For alphabetical characters only, use the following javascript on the validate event of the text field

var r = new RegExp("^[a-z]*$");

var result = r.test(this.rawValue);

if (result == true)

    true;           

else               

    false;  

Also, set the Validation Script Message properties in the Object->Value Tab

In Designer, click on the field you need to validate and open the script window, set the Show drop-down list to validate in the drop-down list and place the code inside the script window. Make sure the Language is Javascript and Run At is set to Client. If you cant see the script window goto Window->Script Editor Menu item.

Not sure if you are looking for an actual validation for the field (i.e. show an error and stop the user from moving to the next field) or just a filter in the field (i.e. no message and just stop characters being entered). From the original message i was assuming you were looking for a validation.

If you want to use a message box validation and return to the field containing the error, use the following -

On the exit event

var r = new RegExp("^[a-z]*$");
var result = r.test(this.rawValue);
 
if ( result != true )
{
    xfa.host.messageBox( "Enter alpha characters only" );
    xfa.host.setFocus( this );
}

If you are looking for a filter then, use the following on a change event of the field

var r = new RegExp("^[a-z]*$");
var result = r.test(xfa.event.change);
 
if ( result != true )
{
    xfa.event.change = "";
}

this will probably only work in pdf and some versions of reader had a bug with this so make sure you are on 10.1.2 to test this. see http://forums.adobe.com/thread/910259

Try the second example in the previous message. That will disable any non-alpha input. Put it on the change event and remove any other event code.

The change event is fired each time a user interacts with an object using keystroke, paste, list selection, select/deselect check bos, or change radio button. The validate on the other hand only fires when the field losses when you tab to the next field for example