$re = '/(?<=\s)\d(?=\d{0,4}-)(?:-?\d){5}(?=\s)/';
$str = 'то есть, матчится:
111-111
11-1111
1-111-11
не матчится:
111111
-111111
111111-
111--111
11-11
1111-1111
';
preg_match_all($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