Added initial support for recursive expressions.

Updated docs and tests accordingly.

[SVN r54994]

test/regress/main.cpp

@@ -72,6 +72,7 @@ void run_tests()
    RUN_TESTS(test_unicode);
    RUN_TESTS(test_pocessive_repeats);
    RUN_TESTS(test_mark_resets);
+   RUN_TESTS(test_recursion);
 }
 
 int cpp_main(int /*argc*/, char * /*argv*/[])