diff --git a/src/api.cc b/src/api.cc
index 23f934751e..88cf2bcde0 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -1046,7 +1046,7 @@ void ObjectTemplate::SetInternalFieldCount(int value) {
 
 ScriptData* ScriptData::PreCompile(const char* input, int length) {
   unibrow::Utf8InputBuffer<> buf(input, length);
-  return i::PreParse(&buf, NULL);
+  return i::PreParse(i::Handle<i::String>(), &buf, NULL);
 }
 
 
diff --git a/src/compiler.cc b/src/compiler.cc
index 0cdfc9edf4..feff492404 100644
--- a/src/compiler.cc
+++ b/src/compiler.cc
@@ -266,7 +266,7 @@ Handle<JSFunction> Compiler::Compile(Handle<String> source,
     if (pre_data == NULL && source_length >= FLAG_min_preparse_length) {
       Access<SafeStringInputBuffer> buf(&safe_string_input_buffer);
       buf->Reset(source.location());
-      pre_data = PreParse(buf.value(), extension);
+      pre_data = PreParse(source, buf.value(), extension);
     }
 
     // Create a script object describing the script to be compiled.
diff --git a/src/factory.cc b/src/factory.cc
index 36554df732..bb6987bb8c 100644
--- a/src/factory.cc
+++ b/src/factory.cc
@@ -87,8 +87,10 @@ Handle<String> Factory::NewStringFromUtf8(Vector<const char> string,
 }
 
 
-Handle<String> Factory::NewStringFromTwoByte(Vector<const uc16> string) {
-  CALL_HEAP_FUNCTION(Heap::AllocateStringFromTwoByte(string), String);
+Handle<String> Factory::NewStringFromTwoByte(Vector<const uc16> string,
+                                             PretenureFlag pretenure) {
+  CALL_HEAP_FUNCTION(Heap::AllocateStringFromTwoByte(string, pretenure),
+                     String);
 }
 
 
diff --git a/src/factory.h b/src/factory.h
index 4db5d4e71b..ddf71dec7a 100644
--- a/src/factory.h
+++ b/src/factory.h
@@ -92,7 +92,8 @@ class Factory : public AllStatic {
       Vector<const char> str,
       PretenureFlag pretenure = NOT_TENURED);
 
-  static Handle<String> NewStringFromTwoByte(Vector<const uc16> str);
+  static Handle<String> NewStringFromTwoByte(Vector<const uc16> str,
+      PretenureFlag pretenure = NOT_TENURED);
 
   // Allocates and partially initializes a TwoByte String. The characters of
   // the string are uninitialized. Currently used in regexp code only, where
diff --git a/src/parser.cc b/src/parser.cc
index 348c12a4ac..e3b72a3848 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -97,7 +97,7 @@ class Parser {
 
   // Pre-parse the program from the character stream; returns true on
   // success, false if a stack-overflow happened during parsing.
-  bool PreParseProgram(unibrow::CharacterStream* stream);
+  bool PreParseProgram(Handle<String> source, unibrow::CharacterStream* stream);
 
   void ReportMessage(const char* message, Vector<const char*> args);
   virtual void ReportMessageAt(Scanner::Location loc,
@@ -1167,13 +1167,14 @@ Parser::Parser(Handle<Script> script,
 }
 
 
-bool Parser::PreParseProgram(unibrow::CharacterStream* stream) {
+bool Parser::PreParseProgram(Handle<String> source,
+                             unibrow::CharacterStream* stream) {
   HistogramTimerScope timer(&Counters::pre_parse);
   StackGuard guard;
   AssertNoZoneAllocation assert_no_zone_allocation;
   AssertNoAllocation assert_no_allocation;
   NoHandleAllocation no_handle_allocation;
-  scanner_.Init(Handle<String>(), stream, 0);
+  scanner_.Init(source, stream, 0);
   ASSERT(target_stack_ == NULL);
   mode_ = PARSE_EAGERLY;
   DummyScope top_scope;
@@ -4593,7 +4594,8 @@ unsigned* ScriptDataImpl::Data() {
 }
 
 
-ScriptDataImpl* PreParse(unibrow::CharacterStream* stream,
+ScriptDataImpl* PreParse(Handle<String> source,
+                         unibrow::CharacterStream* stream,
                          v8::Extension* extension) {
   Handle<Script> no_script;
   bool allow_natives_syntax =
@@ -4601,7 +4603,7 @@ ScriptDataImpl* PreParse(unibrow::CharacterStream* stream,
       FLAG_allow_natives_syntax ||
       Bootstrapper::IsActive();
   PreParser parser(no_script, allow_natives_syntax, extension);
-  if (!parser.PreParseProgram(stream)) return NULL;
+  if (!parser.PreParseProgram(source, stream)) return NULL;
   // The list owns the backing store so we need to clone the vector.
   // That way, the result will be exactly the right size rather than
   // the expected 50% too large.
diff --git a/src/parser.h b/src/parser.h
index c029c4b25d..86e1f74fbe 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -143,7 +143,8 @@ FunctionLiteral* MakeAST(bool compile_in_global_context,
                          ScriptDataImpl* pre_data);
 
 
-ScriptDataImpl* PreParse(unibrow::CharacterStream* stream,
+ScriptDataImpl* PreParse(Handle<String> source,
+                         unibrow::CharacterStream* stream,
                          v8::Extension* extension);
 
 
diff --git a/src/scanner.cc b/src/scanner.cc
index 24a6d4be9c..720dc3536c 100644
--- a/src/scanner.cc
+++ b/src/scanner.cc
@@ -92,18 +92,7 @@ void UTF8Buffer::AddCharSlow(uc32 c) {
 
 
 UTF16Buffer::UTF16Buffer()
-  : pos_(0),
-    pushback_buffer_(0),
-    last_(0),
-    stream_(NULL) { }
-
-
-void UTF16Buffer::Initialize(Handle<String> data,
-                             unibrow::CharacterStream* input) {
-  data_ = data;
-  pos_ = 0;
-  stream_ = input;
-}
+    : pos_(0), size_(0) { }
 
 
 Handle<String> UTF16Buffer::SubString(int start, int end) {
@@ -111,14 +100,27 @@ Handle<String> UTF16Buffer::SubString(int start, int end) {
 }
 
 
-void UTF16Buffer::PushBack(uc32 ch) {
+// CharacterStreamUTF16Buffer
+CharacterStreamUTF16Buffer::CharacterStreamUTF16Buffer()
+    : pushback_buffer_(0), last_(0), stream_(NULL) { }
+
+
+void CharacterStreamUTF16Buffer::Initialize(Handle<String> data,
+                                            unibrow::CharacterStream* input) {
+  data_ = data;
+  pos_ = 0;
+  stream_ = input;
+}
+
+
+void CharacterStreamUTF16Buffer::PushBack(uc32 ch) {
   pushback_buffer()->Add(last_);
   last_ = ch;
   pos_--;
 }
 
 
-uc32 UTF16Buffer::Advance() {
+uc32 CharacterStreamUTF16Buffer::Advance() {
   // NOTE: It is of importance to Persian / Farsi resources that we do
   // *not* strip format control characters in the scanner; see
   //
@@ -135,7 +137,7 @@ uc32 UTF16Buffer::Advance() {
     uc32 next = stream_->GetNext();
     return last_ = next;
   } else {
-    // note: currently the following increment is necessary to avoid a
+    // Note: currently the following increment is necessary to avoid a
     // test-parser problem!
     pos_++;
     return last_ = static_cast<uc32>(-1);
@@ -143,13 +145,53 @@ uc32 UTF16Buffer::Advance() {
 }
 
 
-void UTF16Buffer::SeekForward(int pos) {
+void CharacterStreamUTF16Buffer::SeekForward(int pos) {
   pos_ = pos;
   ASSERT(pushback_buffer()->is_empty());
   stream_->Seek(pos);
 }
 
 
+// TwoByteStringUTF16Buffer
+TwoByteStringUTF16Buffer::TwoByteStringUTF16Buffer()
+    : raw_data_(NULL) { }
+
+
+void TwoByteStringUTF16Buffer::Initialize(
+     Handle<ExternalTwoByteString> data) {
+  ASSERT(!data.is_null());
+
+  data_ = data;
+  pos_ = 0;
+
+  raw_data_ = data->resource()->data();
+  size_ = data->length();
+}
+
+
+uc32 TwoByteStringUTF16Buffer::Advance() {
+  if (pos_ < size_) {
+    return raw_data_[pos_++];
+  } else {
+    // note: currently the following increment is necessary to avoid a
+    // test-parser problem!
+    pos_++;
+    return static_cast<uc32>(-1);
+  }
+}
+
+
+void TwoByteStringUTF16Buffer::PushBack(uc32 ch) {
+  pos_--;
+  ASSERT(pos_ >= 0 && raw_data_[pos_] == ch);
+}
+
+
+void TwoByteStringUTF16Buffer::SeekForward(int pos) {
+  pos_ = pos;
+}
+
+
 // ----------------------------------------------------------------------------
 // Scanner
 
@@ -161,7 +203,15 @@ Scanner::Scanner(bool pre) : stack_overflow_(false), is_pre_parsing_(pre) {
 void Scanner::Init(Handle<String> source, unibrow::CharacterStream* stream,
     int position) {
   // Initialize the source buffer.
-  source_.Initialize(source, stream);
+  if (!source.is_null() && StringShape(*source).IsExternalTwoByte()) {
+    two_byte_string_buffer_.Initialize(
+        Handle<ExternalTwoByteString>::cast(source));
+    source_ = &two_byte_string_buffer_;
+  } else {
+    char_stream_buffer_.Initialize(source, stream);
+    source_ = &char_stream_buffer_;
+  }
+
   position_ = position;
 
   // Reset literals buffer
@@ -180,7 +230,7 @@ void Scanner::Init(Handle<String> source, unibrow::CharacterStream* stream,
 
 
 Handle<String> Scanner::SubString(int start, int end) {
-  return source_.SubString(start - position_, end - position_);
+  return source_->SubString(start - position_, end - position_);
 }
 
 
@@ -223,17 +273,6 @@ void Scanner::AddCharAdvance() {
 }
 
 
-void Scanner::Advance() {
-  c0_ = source_.Advance();
-}
-
-
-void Scanner::PushBack(uc32 ch) {
-  source_.PushBack(ch);
-  c0_ = ch;
-}
-
-
 static inline bool IsByteOrderMark(uc32 c) {
   // The Unicode value U+FFFE is guaranteed never to be assigned as a
   // Unicode character; this implies that in a Unicode context the
@@ -583,7 +622,7 @@ void Scanner::Scan() {
 
 
 void Scanner::SeekForward(int pos) {
-  source_.SeekForward(pos - 1);
+  source_->SeekForward(pos - 1);
   Advance();
   Scan();
 }
diff --git a/src/scanner.h b/src/scanner.h
index eea23a70c2..340da86ed7 100644
--- a/src/scanner.h
+++ b/src/scanner.h
@@ -73,24 +73,53 @@ class UTF8Buffer {
 class UTF16Buffer {
  public:
   UTF16Buffer();
+  virtual ~UTF16Buffer() {}
+
+  virtual void PushBack(uc32 ch) = 0;
+  // returns a value < 0 when the buffer end is reached
+  virtual uc32 Advance() = 0;
+  virtual void SeekForward(int pos) = 0;
 
-  void Initialize(Handle<String> data, unibrow::CharacterStream* stream);
-  void PushBack(uc32 ch);
-  uc32 Advance();  // returns a value < 0 when the buffer end is reached
-  uint16_t CharAt(int index);
   int pos() const { return pos_; }
   int size() const { return size_; }
   Handle<String> SubString(int start, int end);
-  List<uc32>* pushback_buffer() { return &pushback_buffer_; }
-  void SeekForward(int pos);
 
- private:
+ protected:
   Handle<String> data_;
   int pos_;
   int size_;
+};
+
+
+class CharacterStreamUTF16Buffer: public UTF16Buffer {
+ public:
+  CharacterStreamUTF16Buffer();
+  virtual ~CharacterStreamUTF16Buffer() {}
+  void Initialize(Handle<String> data, unibrow::CharacterStream* stream);
+  virtual void PushBack(uc32 ch);
+  virtual uc32 Advance();
+  virtual void SeekForward(int pos);
+
+ private:
   List<uc32> pushback_buffer_;
   uc32 last_;
   unibrow::CharacterStream* stream_;
+
+  List<uc32>* pushback_buffer() { return &pushback_buffer_; }
+};
+
+
+class TwoByteStringUTF16Buffer: public UTF16Buffer {
+ public:
+  TwoByteStringUTF16Buffer();
+  virtual ~TwoByteStringUTF16Buffer() {}
+  void Initialize(Handle<ExternalTwoByteString> data);
+  virtual void PushBack(uc32 ch);
+  virtual uc32 Advance();
+  virtual void SeekForward(int pos);
+
+ private:
+  const uint16_t* raw_data_;
 };
 
 
@@ -184,8 +213,11 @@ class Scanner {
   static unibrow::Predicate<unibrow::WhiteSpace, 128> kIsWhiteSpace;
 
  private:
+  CharacterStreamUTF16Buffer char_stream_buffer_;
+  TwoByteStringUTF16Buffer two_byte_string_buffer_;
+
   // Source.
-  UTF16Buffer source_;
+  UTF16Buffer* source_;
   int position_;
 
   // Buffer to hold literal values (identifiers, strings, numbers)
@@ -219,8 +251,11 @@ class Scanner {
   void TerminateLiteral();
 
   // Low-level scanning support.
-  void Advance();
-  void PushBack(uc32 ch);
+  void Advance() { c0_ = source_->Advance(); }
+  void PushBack(uc32 ch) {
+    source_->PushBack(ch);
+    c0_ = ch;
+  }
 
   bool SkipWhiteSpace();
   Token::Value SkipSingleLineComment();
@@ -243,7 +278,7 @@ class Scanner {
 
   // Return the current source position.
   int source_pos() {
-    return source_.pos() - kCharacterLookaheadBufferSize + position_;
+    return source_->pos() - kCharacterLookaheadBufferSize + position_;
   }
 
   // Decodes a unicode escape-sequence which is part of an identifier.