Permitting only letters, no spaces, in a text field

Hi all,
I've been trying everything I can to make this work and can't figure it out. I'm trying to correct the trailing space that iPhones create when responders autocomplete a text field. (e.g. I want "ABCD" not "ABCD ")
The solution that looks most promising is to use custom validation for the field and I thought I had it correct with "^[A-Za-z]$" but unfortunately, that won't permit anything to be accepted. Can you point out what I'm doing wrong?
Thanks for any and all help,
JMag