That can be used as is to increment font sizes 0 thru 99 to 1 thru 100. I stopped there because I rarely see font sizes larger than that in anything. If there is more that you need it to replace, I can help with that. In some cases not just one regex replacement, but a series of them can do the job.
Regex can't do math. Notepad can if you load scripts into it. I think it uses Python. If it gets too crazy to do with regex, I recommend that. This regex is pretty much on the border of insanity. In fact, if you tried to find this on the Internet, you probably wouldn't find much similar... it just isn't something you would normally use it for.