Index: include/llvm/Support/Regex.h
===================================================================
--- include/llvm/Support/Regex.h
+++ include/llvm/Support/Regex.h
@@ -43,6 +43,7 @@
       BasicRegex=4
     };
 
+    Regex();
     /// Compiles the given regular expression \p Regex.
     Regex(StringRef Regex, unsigned Flags = NoFlags);
     Regex(const Regex &) = delete;
Index: lib/Support/Regex.cpp
===================================================================
--- lib/Support/Regex.cpp
+++ lib/Support/Regex.cpp
@@ -19,6 +19,8 @@
 #include <string>
 using namespace llvm;
 
+Regex::Regex() : error(REG_BADPAT), preg(nullptr) {}
+
 Regex::Regex(StringRef regex, unsigned Flags) {
   unsigned flags = 0;
   preg = new llvm_regex();
Index: unittests/Support/RegexTest.cpp
===================================================================
--- unittests/Support/RegexTest.cpp
+++ unittests/Support/RegexTest.cpp
@@ -100,7 +100,7 @@
 
   EXPECT_EQ("aber", Regex("[0-9]+").sub("\\", "a1234ber", &Error));
   EXPECT_EQ(Error, "replacement string contained trailing backslash");
-  
+
   // Backreferences
   EXPECT_EQ("aa1234bber", Regex("a[0-9]+b").sub("a\\0b", "a1234ber", &Error));
   EXPECT_EQ("", Error);
@@ -153,4 +153,13 @@
   EXPECT_TRUE(r2.match("916"));
 }
 
+TEST_F(RegexTest, NoArgConstructor) {
+  std::string Error;
+  Regex r1;
+  EXPECT_FALSE(r1.isValid(Error));
+  EXPECT_EQ("invalid regular expression", Error);
+  r1 = Regex("abc");
+  EXPECT_TRUE(r1.isValid(Error));
+}
+
 }