$re = '/^\+(?=.{15})\d{10,15}_{0,5}$/m';
$str = '+2345678901_____
+23456789012____
+234567890123___
+2345678901234__
+23456789012345_
+234567890123456';
preg_match_all($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