What I have used is:
Code:
//*[re:test(., '^[A-Z][A-Z][ A-Z-]+$')]
This is the start of an element (^) followed by two capital letters followed by at least one more capital letter or space or dash ([ A-Z-]+) with this pattern all the way to the end of the element ($).
Sometimes the ^ and/or the $ needs to be left off, but they prevent false positives when they work at all.