This allows using an email address (as is) as part of a tag. The main
problem that could arise is when parsing tags from a shell script, but
(AFAIK) '@' is not a special character when used in values (happy to be
corrected if not true).

The patch also moves the re to be compiled at class init time, should
use less resources; in my tests it is fine to use a compiled re from
multiple threads.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>