title: How to write regex that match start with a letter, can contain letters, numbers, hyphens and 1 underscore?
body: <p>I need to write regex(.NET) that match string:
  start with a letter, can contain letters, numbers, periods, hyphens, <strong>1 underscore</strong> and follow '@'. I tried the follow one, but it doesn't match 'a_bc12@' for example. </p>
  
  <p>How to fix it?</p>
  
  <p><code>^[A-Za-z][A-Za-z0-9-]+_{0,1}(?=@)</code></p>
  
  <p>EDITED:
   it should contain {3,10} length.</p>