Value Set for DFF. How to restrict to enter alphanumeric only in DFF text field.

Summary

Value Set for DFF. How to restrict to enter alphanumeric only in DFF text field.

Content

I want to restrict to alphanumeric only when entering values in format only DFF text field.
But when I define a format only value set with char type, system allows to enter double bytes chars in DFF field. 
How can I restrict to enter alphanumeric only in DFF field?