title: How can I validate an email address using a regular expression?
body: <p>Over the years I have slowly developed a <a href="http://en.wikipedia.org/wiki/Regular_expression" rel="noreferrer">regular expression</a> that validates <em>most</em> email addresses correctly, assuming they don't use an IP address as the server part.</p>
  <p>I use it in several PHP programs, and it works most of the time.  However, from time to time I get contacted by someone that is having trouble with a site that uses it, and I end up having to make some adjustment (most recently I realized that I wasn't allowing four-character <a href="https://en.wikipedia.org/wiki/Top-level_domain" rel="noreferrer">TLDs</a>).</p>
  <p><em>What is the best regular expression you have or have seen for validating emails?</em></p>
  <p>I've seen several solutions that use functions that use several shorter expressions, but I'd rather have one long complex expression in a simple function instead of several short expression in a more complex function.</p>