Ask HN: Is there an offline RegEx training app for iOS or Android?

Just wondering if there is something like Duolingo or similar for RegEx. Basically, it throws challenges at you that increase in difficulty, and you try to come up with the correct Regex.

It would be perfect for being on planes or trains, and I've had a quick glance at both of the app stores but couldn't see anything interesting. Would possibly have a great monetisation opportunity (in-app upgrades of higher levels).