Textfield in EditorWindow only updates when it loses focus

I have a textfield that I want to sanitize as the user is typing into it.
I want to strip out non-letter characters and l want to limit the length.

I would expect something like this to work but it doesn't:

Code (CSharp):

1. string tmp = EditorGUILayout.TextField("ACCESS TOKEN:", accessToken);
2. tmp = alphaNumeric.Replace(tmp, "");
3. if (tmp.Length > 6) tmp = tmp.Substring(0, 6);
4. accessToken = tmp.ToUpper();

I can still type whatever I want into the textfield and it only updates once the textfield loses focus. Is this a bug?