catastrophic backtracking in BACKSLASH_URL_RE

[DaveCrim]

Pretty much the title, discovered this in a downstream project, https://github.com/s0md3v/Photon, commented on it there as well. Thought I'd leave the comment here too, the rest of the defang RE seem to work fine, but the backslash one seemed to cause a lot of hangs when I was using it.

Test against: http://myexample.com/dir/../path/escaping/../too/many/../dots/../in/../the/path/../cause/this/to/fail