Regular Expressions

In a recent forum message, a user wrote:

"I am trying to write a regular expression that will match "tester" for the following strings - the idea of this reg-ex is to get the SLD from a user passed in domain name string: tester, tester.com, tester.eu. I've already spent about an hour, and can't get it to work perfect, but I'm not regular expression expert either. Anyone else want to give it a try?"

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