$re = '/^(?=.{3,16}$)[a-z]([0-9a-z]|[0-9a-z\-](?=[0-9a-z])){1,}([\.](?=[a-z][0-9a-z\-][0-9a-z\-])[a-z]([0-9a-z]|[0-9a-z\-](?=[0-9a-z])){1,}){0,}$/m';
$str = 'cryptosharon
cry.pto
-----
cr
sh.ar
-----
cryptosharon9
cry.pto9
c-r.y-p.t-o9
-----
9cryptosharon
9sh.aron
sha.9ron
-----
cry-9-pto-5
s-h-a-r-o-n9
a12.b-3
-----
sh--aron
crptshrn-
otp-.yrc
-----
a12345678901234567
-test-123
.test.123
123.test
tes';
preg_match_all($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