Experimental support for RegExp lookbehind.

src/ast.h

Index: src/ast.h
diff --git a/src/ast.h b/src/ast.h
index 14f71a6cc28bfb03e95f2f767c97cce90727542d..3d02debcd3d127fc2948216dd2460c9f7d314f63 100644
--- a/src/ast.h
+++ b/src/ast.h
@@ -119,7 +119,7 @@ class RegExpCharacterClass;
 class RegExpCompiler;
 class RegExpDisjunction;
 class RegExpEmpty;
-class RegExpLookahead;
+class RegExpLookaround;
 class RegExpQuantifier;
 class RegExpText;
 
@@ -3075,8 +3075,7 @@ class RegExpQuantifier final : public RegExpTree {
 
 class RegExpCapture final : public RegExpTree {
  public:
-  explicit RegExpCapture(RegExpTree* body, int index)
-      : body_(body), index_(index) { }
+  explicit RegExpCapture(int index) : body_(NULL), index_(index) {}
   void* Accept(RegExpVisitor* visitor, void* data) override;
   RegExpNode* ToNode(RegExpCompiler* compiler, RegExpNode* on_success) override;
   static RegExpNode* ToNode(RegExpTree* body,
@@ -3091,6 +3090,7 @@ class RegExpCapture final : public RegExpTree {
   int min_match() override { return body_->min_match(); }
   int max_match() override { 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; }
@@ -3101,22 +3101,23 @@ class RegExpCapture final : public RegExpTree {
 };
 
 
-class RegExpLookahead final : public RegExpTree {
+class RegExpLookaround final : public RegExpTree {
  public:
-  RegExpLookahead(RegExpTree* body,
-                  bool is_positive,
-                  int capture_count,
-                  int capture_from)
+  enum Type { LOOKAHEAD, LOOKBEHIND };
+
+  RegExpLookaround(RegExpTree* body, bool is_positive, int capture_count,
+                   int capture_from, Type type)
       : body_(body),
         is_positive_(is_positive),
         capture_count_(capture_count),
-        capture_from_(capture_from) { }
+        capture_from_(capture_from),
+        type_(type) {}
 
   void* Accept(RegExpVisitor* visitor, void* data) override;
   RegExpNode* ToNode(RegExpCompiler* compiler, RegExpNode* on_success) override;
-  RegExpLookahead* AsLookahead() override;
+  RegExpLookaround* AsLookaround() override;
   Interval CaptureRegisters() override;
-  bool IsLookahead() override;
+  bool IsLookaround() override;
   bool IsAnchoredAtStart() override;
   int min_match() override { return 0; }
   int max_match() override { return 0; }
@@ -3124,12 +3125,14 @@ class RegExpLookahead final : 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_;
 };
 
 
@@ -3142,7 +3145,14 @@ class RegExpBackReference final : public RegExpTree {
   RegExpBackReference* AsBackReference() override;
   bool IsBackReference() override;
   int min_match() override { return 0; }
-  int max_match() override { return capture_->max_match(); }
+  // 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
+  // 0 since lookbehinds have a length of 0.
+  int max_match() override {
+    return capture_->body() ? capture_->max_match() : 0;
+  }
   int index() { return capture_->index(); }
   RegExpCapture* capture() { return capture_; }
  private: