Replace multiple occurrences of same symbol using preg_replace?

Let's say I have a string like this:

$string = "hello---world";

How would I go about replacing the --- with a single hyphen? The stri开发者_如何学Pythonng could easily look like this instead:

$string = "hello--world----what-up";

The desired result should be:

$string = "hello-world-what-up";

$string = preg_replace('/-{2,}/','-',$string);

To remove them from the beginning and the end:

$string = trim($string, '-');

try $string = preg_replace('/-+/', '-', $string)

$string = preg_replace('/--+/', '-', $string);

Here's the function I'm using - works like a charm :)

public static function setString($phrase, $length = null) {
    $result = strtolower($phrase);
    $result = trim(preg_replace("/[^0-9a-zA-Z-]/", "-", $result));
    $result = preg_replace("/--+/", "-", $result);
    $result = !empty($length) ? substr($result, 0, $length) : $result;
    // remove hyphen from the beginning (if exists)
    $first_char = substr($result, 0, 1);
    $result = $first_char == "-" ? substr($result, 1) : $result;
    // remove hyphen from the end (if exists)
    $last_char = substr($result, -1);
    $result = $last_char == "-" ? substr($result, 0, -1) : $result;     
    return $result;
}