[regexp] break recursion in mutually recursive capture/back references.

test/cctest/test-regexp.cc

Index: test/cctest/test-regexp.cc
diff --git a/test/cctest/test-regexp.cc b/test/cctest/test-regexp.cc
index ca432cfde6035fc4d8417a1f659e9b3f7fb2fa53..cdd9ca0dfdf8cf3b7de0786ac84055f9d784416e 100644
--- a/test/cctest/test-regexp.cc
+++ b/test/cctest/test-regexp.cc
@@ -110,6 +110,9 @@ static void CheckParseEq(const char* input, const char* expected) {
   CHECK(result.error.is_null());
   std::ostringstream os;
   result.tree->Print(os, &zone);
+  if (strcmp(expected, os.str().c_str()) != 0) {
+    printf("%s | %s\n", expected, os.str().c_str());
+  }
   CHECK_EQ(0, strcmp(expected, os.str().c_str()));
 }
 
@@ -269,6 +272,7 @@ void TestRegExpParser(bool lookbehind) {
   CheckParseEq("(a)\\1", "(: (^ 'a') (<- 1))");
   CheckParseEq("(a\\1)", "(^ 'a')");
   CheckParseEq("(\\1a)", "(^ 'a')");
+  CheckParseEq("(\\2)(\\1)", "(: (^ (<- 2)) (^ (<- 1)))");
   CheckParseEq("(?=a)?a", "'a'");
   CheckParseEq("(?=a){0,10}a", "'a'");
   CheckParseEq("(?=a){1,10}a", "(: (-> + 'a') 'a')");