I want to know which RegEx-flavour is used for Python? Is it PCRE, Perl compatible or is it ICU or something else?
Unfortunately I cannot answer directly to the comment, but atomic blocks are an important feature (although few people understand their power), since you can create multibyte character sequences with it. I.e. in Windows a newline is \r\n.
Example: /(?>\r\n|\n|\r)\p{Any}/ matches to \n\r or \r., because that is a combination of a newline and any character literal, but it does not match to \r\n since nothing follows the newline.