title: JS regex freezes my browser(s)
body: <p>So I wrote this Regex for validating e-mail addresses:</p>
  
  <pre><code>(?=^([A-Za-z\xC0-\xFF0-9\!\#\$\%\&amp;\'\*\+\-\/\=\?\^\\_\`\{\|\}\~]\.?){0,63}[A-Za-z\xC0-\xFF0-9\!\#\$\%\&amp;\'\*\+\-\/\=\?\^\\_\`\{\|\}\~]@[A-Za-z\xC0-\xFF0-9]([A-Za-z\xC0-\xFF0-9-]{0,61}[A-Za-z\xC0-\xFF0-9])?(\.[A-Za-z\xC0-\xFF]([A-Za-z\xC0-\xFF0-9-]{0,61}[A-Za-z\xC0-\xFF0-9])?)*$)(?=^.{3,254}$)
  </code></pre>
  
  <p>And I wanted to test it in js console:</p>
  
  <pre><code>var patt = new RegExp("(?=^([A-Za-z\xC0-\xFF0-9\!\#\$\%\&amp;\'\*\+\-\/\=\?\^\\_\`\{\|\}\~]\.?){0,63}[A-Za-z\xC0-\xFF0-9\!\#\$\%\&amp;\'\*\+\-\/\=\?\^\\_\`\{\|\}\~]@[A-Za-z\xC0-\xFF0-9]([A-Za-z\xC0-\xFF0-9-]{0,61}[A-Za-z\xC0-\xFF0-9])?(\.[A-Za-z\xC0-\xFF]([A-Za-z\xC0-\xFF0-9-]{0,61}[A-Za-z\xC0-\xFF0-9])?)*$)(?=^.{3,254}$)")
  
  patt.test("jake@domain.domain.domain.domain.domai.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domain.domai")
  </code></pre>
  
  <p>And that freezes my tab. Shorter addresses seem to be working fine.</p>
  
  <p>Tested in C# and online regex tools (e.g. <a href="https://regex101.com/r/pJ2iS4/1" rel="nofollow">regex101.com</a>) without any problems.
  What's going on? Is the JavaScript Regex engine faulty, or is it just my Regex?</p>
  
  <p>(I am aware that this might not be a perfect email validation, but this question is about why doesn't it work)</p>