You're rightfully confused, because that regex must have been written by someone who doesn't know Python regexes well.
In some languages (e.g. JavaScript), regexes are delimited by slashes. That means that if you need an actual slash in your regex, you have to escape it. Since Python doesn't use slashes, there's no need to escape the slash (but it doesn't cause an error, either).
Much more worrisome is that the author failed to use a raw string. In many cases, that won't matter (because Python will treat "\d"
as "\\d"
which then correctly translates to the regex \d
, but in other cases, it will cause problems. One example is "\b"
which means "a backspace character" and not "a word boundary anchor" like the regex \b
would.
Also, the author has escaped a lot of characters that didn't need escaping at all. The entire regex could be rewritten as
metric_pattern = re.compile(r'^([^=]+)=([\d.+eE-]+)([\w/%]*);?([\d.+eE:~@-]+)?;?([\d.+eE:~@-]+)?;?([\d.+eE-]+)?;?([\d.+eE-]+)?;?\s*')
and even then, I'm surprised that it works at all. Looks very chaotic to me and is definitely not foolproof. For example, there appears to be a big potential for catastrophic backtracking meaning that users could freeze your server with malicious input.