One of the minor features I’ve added to the flight log is country flags for tail numbers. Every aircraft is registered to one country, and each country has its own assigned format for tail numbers, so it’s possible to look at each tail number and determine what country it’s from.
Since this operation is matching a string to a pattern, it made sense to create regular expressions for each country. For most countries, whose tail number is a unique prefix followed by a dash and three or four letters, this was easy to do. But the United States rules for valid tail numbers are substantially more complicated.