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my $letter =    q<[a-zA-Z]>;
my $letdig =    q<[a-zA-Z0-9]>;
my $ldh =       q<[-a-zA-Z0-9]>;
my $label =     "$letter(?:$ldh*$letdig)?";
my $domain =    "$label(?:\.$label)*";

my $label =     "$letdig(?:$ldh*$letdig)?";
my $domain =    "$label(?:\.$label)*\.$letter{2,}";

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

my $addr_spec = qr/^$dot_atom\@$domain$/;