cvc-pattern-valid: Value 'A' is not facet-valid with respect to pattern '^[A-Za-z]?$' for type 'whatever'
From the w3 spec Regular Expressions (Appendix D):
...expressions are matched against entire lexical representations rather than user-scoped lexical representations such as line and paragraph. For this reason, the expression language does not contain the metacharacters ^ and $, although ^ is used to express exception, e.g. [^0-9]x
I.e. take out the ^ and $.