When to use re.M and re.X? (re.M versus re.X)

re.M (or re.multiline) relates to how a string is searched. It changes the behaviour of the '^' and '$' characters, allowing you to match against the start and end of lines rather just the start and end of the string. Useful for multiline strings.

re.X (or re.verbose) relates to how you write your regular expression. It lets you spread it over multiple lines to make it more readable.

The full documentation for both can be found here, and is definitely worth a read. There's no way you're going to remember everything relating to regular expressions (at least I can't), and this is one of the areas of the Python documentation I've found myself looking at most frequently:

https://docs.python.org/3.6/library/re.html

Thanks Stuart Wright !!!