Bring bugfix of global regexps to the 2.1 branch.

src/regexp.js

Index: src/regexp.js
diff --git a/src/regexp.js b/src/regexp.js
index dc1b0429f7094c5c4fe80d5af5515c8fd2e5fb91..69cf136d108e1cd21bc30f445dc3919fe4879700 100644
--- a/src/regexp.js
+++ b/src/regexp.js
@@ -213,7 +213,10 @@ function RegExpExec(string) {
   var matchIndices = %_RegExpExec(this, s, i, lastMatchInfo);
 
   if (matchIndices == null) {
-    if (this.global) this.lastIndex = 0;
+    if (this.global) {
+      this.lastIndex = 0;
+      if (lastIndex != 0) return matchIndices;
+    }
     cache.lastIndex = lastIndex;
     cache.regExp = this;
     cache.subject = s;