$a   = '(?<a>\d)';
$b   = '(?<b>\d)';

$rx  = qr/(?:${a}${b}|${a}:${b})/;

#$string = "12";
$string = "1:2";

while (1) {
   $string =~ $rx;
   @tmp = @+{qw(a b)};
}