Enforce User Input "a" to "Z"

[JackieSYK]

In a text Field, How Can I Enforce User Only Ca Input "a" to "z" and "A" to "Z"?

 

[rosito]

field properties->input mask-> "????????????????" for letter Aa-Zz entry optional or
field properties->input mask-> "LLLLLLL" for letter Aa-Zz entry mandatory