Regexp.rightContext was still not quite right. Fixed and

src/macros.py

Index: src/macros.py
===================================================================
--- src/macros.py	(revision 11299)
+++ src/macros.py	(working copy)
@@ -204,6 +204,15 @@
 const CAPTURE0 = 3;
 const CAPTURE1 = 4;
 
+# For the regexp capture override array.  This has the same
+# format as the arguments to a function called from
+# String.prototype.replace.
+macro OVERRIDE_MATCH(override) = ((override)[0]);
+macro OVERRIDE_POS(override) = ((override)[(override).length - 2]);
+macro OVERRIDE_SUBJECT(override) = ((override)[(override).length - 1]);
+# 1-based so index of 1 returns the first capture
+macro OVERRIDE_CAPTURE(override, index) = ((override)[(index)]);
+
 # PropertyDescriptor return value indices - must match
 # PropertyDescriptorIndices in runtime.cc.
 const IS_ACCESSOR_INDEX = 0;