6 +14,7 @@
   IntegerTypesCheck.cpp
   NonConstReferences.cpp
   OverloadedUnaryAndCheck.cpp
+  TestCheck.cpp
   TodoCommentCheck.cpp
   UnnamedNamespaceInHeaderCheck.cpp
   UsingNamespaceDirectiveCheck.cpp
diff --git a/clang-tools-extra/clang-tidy/google/GlobalVariableDeclarationCheck.cpp b/clang-tools-extra/clang-tidy/google/GlobalVariableDeclarationCheck.cpp
--- a/clang-tools-extra/clang-tidy/google/GlobalVariableDeclarationCheck.cpp
+++ b/clang-tools-extra/clang-tidy/google/GlobalVariableDeclarationCheck.cpp
@@ -23,34 +23,40 @@
 
 namespace {
 
-AST_MATCHER(VarDecl, isLocalVariable) {
-  return Node.isLocalVarDecl();
-}
+AST_MATCHER(VarDecl, isLocalVariable) { return Node.isLocalVarDecl(); }
 
 FixItHint generateFixItHint(const VarDecl *Decl, bool IsConst) {
+  if (IsConst && Decl->hasExternalStorage()) {
+    // No fix available if it is a extern global constant, since it is difficult
+    // to determine the proper fix in this case.
+    return FixItHint();
+  }
+
   char FC = Decl->getName()[0];
   if (!llvm::isAlpha(FC) || Decl->getName().size() == 1) {
     // No fix available if first character is not alphabetical character, or it
-    // is a single-character variable, since it is difficult to determine the 
+    // is a single-character variable, since it is difficult to determine the
     // proper fix in this case. Users should create a proper variable name by
     // their own.
     return FixItHint();
   }
   char SC = Decl->getName()[1];
   if ((FC == 'k' || FC == 'g') && !llvm::isAlpha(SC)) {
-    // No fix available if the prefix is correct but the second character is not
-    // alphabetical, since it is difficult to determine the proper fix in this
-    // case.
+    // No fix available if the prefix is correct but the second character is
+    // not alphabetical, since it is difficult to determine the proper fix in
+    // this case.
     return FixItHint();
   }
+
   auto NewName = (IsConst ? "k" : "g") +
                  llvm::StringRef(std::string(1, FC)).upper() +
                  Decl->getName().substr(1).str();
+
   return FixItHint::CreateReplacement(
       CharSourceRange::getTokenRange(SourceRange(Decl->getLocation())),
       llvm::StringRef(NewName));
 }
-}  // namespace
+} // namespace
 
 void GlobalVariableDeclarationCheck::registerMatchers(MatchFinder *Finder) {
   // The relevant Style Guide rule only applies to Objective-C.
@@ -71,7 +77,7 @@
       this);
   Finder->addMatcher(varDecl(hasGlobalStorage(), hasType(isConstQualified()),
                              unless(isLocalVariable()),
-                             unless(matchesName("::(k[A-Z]|[A-Z]{2,})")))
+                             unless(matchesName("::(k[A-Z])|([A-Z][A-Z0-9])")))
                          .bind("global_const"),
                      this);
 }
@@ -96,7 +102,7 @@
   }
 }
 
-}  // namespace objc
-}  // namespace google
-}  // namespace tidy
-}  // namespace clang
+} // namespace objc
+} // namespace google
+} // namespace tidy
+} // namespace clang
diff --git a/clang-tools-extra/test/clang-tidy/google-objc-global-variable-declaration.m b/clang-tools-extra/test/clang-tidy/google-objc-global-variable-declaration.m
--- a/clang-tools-extra/test/clang-tidy/google-objc-global-variable-declaration.m
+++ b/clang-tools-extra/test/clang-tidy/google-objc-global-variable-declaration.m
@@ -1,10 +1,14 @@
 // RUN: %check_clang_tidy %s google-objc-global-variable-declaration %t
 
 @class NSString;
+
 static NSString* const myConstString = @"hello";
 // CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'myConstString' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
 // CHECK-FIXES: static NSString* const kMyConstString = @"hello";
 
+extern NSString* const GlobalConstant = @"hey";
+// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'GlobalConstant' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
+
 static NSString* MyString = @"hi";
 // CHECK-MESSAGES: :[[@LINE-1]]:18: warning: non-const global variable 'MyString' must have a name which starts with 'g[A-Z]' [google-objc-global-variable-declaration]
 // CHECK-FIXES: static NSString* gMyString = @"hi";
@@ -25,13 +29,21 @@
 // CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable '_notAlpha' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
 // CHECK-FIXES: static NSString* const _notAlpha = @"NotBeginWithAlpha";
 
+static NSString* const notCap = @"NotBeginWithCap";
+// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'notCap' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
+
 static NSString* const k_Alpha = @"SecondNotAlpha";
 // CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'k_Alpha' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
 // CHECK-FIXES: static NSString* const k_Alpha = @"SecondNotAlpha";
 
+static NSString* const SecondNotCap = @"SecondNotCapOrNumber";
+// CHECK-MESSAGES: :[[@LINE-1]]:24: warning: const global variable 'SecondNotCap' must have a name which starts with an appropriate prefix [google-objc-global-variable-declaration]
+
 static NSString* const kGood = @"hello";
 static NSString* const XYGood = @"hello";
+static NSString* const X1Good = @"hello";
 static NSString* gMyIntGood = 0;
+extern NSString* Y2Good;
 
 extern NSString* const GTLServiceErrorDomain;
 
@@ -42,8 +54,8 @@
 
 @implementation Foo
 - (void)f {
-    int x = 0;
-    static int bar;
-    static const int baz = 42;
+  int x = 0;
+  static int bar;
+  static const int baz = 42;
 }
 @end