$re = '/^(?:\+?7|8)?(?:[\s\-(_]+)?(\d{3})(?:[\s\-_)]+)?(\d{3})(?:[\s\-_]+)?(\d{2})(?:[\s\-_]+)?(\d{2})$/sm';
$str = '+78005553535
88005553535
8005553535
78005553535
+7(800)555-35-35
+7(800)5553535
+7(800)555 35 35
+7 (800) 555 35 35
+7 (800) 555-35-35
+7-(800)-555-35-35
8 (800) 555 35 35
800 555 35 35
800-555-35-35
8(800)5553535
8(800)555 35 35
8(800) 555-35-35
8-800-555-35-35';
$subst = "$1$2$3$4";
$result = preg_replace($re, $subst, $str);
echo "The result of the substitution is ".$result;
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