Use this Regular Expression (^[A-Z a-z]+$)
Hope it Help you
You can write a microflow for validation of the entity:-
First, create a regular expression “^[a-zA-Z]*$” in Mendix and call a microflow on the save button of the page where you are filling these values.
In the microflow add a decision box and check for the value that is equal to the regex or not if equal then commit and if not equal then Show a validation feedback message there.
Hope it helps!