Remove unnecessary code for non-zero-length global regexps.

test/mjsunit/regexp-global.js

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Redistribution and use in source and binary forms, with or without
 // modification, are permitted provided that the following conditions are
 // met:
 //
 //     * Redistributions of source code must retain the above copyright
 //       notice, this list of conditions and the following disclaimer.
 //     * Redistributions in binary form must reproduce the above
 //       copyright notice, this list of conditions and the following
 //       disclaimer in the documentation and/or other materials provided
@@ skipping 107 matching lines @@
 str = "FOUR \u817f GOOD, TWO \u817f BAD!";
 str = str.replace(/(FOUR|TWO) \u817f (GOOD|BAD)/g,
                   function(match, num_legs, likeability) {
                     assertTrue(num_legs !== undefined);
                     assertTrue(likeability !== undefined);
                     if (num_legs == "FOUR") assertTrue(likeability == "GOOD");
                     if (num_legs == "TWO") assertTrue(likeability == "BAD");
                     return match.length - 7;
                   });
 assertEquals("4, 2!", str);
+
+// Test zero-length matches that have non-zero-length sub-captures that do not
+// start at the match start position.
+str = "up up up up";
+str = str.replace(/\b(?=u(p))/g, function(match, capture) {
+                                    return capture.length;
+                                  });
+
+assertEquals("1up 1up 1up 1up", str);