Why not use URI? It will tell you what you need to know:

    my ($uri) = new URI("http://www.mydomain.com.au/test.html");
    print $uri->host,"\n";
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It's probably far better to use that than a regexp, though, of course, you would not be able to validate the validity of any given Top Level Domain (TLD).

It used to be that top level domains (TLDs) were for countries (i.e. ".ca", ".au", ".uk") or types of companies (".com", ".net", ".org", or even ".mil") and were fairly predictable. These days, with countries being invaded and assimilated, or spliting up because of civil war (frighteningly frequent in Eastern Europe), the TLDs are always changing. The ISO-3166 specifies the country codes for various national entities.

Added to this is the likes of Esther Dyson, chairperson of ICANN, which is proposing to add things like ".museum" to the TLD namespace.

Don't forget the UTF-5 encoded "iTLDs" which are being issued by VeriSign and others. These look really wacky unless your browser supports them, but they mean things like ".com" in Japanese, Chinese, Korean, and other languages that aren't based on the Latin character set.

The only way to know for sure, if only for a short period of time (i.e. a month or so) before requiring an update, is to process the root zone file which lists all the servers for all the active domains.