>>We have an application which negotiates with license plates. It is focused to be international. It currently supports Canada, United States and another country. I would like to know how many characters you are usually reserving for such a field in your application knowing that it could be used to store license plates from all over the world.
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>In my country longest is 11 characters. Unicode not needed. We don't use turkish special letters for plates. Just use numbers and american letters excluded X,W,Q.

The only one of 11 characters I can see here http://en.wikipedia.org/wiki/Vehicle_registration_plates_of_Turkey is a 'vanity' plate.

Don't know what regex you could use for the president's car either :-}