Extracting text from variable

Hi,

Bit of a strange question but I hope someone can help me with this.

What I have is a variable that's called: this.is.a.test.s54352.test
Now the, this.is.a.test part is dynamic and changes all the time also the character count changes. but that's the part I need to have.

So I want to strip: this.is.a.test.s54352.test TO this.is.a.test

Thank you for looking in to this.
moh-san