SECURITY ALERT: Corrected regular expressions for URL and email fields.

Certain email addresses/URLs could trigger a catastrophic backtracking situation, causing 100% CPU and server overload. If deliberately triggered, this could be the basis of a denial-of-service attack.

This security vulnerability was disclosed in public, so we're skipping our
normal security release process to get the fix out as soon as possible.

This is a security related update. A full announcement, as well as backports for the 1.1.X and 1.0.X series will follow.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@11603 bcc190cf-cafb-0310-a4f2-bffc1f526a37