UK Postcode validation is the theme for me today, and the standard has been very useful. Digging a little deeper even provides a couple of regular expression patterns for doing the validation with, both a simple one and a more complicated one.
Examining the patterns has led me to a construct that I can't find documented anywhere. I've looked through all the linked pages in the "Regular Expression Language Elements" section in the VS2005 help, but with no joy. It seems to work though.
Fairly self-explanatory hopefully, with this particular example matching any one of twenty capital letters. If it is actually documented somewhere, please do point me at it.