<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def sub(pattern, repl, string, *args, **kwargs):
 """Apply `sub` after applying backrefs."""


    flags = args[4] if len(args) > 4 else kwargs.get('flags', 0)
    is_replace = _is_replace(repl)
    is_string = isinstance(repl, (str, bytes))
    if is_replace and repl.use_format:
        raise ValueError("Compiled replace cannot be a format object!")

    pattern = compile_search(pattern, flags)
    return _re.sub(
        pattern, (compile_replace(pattern, repl) if is_replace or is_string else repl), string, *args, **kwargs
    )