support capital X for hex escapes in string literals

src/google/protobuf/io/tokenizer.cc

@@ -422,7 +422,7 @@ void Tokenizer::ConsumeString(char delimiter) {
           // Possibly followed by two more octal digits, but these will
           // just be consumed by the main loop anyway so we don't need
           // to do so explicitly here.
-        } else if (TryConsume('x')) {
+        } else if (TryConsume('x') || TryConsume('X')) {
           if (!TryConsumeOne<HexDigit>()) {
             AddError("Expected hex digits for escape sequence.");
           }
@@ -1216,7 +1216,7 @@ void Tokenizer::ParseStringAppend(const std::string& text,
         }
         output->push_back(static_cast<char>(code));
 
-      } else if (*ptr == 'x') {
+      } else if (*ptr == 'x' || *ptr == 'X') {
         // A hex escape.  May zero, one, or two digits.  (The zero case
         // will have been caught as an error earlier.)
         int code = 0;

src/google/protobuf/io/tokenizer_unittest.cc

@@ -1045,6 +1045,8 @@ TEST_F(TokenizerTest, ParseString) {
   EXPECT_EQ("\1x\1\123\739\52\334n\3", output);
   Tokenizer::ParseString("'\\x20\\x4'", &output);
   EXPECT_EQ("\x20\x4", output);
+  Tokenizer::ParseString("'\\X20\\X4'", &output);
+  EXPECT_EQ("\x20\x4", output);
 
   // Test invalid strings that may still be tokenized as strings.
   Tokenizer::ParseString("\"\\a\\l\\v\\t", &output);  // \l is invalid
@@ -1110,7 +1112,7 @@ inline std::ostream& operator<<(std::ostream& out, const ErrorCase& test_case) {
 ErrorCase kErrorCases[] = {
     // String errors.
     {"'\\l' foo", true, "0:2: Invalid escape sequence in string literal.\n"},
-    {"'\\X' foo", true, "0:2: Invalid escape sequence in string literal.\n"},
+    {"'\\X' foo", true, "0:3: Expected hex digits for escape sequence.\n"},
     {"'\\x' foo", true, "0:3: Expected hex digits for escape sequence.\n"},
     {"'foo", false, "0:4: Unexpected end of string.\n"},
     {"'bar\nfoo", true,