How to conditional regex

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

aaa.bbb.ccc.ddd // one part of the regex

aaa.bbb.ccc.ddd.eee // the second part of the regex

how do I achieve this in either js or c#?

something like

?(\.){4} then THIS else THAT

within the regex...

Update

Ok basically what I'm doing is this:

I want to switch, for any given System.Uri, to another subdomain in an extension method.

The problem I came across is that my domains are usually of the form http://subdomain.domain.TLD.TLD/more/url, but sometimes, it can be just http://domain.TLD.TLD/more/url (which just points to www)

So this is what I came up with:

public static class UriExtensions
{
    private const string TopLevelDomainRegex = @"(\.[^\.]{2,3}|\.[^\.]{2,3}\.[^\.]{2,3})$";
    private const string UnspecifiedSubdomainRegex = @"^((http[s]?|ftp):\/\/)(()([^:\/\s]+))(:([^\/]*))?((?:\/)?|(?:\/)(((\w+)*\/)([\w\-\.]+[^#?\s]+)(\?([^#]*))?(#(.*))?))?$";
    private const string SpecifiedSubdomainRegex = @"^((http[s]?|ftp):\/\/)(([^.:\/\s]*)[\.]([^:\/\s]+))(:([^\/]*))?((?:\/)?|(?:\/)(((\w+)*\/)([\w\-\.]+[^#?\s]+)(\?([^#]*))?(#(.*))?))?$";

    public static string AbsolutePathToSubdomain(this Uri uri, string subdomain)
    {
        subdomain = subdomain == "www" ? string.Empty : string.Concat(subdomain, ".");

        var replacement = "$1{0}$5$6".FormatWith(subdomain);

        var spec = Regex.Replace(uri.Authority, TopLevelDomainRegex, string.Empty).Distinct().Count(c => c == '.') != 0;
        return Regex.Replace(uri.AbsoluteUri, spec ? SpecifiedSubdomainRegex : UnspecifiedSubdomainRegex, replacement);
    }
}

Basically with this code I take the Sys