Index: include/llvm/Support/Regex.h
===================================================================
--- include/llvm/Support/Regex.h
+++ include/llvm/Support/Regex.h
@@ -74,7 +74,8 @@
     /// the first group is always the entire pattern.
     ///
     /// This returns true on a successful match.
-    bool match(StringRef String, SmallVectorImpl<StringRef> *Matches = nullptr);
+    bool match(StringRef String,
+               SmallVectorImpl<StringRef> *Matches = nullptr) const;
 
     /// sub - Return the result of replacing the first match of the regex in
     /// \p String with the \p Repl string. Backreferences like "\0" in the
@@ -98,7 +99,7 @@
 
   private:
     struct llvm_regex *preg;
-    int error;
+    mutable int error;
   };
 }
 
Index: lib/Support/Regex.cpp
===================================================================
--- lib/Support/Regex.cpp
+++ lib/Support/Regex.cpp
@@ -56,7 +56,7 @@
   return preg->re_nsub;
 }
 
-bool Regex::match(StringRef String, SmallVectorImpl<StringRef> *Matches){
+bool Regex::match(StringRef String, SmallVectorImpl<StringRef> *Matches) const {
   unsigned nmatch = Matches ? preg->re_nsub+1 : 0;
 
   // pmatch needs to have at least one element.