title: Can it cause harm to validate email addresses with a regex?
body: <p>I've heard that it is a bad thing to validate email addresses with a regex, and that it actually can cause harm. Why is that?</p>
  <p>I thought it never could be a bad thing to validate data. Maybe unnecessary, but never a bad thing provided that you perform the validation correctly. Why is this right or wrong? If it can cause harm, please give an example.</p>