How does Stack Overflow generate its SEO-friendly URLs?

Here’s how we do it. Note that there are probably more edge conditions than you realize at first glance.

This is the second version, unrolled for 5x more performance (and yes, I benchmarked it). I figured I’d optimize it because this function can be called hundreds of times per page.

/// <summary>
/// Produces optional, URL-friendly version of a title, "like-this-one". 
/// hand-tuned for speed, reflects performance refactoring contributed
/// by John Gietzen (user otac0n) 
/// </summary>
public static string URLFriendly(string title)
{
    if (title == null) return "";

    const int maxlen = 80;
    int len = title.Length;
    bool prevdash = false;
    var sb = new StringBuilder(len);
    char c;

    for (int i = 0; i < len; i++)
    {
        c = title[i];
        if ((c >= 'a' && c <= 'z') || (c >= '0' && c <= '9'))
        {
            sb.Append(c);
            prevdash = false;
        }
        else if (c >= 'A' && c <= 'Z')
        {
            // tricky way to convert to lowercase
            sb.Append((char)(c | 32));
            prevdash = false;
        }
        else if (c == ' ' || c == ',' || c == '.' || c == "https://stackoverflow.com/" || 
            c == '\\' || c == '-' || c == '_' || c == '=')
        {
            if (!prevdash && sb.Length > 0)
            {
                sb.Append('-');
                prevdash = true;
            }
        }
        else if ((int)c >= 128)
        {
            int prevlen = sb.Length;
            sb.Append(RemapInternationalCharToAscii(c));
            if (prevlen != sb.Length) prevdash = false;
        }
        if (i == maxlen) break;
    }

    if (prevdash)
        return sb.ToString().Substring(0, sb.Length - 1);
    else
        return sb.ToString();
}

To see the previous version of the code this replaced (but is functionally equivalent to, and 5x faster), view revision history of this post (click the date link).

Also, the RemapInternationalCharToAscii method source code can be found here.