$re = '/(^[A-Za-z][A-Za-z0-9\.\-\_]+$)|(^[\w\-\.\+]+@([\w-]+\.)+[\w-]{2,4}$)/m';
$str = 'User1.2.3
simpleuser123
simpleuser_123
simpleuser-123
1baduser
.baduser
_baduser
1bob-is.great_999+456@gmail.com';
preg_match_all($re, $str, $matches, PREG_SET_ORDER, 0);
// Print the entire match result
var_dump($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