$re = '/([^\r\n]{5})/';
$str = ' aaa
bbbbbbb
cc
dddddddddddd';
$subst = "$1\r\n";
$result = preg_replace($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