Issue mcp-graveyard/UXP#1362 - Revert "Implement regular expression lookbehind"

This reverts commit fa473930f424bf17a9e545b601c84dd2e61364e3.

12 files changed, 157 insertions, 359 deletions

diff --git a/js/src/irregexp/NativeRegExpMacroAssembler.cpp b/js/src/irregexp/NativeRegExpMacroAssembler.cpp
index e17eecb9bb..0fb5072973 100644
--- a/js/src/irregexp/NativeRegExpMacroAssembler.cpp
+++ b/js/src/irregexp/NativeRegExpMacroAssembler.cpp
@@ -582,7 +582,7 @@ NativeRegExpMacroAssembler::CheckAtStart(Label* on_at_start)
 }
 
 void
-NativeRegExpMacroAssembler::CheckNotAtStart(int cp_offset, Label* on_not_at_start)
+NativeRegExpMacroAssembler::CheckNotAtStart(Label* on_not_at_start)
 {
     JitSpew(SPEW_PREFIX "CheckNotAtStart");
 
@@ -673,7 +673,7 @@ NativeRegExpMacroAssembler::CheckGreedyLoop(Label* on_tos_equals_current_positio
 }
 
 void
-NativeRegExpMacroAssembler::CheckNotBackReference(int start_reg, bool read_backward, Label* on_no_match)
+NativeRegExpMacroAssembler::CheckNotBackReference(int start_reg, Label* on_no_match)
 {
     JitSpew(SPEW_PREFIX "CheckNotBackReference(%d)", start_reg);
 
@@ -744,8 +744,8 @@ NativeRegExpMacroAssembler::CheckNotBackReference(int start_reg, bool read_backw
 }
 
 void
-NativeRegExpMacroAssembler::CheckNotBackReferenceIgnoreCase(int start_reg, bool read_backward,
-                                                            Label* on_no_match, bool unicode)
+NativeRegExpMacroAssembler::CheckNotBackReferenceIgnoreCase(int start_reg, Label* on_no_match,
+                                                            bool unicode)
 {
     JitSpew(SPEW_PREFIX "CheckNotBackReferenceIgnoreCase(%d, %d)", start_reg, unicode);
 
diff --git a/js/src/irregexp/NativeRegExpMacroAssembler.h b/js/src/irregexp/NativeRegExpMacroAssembler.h
index fc582dccf2..7a72e252ff 100644
--- a/js/src/irregexp/NativeRegExpMacroAssembler.h
+++ b/js/src/irregexp/NativeRegExpMacroAssembler.h
@@ -105,10 +105,9 @@ class MOZ_STACK_CLASS NativeRegExpMacroAssembler final : public RegExpMacroAssem
     void CheckCharacterGT(char16_t limit, jit::Label* on_greater);
     void CheckCharacterLT(char16_t limit, jit::Label* on_less);
     void CheckGreedyLoop(jit::Label* on_tos_equals_current_position);
-    void CheckNotAtStart(int cp_offset, jit::Label* on_not_at_start);
-    void CheckNotBackReference(int start_reg, bool read_backward, jit::Label* on_no_match);
-    void CheckNotBackReferenceIgnoreCase(int start_reg, bool read_backward,
-                                         jit::Label* on_no_match, bool unicode);
+    void CheckNotAtStart(jit::Label* on_not_at_start);
+    void CheckNotBackReference(int start_reg, jit::Label* on_no_match);
+    void CheckNotBackReferenceIgnoreCase(int start_reg, jit::Label* on_no_match, bool unicode);
     void CheckNotCharacter(unsigned c, jit::Label* on_not_equal);
     void CheckNotCharacterAfterAnd(unsigned c, unsigned and_with, jit::Label* on_not_equal);
     void CheckNotCharacterAfterMinusAnd(char16_t c, char16_t minus, char16_t and_with,
diff --git a/js/src/irregexp/RegExpAST.cpp b/js/src/irregexp/RegExpAST.cpp
index 43867c3123..8dfd99057b 100644
--- a/js/src/irregexp/RegExpAST.cpp
+++ b/js/src/irregexp/RegExpAST.cpp
@@ -250,16 +250,16 @@ RegExpCapture::CaptureRegisters()
 }
 
 // ----------------------------------------------------------------------------
-// RegExpLookaround
+// RegExpLookahead
 
 Interval
-RegExpLookaround::CaptureRegisters()
+RegExpLookahead::CaptureRegisters()
 {
     return body()->CaptureRegisters();
 }
 
 bool
-RegExpLookaround::IsAnchoredAtStart()
+RegExpLookahead::IsAnchoredAtStart()
 {
-    return is_positive() && type() == LOOKAHEAD && body()->IsAnchoredAtStart();
+    return is_positive() && body()->IsAnchoredAtStart();
 }
diff --git a/js/src/irregexp/RegExpAST.h b/js/src/irregexp/RegExpAST.h
index 6f59842bcb..7bda6fc7e6 100644
--- a/js/src/irregexp/RegExpAST.h
+++ b/js/src/irregexp/RegExpAST.h
@@ -360,7 +360,6 @@ class RegExpCapture : public RegExpTree
     virtual int min_match() { return body_->min_match(); }
     virtual int max_match() { return body_->max_match(); }
     RegExpTree* body() { return body_; }
-    void set_body(RegExpTree* body) { body_ = body; }
     int index() { return index_; }
     static int StartRegister(int index) { return index * 2; }
     static int EndRegister(int index) { return index * 2 + 1; }
@@ -370,29 +369,25 @@ class RegExpCapture : public RegExpTree
     int index_;
 };
 
-class RegExpLookaround : public RegExpTree
+class RegExpLookahead : public RegExpTree
 {
   public:
-    enum Type { LOOKAHEAD, LOOKBEHIND };
-
-    RegExpLookaround(RegExpTree* body,
-                     bool is_positive,
-                     int capture_count,
-                     int capture_from,
-                     Type type)
+    RegExpLookahead(RegExpTree* body,
+                    bool is_positive,
+                    int capture_count,
+                    int capture_from)
       : body_(body),
         is_positive_(is_positive),
         capture_count_(capture_count),
-        capture_from_(capture_from),
-        type_(type)
+        capture_from_(capture_from)
     {}
 
     virtual void* Accept(RegExpVisitor* visitor, void* data);
     virtual RegExpNode* ToNode(RegExpCompiler* compiler,
                                RegExpNode* on_success);
-    virtual RegExpLookaround* AsLookaround();
+    virtual RegExpLookahead* AsLookahead();
     virtual Interval CaptureRegisters();
-    virtual bool IsLookaround();
+    virtual bool IsLookahead();
     virtual bool IsAnchoredAtStart();
     virtual int min_match() { return 0; }
     virtual int max_match() { return 0; }
@@ -400,14 +395,12 @@ class RegExpLookaround : public RegExpTree
     bool is_positive() { return is_positive_; }
     int capture_count() { return capture_count_; }
     int capture_from() { return capture_from_; }
-    Type type() { return type_; }
 
   private:
     RegExpTree* body_;
     bool is_positive_;
     int capture_count_;
     int capture_from_;
-    Type type_;
 };
 
 typedef InfallibleVector<RegExpCapture*, 1> RegExpCaptureVector;
@@ -424,14 +417,8 @@ class RegExpBackReference : public RegExpTree
                                RegExpNode* on_success);
     virtual RegExpBackReference* AsBackReference();
     virtual bool IsBackReference();
-    virtual int min_match() override { return 0; }
-    // The capture may not be completely parsed yet, if the reference occurs
-    // before the capture. In the ordinary case, nothing has been captured yet,
-    // so the back reference must have the length 0. If the back reference is
-    // inside a lookbehind, effectively making it a forward reference, we return
-    virtual int max_match() override {
-      return capture_->body() ? capture_->max_match() : 0;
-    }
+    virtual int min_match() { return 0; }
+    virtual int max_match() { return capture_->max_match(); }
     int index() { return capture_->index(); }
     RegExpCapture* capture() { return capture_; }
   private:
diff --git a/js/src/irregexp/RegExpBytecode.h b/js/src/irregexp/RegExpBytecode.h
index ea3f80b4f0..f31b78c593 100644
--- a/js/src/irregexp/RegExpBytecode.h
+++ b/js/src/irregexp/RegExpBytecode.h
@@ -82,19 +82,16 @@ V(CHECK_LT,          35, 8)   /* bc8 pad8 uc16 addr32                       */ \
 V(CHECK_GT,          36, 8)   /* bc8 pad8 uc16 addr32                       */ \
 V(CHECK_NOT_BACK_REF, 37, 8)  /* bc8 reg_idx24 addr32                       */ \
 V(CHECK_NOT_BACK_REF_NO_CASE, 38, 8) /* bc8 reg_idx24 addr32                */ \
-V(CHECK_NOT_BACK_REF_BACKWARD, 39, 8)         /* bc8 reg_idx24 addr32       */ \
-V(CHECK_NOT_BACK_REF_NO_CASE_BACKWARD, 40, 8) /* bc8 reg_idx24 addr32       */ \
-V(CHECK_NOT_REGS_EQUAL, 41, 12) /* bc8 regidx24 reg_idx32 addr32            */ \
-V(CHECK_REGISTER_LT, 42, 12)  /* bc8 reg_idx24 value32 addr32               */ \
-V(CHECK_REGISTER_GE, 43, 12)  /* bc8 reg_idx24 value32 addr32               */ \
-V(CHECK_REGISTER_EQ_POS, 44, 8) /* bc8 reg_idx24 addr32                     */ \
-V(CHECK_AT_START,    45, 8)   /* bc8 pad24 addr32                           */ \
-V(CHECK_NOT_AT_START, 46, 8)  /* bc8 pad24 addr32                           */ \
-V(CHECK_GREEDY,      47, 8)   /* bc8 pad24 addr32                           */ \
-V(ADVANCE_CP_AND_GOTO, 48, 8) /* bc8 offset24 addr32                        */ \
-V(SET_CURRENT_POSITION_FROM_END, 49, 4) /* bc8 idx24                        */ \
-V(CHECK_NOT_BACK_REF_NO_CASE_UNICODE, 50, 8) /* bc8 reg_idx24 addr32        */ \
-V(CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_UNICODE, 51, 8) /* bc8 reg_idx24 addr32 */
+V(CHECK_NOT_REGS_EQUAL, 39, 12) /* bc8 regidx24 reg_idx32 addr32            */ \
+V(CHECK_REGISTER_LT, 40, 12)  /* bc8 reg_idx24 value32 addr32               */ \
+V(CHECK_REGISTER_GE, 41, 12)  /* bc8 reg_idx24 value32 addr32               */ \
+V(CHECK_REGISTER_EQ_POS, 42, 8) /* bc8 reg_idx24 addr32                     */ \
+V(CHECK_AT_START,    43, 8)   /* bc8 pad24 addr32                           */ \
+V(CHECK_NOT_AT_START, 44, 8)  /* bc8 pad24 addr32                           */ \
+V(CHECK_GREEDY,      45, 8)   /* bc8 pad24 addr32                           */ \
+V(ADVANCE_CP_AND_GOTO, 46, 8) /* bc8 offset24 addr32                        */ \
+V(SET_CURRENT_POSITION_FROM_END, 47, 4) /* bc8 idx24                        */ \
+V(CHECK_NOT_BACK_REF_NO_CASE_UNICODE, 48, 8) /* bc8 reg_idx24 addr32        */
 
 #define DECLARE_BYTECODES(name, code, length) \
   static const int BC_##name = code;
diff --git a/js/src/irregexp/RegExpEngine.cpp b/js/src/irregexp/RegExpEngine.cpp
index 62f94c3e72..4d691a5dc9 100644
--- a/js/src/irregexp/RegExpEngine.cpp
+++ b/js/src/irregexp/RegExpEngine.cpp
@@ -721,8 +721,6 @@ ActionNode::EmptyMatchCheck(int start_register,
 int
 TextNode::EatsAtLeast(int still_to_find, int budget, bool not_at_start)
 {
-    if (read_backward())
-        return 0;
     int answer = Length();
     if (answer >= still_to_find)
         return answer;
@@ -738,7 +736,8 @@ TextNode::EatsAtLeast(int still_to_find, int budget, bool not_at_start)
 int
 TextNode::GreedyLoopTextLength()
 {
-    return Length();
+    TextElement elm = elements()[elements().length() - 1];
+    return elm.cp_offset() + elm.length();
 }
 
 RegExpNode*
@@ -888,8 +887,6 @@ AssertionNode::FillInBMInfo(int offset, int budget, BoyerMooreLookahead* bm, boo
 int
 BackReferenceNode::EatsAtLeast(int still_to_find, int budget, bool not_at_start)
 {
-    if (read_backward())
-        return 0;
     if (budget <= 0)
         return 0;
     return on_success()->EatsAtLeast(still_to_find, budget - 1, not_at_start);
@@ -1581,9 +1578,6 @@ class irregexp::RegExpCompiler
         current_expansion_factor_ = value;
     }
 
-    bool read_backward() { return read_backward_; }
-    void set_read_backward(bool value) { read_backward_ = value; }
-
     JSContext* cx() const { return cx_; }
     LifoAlloc* alloc() const { return alloc_; }
 
@@ -1601,7 +1595,6 @@ class irregexp::RegExpCompiler
     bool unicode_;
     bool reg_exp_too_big_;
     int current_expansion_factor_;
-    bool read_backward_;
     FrequencyCollator frequency_collator_;
     JSContext* cx_;
     LifoAlloc* alloc_;
@@ -1631,7 +1624,6 @@ RegExpCompiler::RegExpCompiler(JSContext* cx, LifoAlloc* alloc, int capture_coun
     unicode_(unicode),
     reg_exp_too_big_(false),
     current_expansion_factor_(1),
-    read_backward_(false),
     frequency_collator_(),
     cx_(cx),
     alloc_(alloc)
@@ -1755,7 +1747,7 @@ irregexp::CompilePattern(JSContext* cx, RegExpShared* shared, RegExpCompileData*
             // at the start of input.
             ChoiceNode* first_step_node = alloc.newInfallible<ChoiceNode>(&alloc, 2);
             RegExpNode* char_class =
-                alloc.newInfallible<TextNode>(alloc.newInfallible<RegExpCharacterClass>('*'), false, loop_node);
+                alloc.newInfallible<TextNode>(alloc.newInfallible<RegExpCharacterClass>('*'), loop_node);
             first_step_node->AddAlternative(GuardedAlternative(captured_body));
             first_step_node->AddAlternative(GuardedAlternative(char_class));
             node = first_step_node;
@@ -1858,19 +1850,19 @@ RegExpAtom::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
     TextElementVector* elms =
         compiler->alloc()->newInfallible<TextElementVector>(*compiler->alloc());
     elms->append(TextElement::Atom(this));
-    return compiler->alloc()->newInfallible<TextNode>(elms, compiler->read_backward(), on_success);
+    return compiler->alloc()->newInfallible<TextNode>(elms, on_success);
 }
 
 RegExpNode*
 RegExpText::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
 {
-    return compiler->alloc()->newInfallible<TextNode>(&elements_, compiler->read_backward(), on_success);
+    return compiler->alloc()->newInfallible<TextNode>(&elements_, on_success);
 }
 
 RegExpNode*
 RegExpCharacterClass::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
 {
-    return compiler->alloc()->newInfallible<TextNode>(this, compiler->read_backward(), on_success);
+    return compiler->alloc()->newInfallible<TextNode>(this, on_success);
 }
 
 RegExpNode*
@@ -2011,8 +2003,7 @@ RegExpQuantifier::ToNode(int min,
                         alternation->AddAlternative(GuardedAlternative(body->ToNode(compiler, answer)));
                     }
                     answer = alternation;
-                    if (not_at_start && !compiler->read_backward())
-                        alternation->set_not_at_start();
+                    if (not_at_start) alternation->set_not_at_start();
                 }
                 return answer;
             }
@@ -2024,9 +2015,8 @@ RegExpQuantifier::ToNode(int min,
     int reg_ctr = needs_counter
         ? compiler->AllocateRegister()
         : RegExpCompiler::kNoRegister;
-    LoopChoiceNode* center = alloc->newInfallible<LoopChoiceNode>(alloc, body->min_match() == 0,
-                                                                  compiler->read_backward());
-    if (not_at_start && !compiler->read_backward())
+    LoopChoiceNode* center = alloc->newInfallible<LoopChoiceNode>(alloc, body->min_match() == 0);
+    if (not_at_start)
         center->set_not_at_start();
     RegExpNode* loop_return = needs_counter
         ? static_cast<RegExpNode*>(ActionNode::IncrementRegister(reg_ctr, center))
@@ -2102,7 +2092,7 @@ RegExpAssertion::ToNode(RegExpCompiler* compiler,
         CharacterRange::AddClassEscape(alloc, 'n', newline_ranges);
         RegExpCharacterClass* newline_atom = alloc->newInfallible<RegExpCharacterClass>('n');
         TextNode* newline_matcher =
-            alloc->newInfallible<TextNode>(newline_atom, false,
+            alloc->newInfallible<TextNode>(newline_atom,
                 ActionNode::PositiveSubmatchSuccess(stack_pointer_register,
                                                     position_register,
                                                     0,  // No captures inside.
@@ -2134,7 +2124,6 @@ RegExpBackReference::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
 {
     return compiler->alloc()->newInfallible<BackReferenceNode>(RegExpCapture::StartRegister(index()),
                                                                RegExpCapture::EndRegister(index()),
-                                                               compiler->read_backward(),
                                                                on_success);
 }
 
@@ -2145,7 +2134,7 @@ RegExpEmpty::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
 }
 
 RegExpNode*
-RegExpLookaround::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
+RegExpLookahead::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
 {
     int stack_pointer_register = compiler->AllocateRegister();
     int position_register = compiler->AllocateRegister();
@@ -2156,10 +2145,6 @@ RegExpLookaround::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
     int register_start =
         register_of_first_capture + capture_from_ * registers_per_capture;
 
-    RegExpNode* result;
-    bool was_reading_backward = compiler->read_backward();
-    compiler->set_read_backward(type() == LOOKBEHIND);
-
     if (is_positive()) {
         RegExpNode* bodyNode =
             body()->ToNode(compiler,
@@ -2168,39 +2153,37 @@ RegExpLookaround::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
                                                                register_count,
                                                                register_start,
                                                                on_success));
-        result = ActionNode::BeginSubmatch(stack_pointer_register,
-                                           position_register,
-                                           bodyNode);
-    } else {
-        // We use a ChoiceNode for a negative lookahead because it has most of
-        // the characteristics we need. It has the body of the lookahead as its
-        // first alternative and the expression after the lookahead of the second
-        // alternative. If the first alternative succeeds then the
-        // NegativeSubmatchSuccess will unwind the stack including everything the
-        // choice node set up and backtrack. If the first alternative fails then
-        // the second alternative is tried, which is exactly the desired result
-        // for a negative lookahead. The NegativeLookaheadChoiceNode is a special
-        // ChoiceNode that knows to ignore the first exit when calculating quick
-        // checks.
-        LifoAlloc* alloc = compiler->alloc();
-
-        RegExpNode* success =
-            alloc->newInfallible<NegativeSubmatchSuccess>(alloc,
-                                                          stack_pointer_register,
-                                                          position_register,
-                                                          register_count,
-                                                          register_start);
-        GuardedAlternative body_alt(body()->ToNode(compiler, success));
-
-        ChoiceNode* choice_node =
-            alloc->newInfallible<NegativeLookaheadChoiceNode>(alloc, body_alt, GuardedAlternative(on_success));
-
-        result = ActionNode::BeginSubmatch(stack_pointer_register,
+        return ActionNode::BeginSubmatch(stack_pointer_register,
                                          position_register,
-                                         choice_node);
-    }
-    compiler->set_read_backward(was_reading_backward);
-    return result;
+                                         bodyNode);
+    }
+
+    // We use a ChoiceNode for a negative lookahead because it has most of
+    // the characteristics we need.  It has the body of the lookahead as its
+    // first alternative and the expression after the lookahead of the second
+    // alternative.  If the first alternative succeeds then the
+    // NegativeSubmatchSuccess will unwind the stack including everything the
+    // choice node set up and backtrack.  If the first alternative fails then
+    // the second alternative is tried, which is exactly the desired result
+    // for a negative lookahead.  The NegativeLookaheadChoiceNode is a special
+    // ChoiceNode that knows to ignore the first exit when calculating quick
+    // checks.
+    LifoAlloc* alloc = compiler->alloc();
+
+    RegExpNode* success =
+        alloc->newInfallible<NegativeSubmatchSuccess>(alloc,
+                                                      stack_pointer_register,
+                                                      position_register,
+                                                      register_count,
+                                                      register_start);
+    GuardedAlternative body_alt(body()->ToNode(compiler, success));
+
+    ChoiceNode* choice_node =
+        alloc->newInfallible<NegativeLookaheadChoiceNode>(alloc, body_alt, GuardedAlternative(on_success));
+
+    return ActionNode::BeginSubmatch(stack_pointer_register,
+                                     position_register,
+                                     choice_node);
 }
 
 RegExpNode*
@@ -2215,14 +2198,8 @@ RegExpCapture::ToNode(RegExpTree* body,
                       RegExpCompiler* compiler,
                       RegExpNode* on_success)
 {
-    MOZ_ASSERT(body);
     int start_reg = RegExpCapture::StartRegister(index);
     int end_reg = RegExpCapture::EndRegister(index);
-    if (compiler->read_backward()) {
-        // std::swap(start_reg, end_reg);
-        start_reg = RegExpCapture::EndRegister(index);
-        end_reg = RegExpCapture::StartRegister(index);
-    }
     RegExpNode* store_end = ActionNode::StorePosition(end_reg, true, on_success);
     RegExpNode* body_node = body->ToNode(compiler, store_end);
     return ActionNode::StorePosition(start_reg, true, body_node);
@@ -2233,15 +2210,8 @@ RegExpAlternative::ToNode(RegExpCompiler* compiler, RegExpNode* on_success)
 {
     const RegExpTreeVector& children = nodes();
     RegExpNode* current = on_success;
-    if (compiler->read_backward()) {
-        for (int i = 0; i < children.length(); i++) {
-            current = children[i]->ToNode(compiler, current);
-        }
-    } else {
-        for (int i = children.length() - 1; i >= 0; i--) {
-            current = children[i]->ToNode(compiler, current);
-        }
-    }
+    for (int i = children.length() - 1; i >= 0; i--)
+        current = children[i]->ToNode(compiler, current);
     return current;
 }
 
@@ -2794,6 +2764,7 @@ Trace::InvalidateCurrentCharacter()
 void
 Trace::AdvanceCurrentPositionInTrace(int by, RegExpCompiler* compiler)
 {
+    MOZ_ASSERT(by > 0);
     // We don't have an instruction for shifting the current character register
     // down or for using a shifted value for anything so lets just forget that
     // we preloaded any characters into it.
@@ -3138,9 +3109,9 @@ AssertionNode::Emit(RegExpCompiler* compiler, Trace* trace)
             return;
         }
         if (trace->at_start() == Trace::UNKNOWN) {
-            assembler->CheckNotAtStart(trace->cp_offset(), trace->backtrack());
+            assembler->CheckNotAtStart(trace->backtrack());
             Trace at_start_trace = *trace;
-            at_start_trace.set_at_start(Trace::TRUE_VALUE);
+            at_start_trace.set_at_start(true);
             on_success()->Emit(compiler, &at_start_trace);
             return;
         }
@@ -3843,10 +3814,9 @@ TextNode::TextEmitPass(RegExpCompiler* compiler,
     jit::Label* backtrack = trace->backtrack();
     QuickCheckDetails* quick_check = trace->quick_check_performed();
     int element_count = elements().length();
-    int backward_offset = read_backward() ? -Length() : 0;
     for (int i = preloaded ? 0 : element_count - 1; i >= 0; i--) {
         TextElement elm = elements()[i];
-        int cp_offset = trace->cp_offset() + elm.cp_offset() + backward_offset;
+        int cp_offset = trace->cp_offset() + elm.cp_offset();
         if (elm.text_type() == TextElement::ATOM) {
             const CharacterVector& quarks = elm.atom()->data();
             for (int j = preloaded ? 0 : quarks.length() - 1; j >= 0; j--) {
@@ -3874,12 +3844,11 @@ TextNode::TextEmitPass(RegExpCompiler* compiler,
                     break;
                 }
                 if (emit_function != nullptr) {
-                    bool bounds_check = *checked_up_to < cp_offset + j || read_backward();
                     bool bound_checked = emit_function(compiler,
                                                        quarks[j],
                                                        backtrack,
                                                        cp_offset + j,
-                                                       bounds_check,
+                                                       *checked_up_to < cp_offset + j,
                                                        preloaded);
                     if (bound_checked) UpdateBoundsCheck(cp_offset + j, checked_up_to);
                 }
@@ -3890,14 +3859,13 @@ TextNode::TextEmitPass(RegExpCompiler* compiler,
                 if (first_element_checked && i == 0) continue;
                 if (DeterminedAlready(quick_check, elm.cp_offset())) continue;
                 RegExpCharacterClass* cc = elm.char_class();
-                bool bounds_check = *checked_up_to < cp_offset || read_backward();
                 EmitCharClass(alloc(),
                               assembler,
                               cc,
                               ascii,
                               backtrack,
                               cp_offset,
-                              bounds_check,
+                              *checked_up_to < cp_offset,
                               preloaded);
                 UpdateBoundsCheck(cp_offset, checked_up_to);
             }
@@ -3977,11 +3945,8 @@ TextNode::Emit(RegExpCompiler* compiler, Trace* trace)
     }
 
     Trace successor_trace(*trace);
-    // If we advance backward, we may end up at the start.
-    successor_trace.AdvanceCurrentPositionInTrace(
-        read_backward() ? -Length() : Length(), compiler);
-    successor_trace.set_at_start(read_backward() ? Trace::UNKNOWN
-                                                 : Trace::FALSE_VALUE);
+    successor_trace.set_at_start(false);
+    successor_trace.AdvanceCurrentPositionInTrace(Length(), compiler);
     RecursionCheck rc(compiler);
     on_success()->Emit(compiler, &successor_trace);
 }
@@ -4153,8 +4118,6 @@ ChoiceNode::CalculatePreloadCharacters(RegExpCompiler* compiler, int eats_at_lea
 RegExpNode*
 TextNode::GetSuccessorOfOmnivorousTextNode(RegExpCompiler* compiler)
 {
-    if (read_backward()) return NULL;
-
     if (elements().length() != 1)
         return nullptr;
 
@@ -4202,7 +4165,7 @@ ChoiceNode::GreedyLoopTextLengthForAlternative(GuardedAlternative* alternative)
         SeqRegExpNode* seq_node = static_cast<SeqRegExpNode*>(node);
         node = seq_node->on_success();
     }
-    return read_backward() ? -length : length;
+    return length;
 }
 
 // Creates a list of AlternativeGenerations.  If the list has a reasonable
@@ -4277,7 +4240,7 @@ ChoiceNode::Emit(RegExpCompiler* compiler, Trace* trace)
     jit::Label greedy_loop_label;
     Trace counter_backtrack_trace;
     counter_backtrack_trace.set_backtrack(&greedy_loop_label);
-    if (not_at_start()) counter_backtrack_trace.set_at_start(Trace::FALSE_VALUE);
+    if (not_at_start()) counter_backtrack_trace.set_at_start(false);
 
     if (choice_count > 1 && text_length != kNodeIsTooComplexForGreedyLoops) {
         // Here we have special handling for greedy loops containing only text nodes
@@ -4293,7 +4256,7 @@ ChoiceNode::Emit(RegExpCompiler* compiler, Trace* trace)
         current_trace = &counter_backtrack_trace;
         jit::Label greedy_match_failed;
         Trace greedy_match_trace;
-        if (not_at_start()) greedy_match_trace.set_at_start(Trace::FALSE_VALUE);
+        if (not_at_start()) greedy_match_trace.set_at_start(false);
         greedy_match_trace.set_backtrack(&greedy_match_failed);
         jit::Label loop_label;
         macro_assembler->Bind(&loop_label);
@@ -4642,14 +4605,11 @@ BackReferenceNode::Emit(RegExpCompiler* compiler, Trace* trace)
     MOZ_ASSERT(start_reg_ + 1 == end_reg_);
     if (compiler->ignore_case()) {
         assembler->CheckNotBackReferenceIgnoreCase(start_reg_,
-                                                   read_backward(),
                                                    trace->backtrack(),
                                                    compiler->unicode());
     } else {
-        assembler->CheckNotBackReference(start_reg_, read_backward(), trace->backtrack());
+        assembler->CheckNotBackReference(start_reg_, trace->backtrack());
     }
-    // We are going to advance backward, so we may end up at the start.
-    if (read_backward()) trace->set_at_start(Trace::UNKNOWN);
     on_success()->Emit(compiler, trace);
 }
 
@@ -5017,6 +4977,7 @@ QuickCheckDetails::Clear()
 void
 QuickCheckDetails::Advance(int by, bool ascii)
 {
+    MOZ_ASSERT(by >= 0);
     if (by >= characters_) {
         Clear();
         return;
diff --git a/js/src/irregexp/RegExpEngine.h b/js/src/irregexp/RegExpEngine.h
index c4409dcca0..1a8fd4b220 100644
--- a/js/src/irregexp/RegExpEngine.h
+++ b/js/src/irregexp/RegExpEngine.h
@@ -119,7 +119,7 @@ InterpretCode(JSContext* cx, const uint8_t* byteCode, const CharT* chars, size_t
   VISIT(Atom)                                                        \
   VISIT(Quantifier)                                                  \
   VISIT(Capture)                                                     \
-  VISIT(Lookaround)                                                  \
+  VISIT(Lookahead)                                                   \
   VISIT(BackReference)                                               \
   VISIT(Empty)                                                       \
   VISIT(Text)
@@ -763,19 +763,15 @@ class TextNode : public SeqRegExpNode
 {
   public:
     TextNode(TextElementVector* elements,
-             bool read_backward,
              RegExpNode* on_success)
       : SeqRegExpNode(on_success),
-        elements_(elements),
-        read_backward_(read_backward)
+        elements_(elements)
     {}
 
     TextNode(RegExpCharacterClass* that,
-             bool read_backward,
              RegExpNode* on_success)
       : SeqRegExpNode(on_success),
-        elements_(alloc()->newInfallible<TextElementVector>(*alloc())),
-        read_backward_(read_backward)
+        elements_(alloc()->newInfallible<TextElementVector>(*alloc()))
     {
         elements_->append(TextElement::CharClass(that));
     }
@@ -788,7 +784,6 @@ class TextNode : public SeqRegExpNode
                                       int characters_filled_in,
                                       bool not_at_start);
     TextElementVector& elements() { return *elements_; }
-    bool read_backward() { return read_backward_; }
     void MakeCaseIndependent(bool is_ascii, bool unicode);
     virtual int GreedyLoopTextLength();
     virtual RegExpNode* GetSuccessorOfOmnivorousTextNode(
@@ -819,7 +814,6 @@ class TextNode : public SeqRegExpNode
                       int* checked_up_to);
     int Length();
     TextElementVector* elements_;
-    bool read_backward_;
 };
 
 class AssertionNode : public SeqRegExpNode
@@ -888,18 +882,15 @@ class BackReferenceNode : public SeqRegExpNode
   public:
     BackReferenceNode(int start_reg,
                       int end_reg,
-                      bool read_backward,
                       RegExpNode* on_success)
       : SeqRegExpNode(on_success),
         start_reg_(start_reg),
-        end_reg_(end_reg),
-        read_backward_(read_backward)
+        end_reg_(end_reg)
     {}
 
     virtual void Accept(NodeVisitor* visitor);
     int start_register() { return start_reg_; }
     int end_register() { return end_reg_; }
-    bool read_backward() { return read_backward_; }
     virtual void Emit(RegExpCompiler* compiler, Trace* trace);
     virtual int EatsAtLeast(int still_to_find,
                             int recursion_depth,
@@ -918,7 +909,6 @@ class BackReferenceNode : public SeqRegExpNode
   private:
     int start_reg_;
     int end_reg_;
-    bool read_backward_;
 };
 
 class EndNode : public RegExpNode
@@ -1063,7 +1053,6 @@ class ChoiceNode : public RegExpNode
     void set_being_calculated(bool b) { being_calculated_ = b; }
     virtual bool try_to_emit_quick_check_for_alternative(int i) { return true; }
     virtual RegExpNode* FilterASCII(int depth, bool ignore_case, bool unicode);
-    virtual bool read_backward() { return false; }
 
   protected:
     int GreedyLoopTextLengthForAlternative(GuardedAlternative* alternative);
@@ -1122,13 +1111,11 @@ class NegativeLookaheadChoiceNode : public ChoiceNode
 class LoopChoiceNode : public ChoiceNode
 {
   public:
-    explicit LoopChoiceNode(LifoAlloc* alloc, bool body_can_be_zero_length,
-                            bool read_backward)
+    explicit LoopChoiceNode(LifoAlloc* alloc, bool body_can_be_zero_length)
       : ChoiceNode(alloc, 2),
         loop_node_(nullptr),
         continue_node_(nullptr),
-        body_can_be_zero_length_(body_can_be_zero_length),
-        read_backward_(read_backward)
+        body_can_be_zero_length_(body_can_be_zero_length)
     {}
 
     void AddLoopAlternative(GuardedAlternative alt);
@@ -1146,7 +1133,6 @@ class LoopChoiceNode : public ChoiceNode
     RegExpNode* loop_node() { return loop_node_; }
     RegExpNode* continue_node() { return continue_node_; }
     bool body_can_be_zero_length() { return body_can_be_zero_length_; }
-    virtual bool read_backward() { return read_backward_; }
     virtual void Accept(NodeVisitor* visitor);
     virtual RegExpNode* FilterASCII(int depth, bool ignore_case, bool unicode);
 
@@ -1161,7 +1147,6 @@ class LoopChoiceNode : public ChoiceNode
     RegExpNode* loop_node_;
     RegExpNode* continue_node_;
     bool body_can_be_zero_length_;
-    bool read_backward_;
 };
 
 // Improve the speed that we scan for an initial point where a non-anchored
@@ -1437,8 +1422,8 @@ class Trace
     }
 
     TriBool at_start() { return at_start_; }
-    void set_at_start(TriBool at_start) {
-        at_start_ = at_start;
+    void set_at_start(bool at_start) {
+        at_start_ = at_start ? TRUE_VALUE : FALSE_VALUE;
     }
     jit::Label* backtrack() { return backtrack_; }
     jit::Label* loop_label() { return loop_label_; }
diff --git a/js/src/irregexp/RegExpInterpreter.cpp b/js/src/irregexp/RegExpInterpreter.cpp
index d09b4671e4..7fd2d983a5 100644
--- a/js/src/irregexp/RegExpInterpreter.cpp
+++ b/js/src/irregexp/RegExpInterpreter.cpp
@@ -222,8 +222,8 @@ irregexp::InterpretCode(JSContext* cx, const uint8_t* byteCode, const CharT* cha
             }
             break;
           BYTECODE(LOAD_CURRENT_CHAR) {
-            int pos = current + (insn >> BYTECODE_SHIFT);
-            if (pos >= (int)length || pos < 0) {
+            size_t pos = current + (insn >> BYTECODE_SHIFT);
+            if (pos >= length) {
                 pc = byteCode + Load32Aligned(pc + 4);
             } else {
                 current_char = chars[pos];
@@ -238,8 +238,8 @@ irregexp::InterpretCode(JSContext* cx, const uint8_t* byteCode, const CharT* cha
             break;
           }
           BYTECODE(LOAD_2_CURRENT_CHARS) {
-            int pos = current + (insn >> BYTECODE_SHIFT);
-            if (pos + 2 > (int)length || pos < 0) {
+            size_t pos = current + (insn >> BYTECODE_SHIFT);
+            if (pos + 2 > length) {
                 pc = byteCode + Load32Aligned(pc + 4);
             } else {
                 CharT next = chars[pos + 1];
@@ -425,30 +425,6 @@ irregexp::InterpretCode(JSContext* cx, const uint8_t* byteCode, const CharT* cha
             pc += BC_CHECK_NOT_BACK_REF_LENGTH;
             break;
           }
-          BYTECODE(CHECK_NOT_BACK_REF_BACKWARD) {
-            int from = registers[insn >> BYTECODE_SHIFT];
-            int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
-            if (from < 0 || len <= 0) {
-                pc += BC_CHECK_NOT_BACK_REF_BACKWARD_LENGTH;
-                break;
-            }
-            if (int(current) - len < 0) {
-                pc = byteCode + Load32Aligned(pc + 4);
-                break;
-            } else {
-                int i;
-                for (i = 0; i < len; i++) {
-                    if (chars[from + i] != chars[int(current) - len + i]) {
-                        pc = byteCode + Load32Aligned(pc + 4);
-                        break;
-                    }
-                }
-                if (i < len) break;
-                current -= len;
-            }
-            pc += BC_CHECK_NOT_BACK_REF_BACKWARD_LENGTH;
-            break;
-          }
           BYTECODE(CHECK_NOT_BACK_REF_NO_CASE) {
             int from = registers[insn >> BYTECODE_SHIFT];
             int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
@@ -489,46 +465,6 @@ irregexp::InterpretCode(JSContext* cx, const uint8_t* byteCode, const CharT* cha
             }
             break;
           }
-          BYTECODE(CHECK_NOT_BACK_REF_NO_CASE_BACKWARD) {
-            int from = registers[insn >> BYTECODE_SHIFT];
-            int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
-            if (from < 0 || len <= 0) {
-                pc += BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_LENGTH;
-                break;
-            }
-            if (int(current) - len < 0) {
-                pc = byteCode + Load32Aligned(pc + 4);
-                break;
-            }
-            if (CaseInsensitiveCompareStrings(chars + from, chars + int(current) - len, len * sizeof(CharT))) {
-                current -= len;
-                pc += BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_LENGTH;
-            } else {
-                pc = byteCode + Load32Aligned(pc + 4);
-            }
-            break;
-
-          }
-          BYTECODE(CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_UNICODE) {
-            int from = registers[insn >> BYTECODE_SHIFT];
-            int len = registers[(insn >> BYTECODE_SHIFT) + 1] - from;
-            if (from < 0 || len <= 0) {
-                pc += BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_LENGTH;
-                break;
-            }
-            if (int(current) - len < 0) {
-                pc = byteCode + Load32Aligned(pc + 4);
-                break;
-            }
-            if (CaseInsensitiveCompareUCStrings(chars + from, chars + int(current) - len, len * sizeof(CharT))) {
-                current -= len;
-                pc += BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_LENGTH;
-            } else {
-                pc = byteCode + Load32Aligned(pc + 4);
-            }
-            break;
-
-          }
           BYTECODE(CHECK_AT_START)
             if (current == 0)
                 pc = byteCode + Load32Aligned(pc + 4);
@@ -536,7 +472,7 @@ irregexp::InterpretCode(JSContext* cx, const uint8_t* byteCode, const CharT* cha
                 pc += BC_CHECK_AT_START_LENGTH;
             break;
           BYTECODE(CHECK_NOT_AT_START)
-            if (current + (insn >> BYTECODE_SHIFT) == 0)
+            if (current == 0)
                 pc += BC_CHECK_NOT_AT_START_LENGTH;
             else
                 pc = byteCode + Load32Aligned(pc + 4);
diff --git a/js/src/irregexp/RegExpMacroAssembler.cpp b/js/src/irregexp/RegExpMacroAssembler.cpp
index 6b1ceba8ad..d66d0d204a 100644
--- a/js/src/irregexp/RegExpMacroAssembler.cpp
+++ b/js/src/irregexp/RegExpMacroAssembler.cpp
@@ -226,37 +226,32 @@ InterpretedRegExpMacroAssembler::CheckGreedyLoop(jit::Label* on_tos_equals_curre
 }
 
 void
-InterpretedRegExpMacroAssembler::CheckNotAtStart(int cp_offset, jit::Label* on_not_at_start)
+InterpretedRegExpMacroAssembler::CheckNotAtStart(jit::Label* on_not_at_start)
 {
-    Emit(BC_CHECK_NOT_AT_START, cp_offset);
+    Emit(BC_CHECK_NOT_AT_START, 0);
     EmitOrLink(on_not_at_start);
 }
 
 void
-InterpretedRegExpMacroAssembler::CheckNotBackReference(int start_reg, bool read_backward,
-                                                       jit::Label* on_no_match)
+InterpretedRegExpMacroAssembler::CheckNotBackReference(int start_reg, jit::Label* on_no_match)
 {
     MOZ_ASSERT(start_reg >= 0);
     MOZ_ASSERT(start_reg <= kMaxRegister);
-    Emit(read_backward ? BC_CHECK_NOT_BACK_REF_BACKWARD : BC_CHECK_NOT_BACK_REF,
-         start_reg);
+    Emit(BC_CHECK_NOT_BACK_REF, start_reg);
     EmitOrLink(on_no_match);
 }
 
 void
 InterpretedRegExpMacroAssembler::CheckNotBackReferenceIgnoreCase(int start_reg,
-                                                                 bool read_backward,
                                                                  jit::Label* on_no_match,
                                                                  bool unicode)
 {
     MOZ_ASSERT(start_reg >= 0);
     MOZ_ASSERT(start_reg <= kMaxRegister);
     if (unicode)
-        Emit(read_backward ? BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD_UNICODE : BC_CHECK_NOT_BACK_REF_NO_CASE_UNICODE,
-             start_reg);
+        Emit(BC_CHECK_NOT_BACK_REF_NO_CASE_UNICODE, start_reg);
     else
-        Emit(read_backward ? BC_CHECK_NOT_BACK_REF_NO_CASE_BACKWARD : BC_CHECK_NOT_BACK_REF_NO_CASE,
-             start_reg);
+        Emit(BC_CHECK_NOT_BACK_REF_NO_CASE, start_reg);
     EmitOrLink(on_no_match);
 }
 
diff --git a/js/src/irregexp/RegExpMacroAssembler.h b/js/src/irregexp/RegExpMacroAssembler.h
index c5def92f29..dca2edf905 100644
--- a/js/src/irregexp/RegExpMacroAssembler.h
+++ b/js/src/irregexp/RegExpMacroAssembler.h
@@ -110,10 +110,10 @@ class MOZ_STACK_CLASS RegExpMacroAssembler
     virtual void CheckCharacterGT(char16_t limit, jit::Label* on_greater) = 0;
     virtual void CheckCharacterLT(char16_t limit, jit::Label* on_less) = 0;
     virtual void CheckGreedyLoop(jit::Label* on_tos_equals_current_position) = 0;
-    virtual void CheckNotAtStart(int cp_offset, jit::Label* on_not_at_start) = 0;
-    virtual void CheckNotBackReference(int start_reg, bool read_backward, jit::Label* on_no_match) = 0;
-    virtual void CheckNotBackReferenceIgnoreCase(int start_reg, bool read_backward,
-                                                 jit::Label* on_no_match, bool unicode) = 0;
+    virtual void CheckNotAtStart(jit::Label* on_not_at_start) = 0;
+    virtual void CheckNotBackReference(int start_reg, jit::Label* on_no_match) = 0;
+    virtual void CheckNotBackReferenceIgnoreCase(int start_reg, jit::Label* on_no_match,
+                                                 bool unicode) = 0;
 
     // Check the current character for a match with a literal character.  If we
     // fail to match then goto the on_failure label.  End of input always
@@ -245,10 +245,9 @@ class MOZ_STACK_CLASS InterpretedRegExpMacroAssembler final : public RegExpMacro
     void CheckCharacterGT(char16_t limit, jit::Label* on_greater);
     void CheckCharacterLT(char16_t limit, jit::Label* on_less);
     void CheckGreedyLoop(jit::Label* on_tos_equals_current_position);
-    void CheckNotAtStart(int cp_offset, jit::Label* on_not_at_start);
-    void CheckNotBackReference(int start_reg, bool read_backward, jit::Label* on_no_match);
-    void CheckNotBackReferenceIgnoreCase(int start_reg, bool read_backward,
-                                         jit::Label* on_no_match, bool unicode);
+    void CheckNotAtStart(jit::Label* on_not_at_start);
+    void CheckNotBackReference(int start_reg, jit::Label* on_no_match);
+    void CheckNotBackReferenceIgnoreCase(int start_reg, jit::Label* on_no_match, bool unicode);
     void CheckNotCharacter(unsigned c, jit::Label* on_not_equal);
     void CheckNotCharacterAfterAnd(unsigned c, unsigned and_with, jit::Label* on_not_equal);
     void CheckNotCharacterAfterMinusAnd(char16_t c, char16_t minus, char16_t and_with,
diff --git a/js/src/irregexp/RegExpParser.cpp b/js/src/irregexp/RegExpParser.cpp
index 1ad044e8e2..d4308d0d84 100644
--- a/js/src/irregexp/RegExpParser.cpp
+++ b/js/src/irregexp/RegExpParser.cpp
@@ -227,7 +227,6 @@ RegExpParser<CharT>::RegExpParser(frontend::TokenStream& ts, LifoAlloc* alloc,
     alloc(alloc),
     captures_(nullptr),
     next_pos_(chars),
-    captures_started_(0),
     end_(end),
     current_(kEndMarker),
     capture_count_(0),
@@ -420,8 +419,7 @@ RangeAtom(LifoAlloc* alloc, char16_t from, char16_t to)
 static inline RegExpTree*
 NegativeLookahead(LifoAlloc* alloc, char16_t from, char16_t to)
 {
-    return alloc->newInfallible<RegExpLookaround>(RangeAtom(alloc, from, to), false,
-                                                  0, 0, RegExpLookaround::LOOKAHEAD);
+    return alloc->newInfallible<RegExpLookahead>(RangeAtom(alloc, from, to), false, 0, 0);
 }
 
 static bool
@@ -1216,38 +1214,6 @@ RegExpParser<CharT>::ParseBackReferenceIndex(int* index_out)
     return true;
 }
 
-template <typename CharT>
-RegExpCapture*
-RegExpParser<CharT>::GetCapture(int index) {
-  // The index for the capture groups are one-based. Its index in the list is
-  // zero-based.
-  int known_captures =
-      is_scanned_for_captures_ ? capture_count_ : captures_started_;
-  MOZ_ASSERT(index <= known_captures);
-  if (captures_ == NULL) {
-    captures_ = alloc->newInfallible<RegExpCaptureVector>(*alloc);
-  }
-  while ((int)captures_->length() < known_captures) {
-    RegExpCapture* capture = alloc->newInfallible<RegExpCapture>(nullptr, captures_->length() + 1);
-    captures_->append(capture);
-  }
-  return (*captures_)[index - 1];
-}
-
-
-template <typename CharT>
-bool
-RegExpParser<CharT>::RegExpParserState::IsInsideCaptureGroup(int index) {
-  for (RegExpParserState* s = this; s != NULL; s = s->previous_state()) {
-    if (s->group_type() != CAPTURE) continue;
-    // Return true if we found the matching capture index.
-    if (index == s->capture_index()) return true;
-    // Abort if index is larger than what has been parsed up till this state.
-    if (index > s->capture_index()) return false;
-  }
-  return false;
-}
-
 // QuantifierPrefix ::
 //   { DecimalDigits }
 //   { DecimalDigits , }
@@ -1490,24 +1456,24 @@ RegExpTree*
 RegExpParser<CharT>::ParseDisjunction()
 {
     // Used to store current state while parsing subexpressions.
-    RegExpParserState initial_state(alloc, nullptr, INITIAL, RegExpLookaround::LOOKAHEAD, 0);
-    RegExpParserState* state = &initial_state;
+    RegExpParserState initial_state(alloc, nullptr, INITIAL, 0);
+    RegExpParserState* stored_state = &initial_state;
     // Cache the builder in a local variable for quick access.
     RegExpBuilder* builder = initial_state.builder();
     while (true) {
         switch (current()) {
           case kEndMarker:
-            if (state->IsSubexpression()) {
+            if (stored_state->IsSubexpression()) {
                 // Inside a parenthesized group when hitting end of input.
                 return ReportError(JSMSG_MISSING_PAREN);
             }
-            MOZ_ASSERT(INITIAL == state->group_type());
+            MOZ_ASSERT(INITIAL == stored_state->group_type());
             // Parsing completed successfully.
             return builder->ToRegExp();
           case ')': {
-            if (!state->IsSubexpression())
+            if (!stored_state->IsSubexpression())
                 return ReportError(JSMSG_UNMATCHED_RIGHT_PAREN);
-            MOZ_ASSERT(INITIAL != state->group_type());
+            MOZ_ASSERT(INITIAL != stored_state->group_type());
 
             Advance();
             // End disjunction parsing and convert builder content to new single
@@ -1516,30 +1482,29 @@ RegExpParser<CharT>::ParseDisjunction()
 
             int end_capture_index = captures_started();
 
-            int capture_index = state->capture_index();
-            SubexpressionType group_type = state->group_type();
+            int capture_index = stored_state->capture_index();
+            SubexpressionType group_type = stored_state->group_type();
+
+            // Restore previous state.
+            stored_state = stored_state->previous_state();
+            builder = stored_state->builder();
 
             // Build result of subexpression.
             if (group_type == CAPTURE) {
-                RegExpCapture* capture = GetCapture(capture_index);
-                capture->set_body(body);
+                RegExpCapture* capture = alloc->newInfallible<RegExpCapture>(body, capture_index);
+                (*captures_)[capture_index - 1] = capture;
                 body = capture;
             } else if (group_type != GROUPING) {
-                MOZ_ASSERT(group_type == POSITIVE_LOOKAROUND ||
-                           group_type == NEGATIVE_LOOKAROUND);
-                bool is_positive = (group_type == POSITIVE_LOOKAROUND);
-                body = alloc->newInfallible<RegExpLookaround>(body,
+                MOZ_ASSERT(group_type == POSITIVE_LOOKAHEAD ||
+                           group_type == NEGATIVE_LOOKAHEAD);
+                bool is_positive = (group_type == POSITIVE_LOOKAHEAD);
+                body = alloc->newInfallible<RegExpLookahead>(body,
                                                    is_positive,
                                                    end_capture_index - capture_index,
-                                                   capture_index,
-                                                   state->lookaround_type());
+                                                   capture_index);
             }
-
-            // Restore previous state.
-            state = state->previous_state();
-            builder = state->builder();
             builder->AddAtom(body);
-            if (unicode_ && (group_type == POSITIVE_LOOKAROUND || group_type == NEGATIVE_LOOKAROUND))
+            if (unicode_ && (group_type == POSITIVE_LOOKAHEAD || group_type == NEGATIVE_LOOKAHEAD))
                 continue;
             // For compatability with JSC and ES3, we allow quantifiers after
             // lookaheads, and break in all cases.
@@ -1599,7 +1564,6 @@ RegExpParser<CharT>::ParseDisjunction()
           }
           case '(': {
             SubexpressionType subexpr_type = CAPTURE;
-            RegExpLookaround::Type lookaround_type = state->lookaround_type();
             Advance();
             if (current() == '?') {
                 switch (Next()) {
@@ -1607,39 +1571,26 @@ RegExpParser<CharT>::ParseDisjunction()
                     subexpr_type = GROUPING;
                     break;
                   case '=':
-                    lookaround_type = RegExpLookaround::LOOKAHEAD;
-                    subexpr_type = POSITIVE_LOOKAROUND;
+                    subexpr_type = POSITIVE_LOOKAHEAD;
                     break;
                   case '!':
-                    lookaround_type = RegExpLookaround::LOOKAHEAD;
-                    subexpr_type = NEGATIVE_LOOKAROUND;
+                    subexpr_type = NEGATIVE_LOOKAHEAD;
                     break;
-                  case '<':
-                    Advance();
-                    lookaround_type = RegExpLookaround::LOOKBEHIND;
-                    if (Next() == '=') {
-                      subexpr_type = POSITIVE_LOOKAROUND;
-                      break;
-                    } else if (Next() == '!') {
-                      subexpr_type = NEGATIVE_LOOKAROUND;
-                      break;
-                    }
-                    // We didn't get a positive or negative after '<'.
-                    // That's an error.
-                    return ReportError(JSMSG_INVALID_GROUP);
                   default:
                     return ReportError(JSMSG_INVALID_GROUP);
                 }
                 Advance(2);
             } else {
+                if (captures_ == nullptr)
+                    captures_ = alloc->newInfallible<RegExpCaptureVector>(*alloc);
                 if (captures_started() >= kMaxCaptures)
                     return ReportError(JSMSG_TOO_MANY_PARENS);
-                captures_started_++;
+                captures_->append((RegExpCapture*) nullptr);
             }
             // Store current state and begin new disjunction parsing.
-            state = alloc->newInfallible<RegExpParserState>(alloc, state, subexpr_type,
-                                                            lookaround_type, captures_started_);
-            builder = state->builder();
+            stored_state = alloc->newInfallible<RegExpParserState>(alloc, stored_state, subexpr_type,
+                                                                   captures_started());
+            builder = stored_state->builder();
             continue;
           }
           case '[': {
@@ -1694,18 +1645,19 @@ RegExpParser<CharT>::ParseDisjunction()
               case '7': case '8': case '9': {
                 int index = 0;
                 if (ParseBackReferenceIndex(&index)) {
-                    if (state->IsInsideCaptureGroup(index)) {
-                      // The backreference is inside the capture group it refers to.
-                      // Nothing can possibly have been captured yet.
-                      builder->AddEmpty();
-                    } else {
-                      RegExpCapture* capture = GetCapture(index);
-                      RegExpTree* atom = alloc->newInfallible<RegExpBackReference>(capture);
-                      if (unicode_)
-                          builder->AddAtom(UnicodeBackReferenceAtom(alloc, atom));
-                      else
-                          builder->AddAtom(atom);
+                    RegExpCapture* capture = nullptr;
+                    if (captures_ != nullptr && index <= (int) captures_->length()) {
+                        capture = (*captures_)[index - 1];
+                    }
+                    if (capture == nullptr) {
+                        builder->AddEmpty();
+                        break;
                     }
+                    RegExpTree* atom = alloc->newInfallible<RegExpBackReference>(capture);
+                    if (unicode_)
+                        builder->AddAtom(UnicodeBackReferenceAtom(alloc, atom));
+                    else
+                        builder->AddAtom(atom);
                     break;
                 }
                 if (unicode_)
diff --git a/js/src/irregexp/RegExpParser.h b/js/src/irregexp/RegExpParser.h
index ee57f04365..a58800a910 100644
--- a/js/src/irregexp/RegExpParser.h
+++ b/js/src/irregexp/RegExpParser.h
@@ -229,7 +229,7 @@ class RegExpParser
     bool simple() { return simple_; }
     bool contains_anchor() { return contains_anchor_; }
     void set_contains_anchor() { contains_anchor_ = true; }
-    int captures_started() { return captures_started_; }
+    int captures_started() { return captures_ == nullptr ? 0 : captures_->length(); }
     const CharT* position() { return next_pos_ - 1; }
 
     static const int kMaxCaptures = 1 << 16;
@@ -239,8 +239,8 @@ class RegExpParser
     enum SubexpressionType {
         INITIAL,
         CAPTURE,  // All positive values represent captures.
-        POSITIVE_LOOKAROUND,
-        NEGATIVE_LOOKAROUND,
+        POSITIVE_LOOKAHEAD,
+        NEGATIVE_LOOKAHEAD,
         GROUPING
     };
 
@@ -249,12 +249,10 @@ class RegExpParser
         RegExpParserState(LifoAlloc* alloc,
                           RegExpParserState* previous_state,
                           SubexpressionType group_type,
-                          RegExpLookaround::Type lookaround_type,
                           int disjunction_capture_index)
             : previous_state_(previous_state),
               builder_(alloc->newInfallible<RegExpBuilder>(alloc)),
               group_type_(group_type),
-              lookaround_type_(lookaround_type),
               disjunction_capture_index_(disjunction_capture_index)
         {}
         // Parser state of containing expression, if any.
@@ -264,16 +262,11 @@ class RegExpParser
         RegExpBuilder* builder() { return builder_; }
         // Type of regexp being parsed (parenthesized group or entire regexp).
         SubexpressionType group_type() { return group_type_; }
-        // Lookahead or Lookbehind.
-        RegExpLookaround::Type lookaround_type() { return lookaround_type_; }
         // Index in captures array of first capture in this sub-expression, if any.
         // Also the capture index of this sub-expression itself, if group_type
         // is CAPTURE.
         int capture_index() { return disjunction_capture_index_; }
 
-        // Check whether the parser is inside a capture group with the given index.
-        bool IsInsideCaptureGroup(int index);
-
       private:
         // Linked list implementation of stack of states.
         RegExpParserState* previous_state_;
@@ -281,15 +274,10 @@ class RegExpParser
         RegExpBuilder* builder_;
         // Stored disjunction type (capture, look-ahead or grouping), if any.
         SubexpressionType group_type_;
-        // Stored read direction.
-        RegExpLookaround::Type lookaround_type_;
         // Stored disjunction's capture index (if any).
         int disjunction_capture_index_;
     };
 
-    // Return the 1-indexed RegExpCapture object, allocate if necessary.
-    RegExpCapture* GetCapture(int index);
-
     widechar current() { return current_; }
     bool has_more() { return has_more_; }
     bool has_next() { return next_pos_ < end_; }
@@ -306,7 +294,6 @@ class RegExpParser
     const CharT* next_pos_;
     const CharT* end_;
     widechar current_;
-    int captures_started_;
     // The capture count is only valid after we have scanned for captures.
     int capture_count_;
     bool has_more_;