my $atext =     q<[-!#$%&'*+/0-9=?A-Z^_`a-z{|}~]>;
# or $atext =    q<[-\w!#$%&'*+/=?^`a-z{|}~]>;
# (but that raises the question of non-ASCII letters)
my $atom =      "$atext+";
my $dot_atom =  "$atom(?:.$atom)*";
my $addr_spec = $dot_atom . '@' . $dot_atom;