On different platforms and versions the following expression has different results:

  python -c 'import re; print(re.compile("(.*)", 0).sub("a\\1", "bc"))'

As far is I observed:

  Linux/Python 3.6.9   => abc
  MacOS/Python 3.7.1   => abca
  Repl.it/Python 3.8.1 => abca
  MacOS/Python 2.7.17  => abc
  Linux/Python 2.7.17  => abc

According the the documentation I would guess that "abc" is the correct return value.

The issues also occurs without compiling or capture group:

 re.sub(".*", "a", "cb")  a vs aa