Investigate double-slash logic

[TAbdiukov]

Recall

	
	old = "(\\xD8\\x64\\x24\\x04\\xD9\\x99\\x3C\\x01\\x00\\x00)(.{0,8}?)(\\xC7\\x81\\x3C\\x01\\x00\\x00\\x00\\x00\\x00\\x00\\xD9\\x81\\x3C\\x01\\x00\\x00)"
	new = "\xD8\x64\x24\x04\x90\x90\x90\x90\x90\x90\g<2>\g<3>"

2 questions

1. Why does "old" require double-slashes to work properly?
2. why the script crashes if "new" has double-slash

[TAbdiukov]

re.compile takes a string with double backslashes because it needs to know the exact characters to match in the regular expression. But re.subn takes a string with single backslashes because it needs to know the actual characters to use in the replacement, not their escaped versions.

• re.compile: \\xAB -> r"\xAB"

• re.subn: \xAB -> (byte)