title: Make portion of UITextView undeletable
body: <p>I have a UITextView and need to make a specific portion un-deletable. Its the first 10 characters of the views text.</p>
  
  <p>I just want it so that if the user is tapping the delete key on the keyboard it simply stops when it reaches say the 10th character in.</p>
  
  <h2>Edit</h2>
  
  <p>Let me go into a bit more detail.</p>
  
  <p>Let's say the prefix is '123456789:'. I want to be able to type anywhere after this prefix, it can't be editable at all though, so '123456789:' shouldn't not be altered at all. Fichek's answer does this perfectly, however the prefix isn't always there, so how can I detect when it isn't in the textview? I thought the if statement did this but it seems not to.</p>