substr to find all text before a wildcard substring

What I am trying to do is get all of the characters before a wildcard substring. For example, if I have the following string:

I.Want.This.Bit.D00F00.Non.Of.This

So, I want the output to be I.Want.This.Bit

The D and the F in D00F00 are always going to be there, but the integers inbetween will change. So it could be D13F02, or D01F15. The will not be more than 2 integers after the D and the F.

I had thought about doing the following, but then realised it would not work:

$string = "I.Want.This.Bit.D00F00.Non.Of.This"    
$substring = substr($string, 0, strpos($string, '.D'));

The reason it would not work is because there is a chance the bit of the string I want to keep could have a .D in it, e.g The.Daft.String.D03F12. Using that example, all I would get is The as the output, rather than The.Daft.String.

Any guidance would be much appreciated.