I don’t think you’re going to be able to do this without restricting yourself to certain issuing countries. If you look at the specs of machine readable passports:
http://www.highprogrammer.com/alan/numbers/mrp.html
It says that each country is free to use any format it likes for the actual passport number (including letters or digits). Although in the machine readable schema these seem to be limited to 9 characters (which might cause problems for countries with billions of citizens if they were to just use digits!)