Add support for some non-standard escapes in regular expressions.
This adds support for some non-standard escape sequences observed in Microsoft's MSXML DLLs and used by Windows apps, and thus needed by Wine. Some are also used in other XML implementations, eg. Java's.
This isn't intended to be final. We probably wish to toggle these non-standard escape sequences on and off somehow, as needed by the caller.
Further discussion: GNOME/libxml2#260 (closed)