title: add prefix to UITextField
body: <p>I want to add prefix of <code>UITextfield</code> text. The <code>UITextfield</code> text length less than 7. How many characters are less than 7, that all replace with zeros.</p>
  
  <p>If text is "1234", add prefix like "0001234".
  If text is "12345", add prefix like "0012345".
  If text is "123", add prefix like "0000123".</p>
  
  <p>can any one suggest me, how to implement.</p>