Sorry it took so long to get back to you, I was trying to find a type of some sort.
The app offers this https://github.com/gedoor/legado/blob/master/app/src/main/assets/web/help/md/regexHelp.md Which I hope you find fruitful as an indication.
What do you use to apply your regex?
An ePub Reader called Legado
Is it decimal? One digit or multiple digits? Natural, rational?
In the screenshot above, you can see it differs. I feel like I'm not doing your kindness justice with my sparse information, sorry! 🥺