Put an end to NaN and other tips

When you are using InfoPath you will often come across fields being rendered as ‘NaN’ (Not a Number). This usually occurs when a repeating element does not exist.
I have seen a number of ‘solutions’ to this problem so I thought I would add my two penny worth:
If you convert the number to a string then you cannot get the ‘NaN’ error. Finally you may want to do something with the number so prefix it with a zero:

concat(‘0’,string(<element>))

The number always defaults to zero and you may use round, floor max with the warm feeling that NaN will no longer make an unwanted appearance.

... and what about another tip? The other day I was trying to put a validator on a multi-line textbox to restrict the number of characters, I have done this before but for the life of me I could not remember how I did it. Dutifully I added the regular expression which of course failed, after some puzzling I remembered you can simply use string-length(expression) > 255. Regular expressions in InfoPath should always be a last resort.