Remove extra dash from url

 I have a blog system where user enter title and from it i make url here is the function to create url

function create_slug($string){     
    $replace = '-';         
    $string = strtolower(trim($string));     

    //replace / and . with white space     
    $string = preg_replace("/[\/\.]/", " ", $string);     
    $string = preg_replace("/[^a-z0-9_\s-]/", "", $string);     

    //remove multiple dashes or whitespaces     
    $string = preg_replace("/[\s-]+/", " ", $string);     

    //convert whitespaces and underscore to $replace     
    $string = preg_replace("/[\s_]/", $replace, $string);     

    //limit the slug size     
    $string = substr($string, 0, 100);     

    //slug is generated     
    return $string; 
}