The expected key is a valid identifier, which is already free of \\, <0x20, ".

src/json-parser.h

 // Copyright 2011 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_JSON_PARSER_H_
 #define V8_JSON_PARSER_H_
 
 #include "src/v8.h"
 
 #include "src/char-predicates-inl.h"
@@ skipping 90 matching lines @@
     int length = expected->length();
     if (source_->length() - position_ - 1 > length) {
       DisallowHeapAllocation no_gc;
       String::FlatContent content = expected->GetFlatContent();
       if (content.IsOneByte()) {
         DCHECK_EQ('"', c0_);
         const uint8_t* input_chars = seq_source_->GetChars() + position_ + 1;
         const uint8_t* expected_chars = content.ToOneByteVector().start();
         for (int i = 0; i < length; i++) {
           uint8_t c0 = input_chars[i];
-          if (c0 != expected_chars[i] ||
-              c0 == '"' || c0 < 0x20 || c0 == '\\') {
-            return false;
-          }
+          // The expected string has to be free of \, " and characters < 0x20.
+          if (c0 != expected_chars[i]) return false;
         }
         if (input_chars[length] == '"') {
           position_ = position_ + length + 1;
           AdvanceSkipWhitespace();
           return true;
         }
       }
     }
     return false;
   }
@@ skipping 685 matching lines @@
 
   DCHECK_EQ('"', c0_);
   // Advance past the last '"'.
   AdvanceSkipWhitespace();
   return result;
 }
 
 } }  // namespace v8::internal
 
 #endif  // V8_JSON_PARSER_H_