null values in table

[capriconuser]

i want only null values or dash (' - ')  value  from table on first column

i attached file 

[Arthur_Fong]

What is the expected result?

After clicking the button, what you want to show in the table?

[Arthur_Fong]

Try this in your expression:

=If(Len(B) <= 1 and not(IsNum(B)), C, Null())