A nit-- can you then modify the text so that it is valid and make sure that the save button is re-enabled? 

To me, in an ideal world, I'd expect the error message to go away (and the Save button to re-enable) at the moment that the text becomes valid. However, taking away the error message as soon as you type would cause the editor to shift up, so I'm OK with how you currently have it (error text remains until you press Save). I also realize that you are re-enabling the Save button on edit (even if text is not valid). Not ideal, but seems OK.