(?^:a)

(?:(?|(?:\()(?:[^\\\(]*(?:\\.[^\\\(]*)*)(?:\()|(?:\?)(?:[^\\\?]*(?:\\.[^\\\?]*)*)(?:\?)|(?:\^)(?:[^\\\^]*(?:\\.[^\\\^]*)*)(?:\^)|(?:\:)(?:[^\\\:]*(?:\\.[^\\\:]*)*)(?:\:)|(?:a)(?:[^\\a]*(?:\\.[^\\a]*)*)(?:a)|(?:\))(?:[^\\\)]*(?:\\.[^\\\)]*)*)(?:\))))