diff --git a/llvm/lib/FileCheck/FileCheck.cpp b/llvm/lib/FileCheck/FileCheck.cpp
--- a/llvm/lib/FileCheck/FileCheck.cpp
+++ b/llvm/lib/FileCheck/FileCheck.cpp
@@ -1083,8 +1083,15 @@
           if (IsPseudo) {
             MatchStr = OrigMatchStr;
             IsLegacyLineExpr = IsNumBlock = true;
-          } else
+          } else {
+            if (!MatchStr.empty()) {
+              SM.PrintMessage(SMLoc::getFromPointer(Name.data()),
+                              SourceMgr::DK_Error,
+                              "invalid name in string variable use");
+              return true;
+            }
             SubstStr = Name;
+          }
         }
       }
 
diff --git a/llvm/test/FileCheck/simple-var-capture.txt b/llvm/test/FileCheck/simple-var-capture.txt
--- a/llvm/test/FileCheck/simple-var-capture.txt
+++ b/llvm/test/FileCheck/simple-var-capture.txt
@@ -11,3 +11,15 @@
 ; CHECK-NEXT:   op4 {{r[0-9]+}}, [[REGa]], [[REGb]]
 
 
+// RUN: %ProtectFileCheckOutput \
+// RUN: not FileCheck --check-prefixes INVALID-VARNAME --input-file %s %s 2>&1 \
+// RUN:   | FileCheck --check-prefix INVALID-VARNAME-MSG --strict-whitespace %s
+
+5
+4
+; INVALID-VARNAME: [[X:]]
+; INVALID-VARNAME-NEXT: [[Y:]]
+; INVALID-VARNAME-NEXT: [[X-Y]]
+; INVALID-VARNAME-MSG: simple-var-capture.txt:[[#@LINE-1]]:27: error: invalid name in string variable use
+; INVALID-VARNAME-MSG-NEXT: ; {{I}}NVALID-VARNAME-NEXT: {{\[\[X-Y\]\]}}
+; INVALID-VARNAME-MSG-NEXT:    {{^}}                          ^{{$}}
diff --git a/llvm/unittests/FileCheck/FileCheckTest.cpp b/llvm/unittests/FileCheck/FileCheckTest.cpp
--- a/llvm/unittests/FileCheck/FileCheckTest.cpp
+++ b/llvm/unittests/FileCheck/FileCheckTest.cpp
@@ -1343,6 +1343,9 @@
   // Collision with numeric variable.
   EXPECT_TRUE(Tester.parsePattern("[[FOO:]]"));
 
+  // Invalid use of string variable.
+  EXPECT_TRUE(Tester.parsePattern("[[FOO-BAR]]"));
+
   // Valid use of string variable.
   EXPECT_FALSE(Tester.parsePattern("[[BAR]]"));