How to conditional regex

Question

I want a regex that does one thing if it has 3 instances of .in the string, and something else if it has more than 3 instances.

for example

Answer 1

You don't need Regex for this (as for many other common tasks).

public static string AbsolutePathToSubdomain(this Uri uri, string subdomain)
{
    // Pre-process the new subdomain
    if (subdomain == null || subdomain.Equals("www", StringComparison.CurrentCultureIgnoreCase))
        subdomain = string.Empty;

    // Count number of TLDs (assume at least one)
    List parts = uri.Host.Split('.').ToList();
    int tldCount = 1;
    if (parts.Count >= 2 && parts[parts.Count - 2].Length <= 3)
    {
        tldCount++;
    }

    // Drop all subdomains
    if (parts.Count - tldCount > 1)
        parts.RemoveRange(0, parts.Count - tldCount - 1);

    // Add new subdomain, if applicable
    if (subdomain != string.Empty)
        parts.Insert(0, subdomain);

    // Construct the new URI
    UriBuilder builder = new UriBuilder(uri);
    builder.Host = string.Join(".", parts.ToArray());
    builder.Path = "/";
    builder.Query = "";
    builder.Fragment = "";

    return builder.Uri.ToString();
}