You can put the following in a decisiton:
isMatch($YourObject/ISBN, '^ISBN[A-Za-z0-9]{4}$')
Use a regex for it. See stackoverflow: https://stackoverflow.com/questions/41271613/use-regex-to-verify-an-isbn-number
And see the documentation here for how to use a regex: https://docs.mendix.com/refguide/regular-expressions/
Regards,
Ronald
You can try input mask feature for input fields which will easily take care of prefix, length and type.
For instance, for your requirement use this: ISBN*******
*: for alpha numeric
You can also try marketplace control like IMask - Masked Input.
You can validate this using a Decision with isMatch.
isMatches($Object/ISBN, '^ISBN[A-Za-z0-9]{4}$')
If the condition is false, show a validation message.