That is correct. Below is example to implement ClientSideEvents.KeyPress which under English keyboard allows to enter only letters "abcABC". All other entries are not affected.

function Mask1_KeyPress(oEdit, keyCode, oEvent)
{
 var a = 65, z = 90;
 var A = a + 32, Z = z + 32;
 var cancel = false;
 if(keyCode >= a + 3 && keyCode <= z)
   cancel = true;
 if(keyCode >= A + 3 && keyCode <= Z)
   cancel = true;
 oEvent.cancel = cancel;
}