帮助移植php函数do "clean url"c#

我正在尝试将这个函数移植到c#

http://www.phpsnaps.com/snaps/view/clean-url/

我有问题转换"~[^-a-z0-9_]+~" (php模式命名法)在正则表达式charp命名法

<?php
function cleanURL($string)
{
    $url = str_replace("'", '', $string);
    $url = str_replace('%20', ' ', $url);
    // (PROBLEM) substitutes anything but letters, numbers and '_' with separator
    $url = preg_replace('~[^'pL0-9_]+~u', '-', $url);
    $url = trim($url, "-");
    // you may opt for your own custom character map for encoding.
    $url = iconv("utf-8", "us-ascii//TRANSLIT", $url); 
    $url = strtolower($url);
     (PROBLEM)
    $url = preg_replace('~[^-a-z0-9_]+~', '', $url); // keep only letters, numbers, '_' and separator
    return $url;
} // echo cleanURL("Shelly's%20Greatest%20Poem%20(2008)");  // shellys-greatest-poem-2008
?>

这是c#函数:

static String cleanURL(String url)
{
    url = url.Replace("'", "");
    url = url.Replace("%20", " ");            
    url = System.Text.RegularExpressions.Regex.Replace(url, "~[^'pL0-9_]+~u", "-");           
    url = url.Trim(new char[1]{'-'});         
    Encoding ascii = Encoding.ASCII;           
    Encoding utf8 = Encoding.UTF8;           
    byte[] utf8bytes = utf8.GetBytes(url);           
    byte[] asciiBytes = Encoding.Convert(utf8, ascii, utf8bytes);            
    char[] asciiChars = new char[ascii.GetCharCount(asciiBytes, 0, asciiBytes.Length)];           
    ascii.GetChars(asciiBytes, 0, asciiBytes.Length, asciiChars, 0);           
    url = new string(asciiChars);           
    url = url.ToLower();                    
    url = System.Text.RegularExpressions.Regex.Replace(url, "~[^-a-z0-9_]+~", "");
    return url;           
}    

谢谢。有人能帮我吗?