See, that's exactly what the maxim is about: "I know, I'll use regular expressions, [because regular expressions are the universal text manipulation tool]." Now you indeed have two problems, because regex is not an appropriate tool here.

You are absolutely not looking for a regular expression (as the grammatical rules and their exceptions are not all that easy to reimplement in regexp syntax), you are looking for a dictionary (a set of key-value pairs, i.e. "American zpelling" => "British spelling" ;)). Then iterate through each word in the document, see if it's a dictionary key, if so, replace with the relevant value. No regex needed.