Regex to match something based on what was matched before

Question

I\'m trying to write a regular expression to validate moves in algebraic chess notation. Here\'s what I have so far:

/
    O-O(-O)?                 # Castlin         


        

Answer 1

Matching the adjacent letter is not hard using alternations, but you won't find anything compact as there is no character arithmetic in regex.

Someone just posted a similar question out of sympathy for your plight.

The long way is the only way.