How can I create a SEO friendly dash-delimited url from a string?

C#

public string toFriendly(string subject)
{
    subject = subject.Trim().ToLower();
    subject = Regex.Replace(subject, @"\s+", "-");
    subject = Regex.Replace(subject, @"[^A-Za-z0-9_-]", "");
    return subject;
}

Here is my solution in C#

private string ToSeoFriendly(string title, int maxLength) {
    var match = Regex.Match(title.ToLower(), "[\\w]+");
    StringBuilder result = new StringBuilder("");
    bool maxLengthHit = false;
    while (match.Success && !maxLengthHit) {
        if (result.Length + match.Value.Length <= maxLength) {
            result.Append(match.Value + "-");
        } else {
            maxLengthHit = true;
            // Handle a situation where there is only one word and it is greater than the max length.
            if (result.Length == 0) result.Append(match.Value.Substring(0, maxLength));
        }
        match = match.NextMatch();
    }
    // Remove trailing '-'
    if (result[result.Length - 1] == '-') result.Remove(result.Length - 1, 1);
    return result.ToString();
}

I would follow these steps:

1. convert string to lower case
2. replace unwanted characters by hyphens
3. replace multiple hyphens by one hyphen (not necessary as the preg_replace() function call already prevents multiple hyphens)
4. remove hypens at the begin and end if necessary
5. trim if needed from the last hyphen before position x to the end

So, all together in a function (PHP):

function generateUrlSlug($string, $maxlen=0)
{
    $string = trim(preg_replace('/[^a-z0-9]+/', '-', strtolower($string)), '-');
    if ($maxlen && strlen($string) > $maxlen) {
        $string = substr($string, 0, $maxlen);
        $pos = strrpos($string, '-');
        if ($pos > 0) {
            $string = substr($string, 0, $pos);
        }
    }
    return $string;
}

Here's a solution for php:

function make_uri($input, $max_length) {
  if (function_exists('iconv')) {  
    $input = @iconv('UTF-8', 'ASCII//TRANSLIT', $input);  
  }

  $lower = strtolower($input);


  $without_special = preg_replace_all('/[^a-z0-9 ]/', '', $input);
  $tokens = preg_split('/ +/', $without_special);

  $result = '';

  for ($tokens as $token) {
    if (strlen($result.'-'.$token) > $max_length+1) {
      break;
    }

    $result .= '-'.$token;       
  }

  return substr($result, 1);
}

usage:

echo make_uri('In C#: How do I add "Quotes" around string in a ...', 500);

Unless you need the uris to be typable, they don't need to be small. But you should specify a maximum so that the urls work well with proxies etc.