import re
s = '[*]\r\n[list][*][*][/list][*]text[list][*][/list]'
match = re.compile('\[list].+?\[/list]', re.DOTALL)
print(match.sub(lambda m: m.group().replace('*', '**'), s))
aW1wb3J0IHJlCnMgPSAnWypdXHJcbltsaXN0XVsqXVsqXVsvbGlzdF1bKl10ZXh0W2xpc3RdWypdWy9saXN0XScKbWF0Y2ggPSByZS5jb21waWxlKCdcW2xpc3RdLis/XFsvbGlzdF0nLCByZS5ET1RBTEwpCnByaW50KG1hdGNoLnN1YihsYW1iZGEgbTogbS5ncm91cCgpLnJlcGxhY2UoJyonLCAnKionKSwgcykp