$re = '/(?:<video[^>].*src=\")(?<src>[^"].*)(?=\?)/';
$str = '<video id="video" class="playerVideo" autoplay="1" oncontextmenu="return false;" x-webkit-airplay="allow" src="http://vid.pluralsight.com/expiretime=1426057457/9207dc050972394adfa1b36a10c7fcc4/clip-videos/jon-flanders/ios-dotnet-developers-m01-intro/ios-dotnet-developers-m01-intro-01/1024x768mp4/20130301234848.mp4?_=1" type="video/mp4">';
preg_match($re, $str, $matches, PREG_OFFSET_CAPTURE, 0);
// Print the entire match result
var_dump($matches);
Please keep in mind that these code samples are automatically generated and are not guaranteed to work. If you find any syntax errors, feel free to submit a bug report. For a full regex reference for PHP, please visit: http://php.net/manual/en/ref.pcre.php