diff --git a/llvm/include/llvm/Support/FileCheck.h b/llvm/include/llvm/Support/FileCheck.h
--- a/llvm/include/llvm/Support/FileCheck.h
+++ b/llvm/include/llvm/Support/FileCheck.h
@@ -102,8 +102,10 @@
   llvm::Optional<StringRef> getVarValue(StringRef VarName);
 
   /// Define variables from definitions given on the command line passed as a
-  /// vector of VAR=VAL strings in \p CmdlineDefines.
-  void defineCmdlineVariables(std::vector<std::string> &CmdlineDefines);
+  /// vector of VAR=VAL strings in \p CmdlineDefines. Report any error to \p SM
+  /// and return whether an error occured.
+  bool defineCmdlineVariables(std::vector<std::string> &CmdlineDefines,
+                              SourceMgr &SM);
 
   /// Undefine local variables (variables whose name does not start with a '$'
   /// sign), i.e. remove them from GlobalVariableTable.
@@ -153,6 +155,13 @@
   /// Returns the pointer to the global state for all patterns in this
   /// FileCheck instance.
   FileCheckPatternContext *getContext() const { return Context; }
+  /// Return whether \p is a valid first character for a variable name.
+  static bool isValidVarNameStart(char c);
+  /// Verify that the string at the start of \p Str is a well formed variable.
+  /// Return false if it is and set \p IsPseudo to indicate if it is a pseudo
+  /// variable and \p TrailIdx to the position of the last character that is
+  /// part of the variable name. Otherwise, only return true.
+  static bool parseVariable(StringRef Str, bool &IsPseudo, unsigned &TrailIdx);
   bool ParsePattern(StringRef PatternStr, StringRef Prefix, SourceMgr &SM,
                     unsigned LineNumber, const FileCheckRequest &Req);
   size_t match(StringRef Buffer, size_t &MatchLen) const;
diff --git a/llvm/lib/Support/FileCheck.cpp b/llvm/lib/Support/FileCheck.cpp
--- a/llvm/lib/Support/FileCheck.cpp
+++ b/llvm/lib/Support/FileCheck.cpp
@@ -24,6 +24,38 @@
 
 using namespace llvm;
 
+bool FileCheckPattern::isValidVarNameStart(char c) {
+  return c == '_' || isalpha(c);
+}
+
+bool FileCheckPattern::parseVariable(StringRef Str, bool &IsPseudo,
+                                     unsigned &TrailIdx) {
+  bool ParsedOneChar = false;
+  unsigned i = 0;
+  IsPseudo = false;
+  for (unsigned e = Str.size(); i != e; ++i) {
+    if (i == 0) {
+      // Global vars start with '$'.
+      if (Str[i] == '$')
+        continue;
+      else if (Str[i] == '@') {
+        IsPseudo = true;
+        continue;
+      }
+    }
+    if (!ParsedOneChar && !isValidVarNameStart(Str[i]))
+      return true;
+
+    // Variable names are composed of alphanumeric characters and underscores.
+    if (Str[i] != '_' && !isalnum(Str[i]))
+      break;
+    ParsedOneChar = true;
+  }
+
+  TrailIdx = i;
+  return false;
+}
+
 /// Parses the given string into the Pattern.
 ///
 /// \p Prefix provides which prefix is being matched, \p SM provides the
@@ -117,9 +149,10 @@
     // itself must be of the form "[a-zA-Z_][0-9a-zA-Z_]*", otherwise we reject
     // it.  This is to catch some common errors.
     if (PatternStr.startswith("[[")) {
+      StringRef MatchStr = PatternStr.substr(2);
       // Find the closing bracket pair ending the match.  End is going to be an
       // offset relative to the beginning of the match string.
-      size_t End = FindRegexVarEnd(PatternStr.substr(2), SM);
+      size_t End = FindRegexVarEnd(MatchStr, SM);
 
       if (End == StringRef::npos) {
         SM.PrintMessage(SMLoc::getFromPointer(PatternStr.data()),
@@ -128,55 +161,45 @@
         return true;
       }
 
-      StringRef MatchStr = PatternStr.substr(2, End);
+      MatchStr = MatchStr.substr(0, End);
       PatternStr = PatternStr.substr(End + 4);
 
-      // Get the regex name (e.g. "foo").
-      size_t NameEnd = MatchStr.find(':');
-      StringRef Name = MatchStr.substr(0, NameEnd);
+      // Get the regex name (e.g. "foo") and verify it is well formed.
+      bool IsPseudo;
+      unsigned TrailIdx;
+      if (parseVariable(MatchStr, IsPseudo, TrailIdx)) {
+        SM.PrintMessage(SMLoc::getFromPointer(MatchStr.data()),
+                        SourceMgr::DK_Error, "Invalid name in named regex");
+        return true;
+      }
+
+      StringRef Name = MatchStr.substr(0, TrailIdx);
+      StringRef Trailer = MatchStr.substr(TrailIdx);
+      size_t DefSepIdx = Trailer.find(":");
+      bool IsVarDef = (DefSepIdx != StringRef::npos);
 
-      if (Name.empty()) {
-        SM.PrintMessage(SMLoc::getFromPointer(Name.data()), SourceMgr::DK_Error,
-                        "invalid name in named regex: empty name");
+      if (IsVarDef && (IsPseudo || !Trailer.consume_front(":"))) {
+        SM.PrintMessage(SMLoc::getFromPointer(MatchStr.data()),
+                        SourceMgr::DK_Error,
+                        "Invalid name in named regex definition");
         return true;
       }
 
       // Verify that the name/expression is well formed. FileCheck currently
       // supports @LINE, @LINE+number, @LINE-number expressions. The check here
       // is relaxed, more strict check is performed in \c EvaluateExpression.
-      bool IsExpression = false;
-      for (unsigned i = 0, e = Name.size(); i != e; ++i) {
-        if (i == 0) {
-          if (Name[i] == '$')  // Global vars start with '$'
-            continue;
-          if (Name[i] == '@') {
-            if (NameEnd != StringRef::npos) {
-              SM.PrintMessage(SMLoc::getFromPointer(Name.data()),
-                              SourceMgr::DK_Error,
-                              "invalid name in named regex definition");
-              return true;
-            }
-            IsExpression = true;
-            continue;
+      if (IsPseudo) {
+        for (unsigned i = 0, e = Trailer.size(); i != e; ++i) {
+          if (!isalnum(Trailer[i]) && Trailer[i] != '+' && Trailer[i] != '-') {
+            SM.PrintMessage(SMLoc::getFromPointer(Name.data() + i),
+                            SourceMgr::DK_Error, "invalid name in named regex");
+            return true;
           }
         }
-        if (Name[i] != '_' && !isalnum(Name[i]) &&
-            (!IsExpression || (Name[i] != '+' && Name[i] != '-'))) {
-          SM.PrintMessage(SMLoc::getFromPointer(Name.data() + i),
-                          SourceMgr::DK_Error, "invalid name in named regex");
-          return true;
-        }
-      }
-
-      // Name can't start with a digit.
-      if (isdigit(static_cast<unsigned char>(Name[0]))) {
-        SM.PrintMessage(SMLoc::getFromPointer(Name.data()), SourceMgr::DK_Error,
-                        "invalid name in named regex");
-        return true;
       }
 
       // Handle [[foo]].
-      if (NameEnd == StringRef::npos) {
+      if (!IsVarDef) {
         // Handle variables that were defined earlier on the same line by
         // emitting a backreference.
         if (VariableDefs.find(Name) != VariableDefs.end()) {
@@ -189,7 +212,7 @@
           }
           AddBackrefToRegEx(VarParenNum);
         } else {
-          VariableUses.push_back(std::make_pair(Name, RegExStr.size()));
+          VariableUses.push_back(std::make_pair(MatchStr, RegExStr.size()));
         }
         continue;
       }
@@ -199,7 +222,7 @@
       RegExStr += '(';
       ++CurParen;
 
-      if (AddRegExToRegEx(MatchStr.substr(NameEnd + 1), CurParen, SM))
+      if (AddRegExToRegEx(Trailer, CurParen, SM))
         return true;
 
       RegExStr += ')';
@@ -755,7 +778,8 @@
 bool llvm::FileCheck::ReadCheckFile(
     SourceMgr &SM, StringRef Buffer, Regex &PrefixRE,
     std::vector<FileCheckString> &CheckStrings) {
-  PatternContext.defineCmdlineVariables(Req.GlobalDefines);
+  if (PatternContext.defineCmdlineVariables(Req.GlobalDefines, SM))
+    return true;
 
   std::vector<FileCheckPattern> ImplicitNegativeChecks;
   for (const auto &PatternString : Req.ImplicitCheckNot) {
@@ -1374,12 +1398,36 @@
   return Regex(PrefixRegexStr);
 }
 
-void FileCheckPatternContext::defineCmdlineVariables(
-    std::vector<std::string> &CmdlineDefines) {
+bool FileCheckPatternContext::defineCmdlineVariables(
+    std::vector<std::string> &CmdlineDefines, SourceMgr &SM) {
   assert(GlobalVariableTable.empty() &&
          "Overriding defined variable with command-line variable definitions");
-  for (StringRef CmdlineDef : CmdlineDefines)
-    GlobalVariableTable.insert(CmdlineDef.split('='));
+
+  bool ErrorFound = false;
+  for (const auto &CmdlineDef : CmdlineDefines) {
+    std::string Prefix = "-D";
+    std::string DiagCmdline = Prefix + CmdlineDef;
+    std::unique_ptr<MemoryBuffer> CmdLine =
+        MemoryBuffer::getMemBufferCopy(DiagCmdline, "command line");
+    StringRef CmdlineDefRef = CmdLine->getBuffer().substr(Prefix.size());
+    SM.AddNewSourceBuffer(std::move(CmdLine), SMLoc());
+
+    bool IsPseudo;
+    unsigned TrailIdx;
+    std::pair<StringRef, StringRef> CmdlineNameVal = CmdlineDefRef.split('=');
+    StringRef Name = CmdlineNameVal.first;
+    if (FileCheckPattern::parseVariable(Name, IsPseudo, TrailIdx) || IsPseudo ||
+        TrailIdx != Name.size()) {
+      SM.PrintMessage(SMLoc::getFromPointer(CmdlineDefRef.data()),
+                      SourceMgr::DK_Error,
+                      "Invalid name for variable definition '" + Name + "'");
+      ErrorFound = true;
+      continue;
+    }
+    GlobalVariableTable.insert(CmdlineNameVal);
+  }
+
+  return ErrorFound;
 }
 
 void FileCheckPatternContext::clearLocalVars() {
diff --git a/llvm/test/FileCheck/defines.txt b/llvm/test/FileCheck/defines.txt
--- a/llvm/test/FileCheck/defines.txt
+++ b/llvm/test/FileCheck/defines.txt
@@ -1,6 +1,5 @@
 ; RUN: FileCheck -DVALUE=10 -input-file %s %s
 ; RUN: not FileCheck -DVALUE=20 -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRMSG
-;
 ; RUN: not FileCheck -DVALUE=10 -check-prefix NOT -input-file %s %s 2>&1 | FileCheck %s -check-prefix NOT-ERRMSG
 ; RUN: FileCheck -DVALUE=20 -check-prefix NOT -input-file %s %s
 ; RUN: not FileCheck -DVALUE10 -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIEQ1
@@ -9,6 +8,9 @@
 ; RUN: not FileCheck -D= -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIVAR2
 ; RUN: FileCheck -DVALUE= -check-prefix EMPTY -input-file %s %s 2>&1
 
+; RUN: not FileCheck -D10VALUE=10 -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIFMT
+; RUN: not FileCheck -D@VALUE=10 -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLIPSEUDO
+; RUN: not FileCheck -D'VALUE + 2=10' -input-file %s %s 2>&1 | FileCheck %s -check-prefix ERRCLITRAIL
 Value = 10
 ; CHECK: Value = [[VALUE]]
 ; NOT-NOT: Value = [[VALUE]]
@@ -32,3 +34,15 @@
 
 Empty value = @@
 ; EMPTY: Empty value = @[[VALUE]]@
+
+; ERRCLIFMT: command line:1:3: error: Invalid name for variable definition '10VALUE'
+; ERRCLIFMT-NEXT: -D10VALUE=10
+; ERRCLIFMT-NEXT: {{^  \^$}}
+
+; ERRCLIPSEUDO: command line:1:3: error: Invalid name for variable definition '@VALUE'
+; ERRCLIPSEUDO-NEXT: -D@VALUE=10
+; ERRCLIPSEUDO-NEXT: {{^  \^$}}
+
+; ERRCLITRAIL: command line:1:3: error: Invalid name for variable definition 'VALUE + 2'
+; ERRCLITRAIL-NEXT: -DVALUE + 2=10
+; ERRCLITRAIL-NEXT: {{^  \^$}}
diff --git a/llvm/unittests/Support/FileCheckTest.cpp b/llvm/unittests/Support/FileCheckTest.cpp
--- a/llvm/unittests/Support/FileCheckTest.cpp
+++ b/llvm/unittests/Support/FileCheckTest.cpp
@@ -17,10 +17,11 @@
 TEST_F(FileCheckTest, FileCheckContext) {
   FileCheckPatternContext Cxt;
   std::vector<std::string> GlobalDefines;
+  SourceMgr SM;
 
   // Define local and global variables from command-line.
   GlobalDefines.emplace_back(std::string("LocalVar=FOO"));
-  Cxt.defineCmdlineVariables(GlobalDefines);
+  Cxt.defineCmdlineVariables(GlobalDefines, SM);
 
   // Check defined variables are present and undefined is absent.
   StringRef LocalVarStr = "LocalVar";
@@ -38,7 +39,7 @@
 
   // Redefine global variables and check variables are defined again.
   GlobalDefines.emplace_back(std::string("$GlobalVar=BAR"));
-  Cxt.defineCmdlineVariables(GlobalDefines);
+  Cxt.defineCmdlineVariables(GlobalDefines, SM);
   StringRef GlobalVarStr = "$GlobalVar";
   llvm::Optional<StringRef> GlobalVar = Cxt.getVarValue(GlobalVarStr);
   EXPECT_TRUE(GlobalVar);
@@ -49,4 +50,78 @@
   GlobalVar = Cxt.getVarValue(GlobalVarStr);
   EXPECT_TRUE(GlobalVar);
 }
+
+TEST_F(FileCheckTest, ParseVar) {
+  EXPECT_TRUE(FileCheckPattern::isValidVarNameStart('a'));
+  EXPECT_TRUE(FileCheckPattern::isValidVarNameStart('G'));
+  EXPECT_TRUE(FileCheckPattern::isValidVarNameStart('_'));
+  EXPECT_FALSE(FileCheckPattern::isValidVarNameStart('2'));
+  EXPECT_FALSE(FileCheckPattern::isValidVarNameStart('$'));
+  EXPECT_FALSE(FileCheckPattern::isValidVarNameStart('@'));
+  EXPECT_FALSE(FileCheckPattern::isValidVarNameStart('+'));
+  EXPECT_FALSE(FileCheckPattern::isValidVarNameStart('-'));
+  EXPECT_FALSE(FileCheckPattern::isValidVarNameStart(':'));
+
+  StringRef VarName = "GoodVar42";
+  bool IsPseudo = true;
+  unsigned TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_FALSE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 9U);
+
+  VarName = "$GoodGlobalVar";
+  IsPseudo = true;
+  TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_FALSE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 14U);
+
+  VarName = "@GoodPseudoVar";
+  IsPseudo = true;
+  TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_TRUE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 14U);
+
+  VarName = "42BadVar";
+  EXPECT_TRUE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+
+  VarName = "$@";
+  EXPECT_TRUE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+
+  VarName = "B@dVar";
+  IsPseudo = true;
+  TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_FALSE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 1U);
+
+  VarName = "B$dVar";
+  IsPseudo = true;
+  TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_FALSE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 1U);
+
+  VarName = "BadVar+";
+  IsPseudo = true;
+  TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_FALSE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 6U);
+
+  VarName = "BadVar-";
+  IsPseudo = true;
+  TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_FALSE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 6U);
+
+  VarName = "BadVar:";
+  IsPseudo = true;
+  TrailIdx = 0;
+  EXPECT_FALSE(FileCheckPattern::parseVariable(VarName, IsPseudo, TrailIdx));
+  EXPECT_FALSE(IsPseudo);
+  EXPECT_EQ(TrailIdx, 6U);
+}
 } // namespace