Hi @Olimpia,
I think you missed an quantifier in the regexp, something like [A-Za-z]* if an empty string is allowed or [A-Za-z]+ if at least one letter is required
This resource is quite useful :
https://blog.cartong.org/2018/03/15/all-about-regex-in-xls-form-when-how-and-examples-in-the-humanitarian-and-development-fields/
And this documentation page :
https://docs.getodk.org/form-regex/