Choice based on field content

This is a typical MAC address for a Dante device:  

00:1D:C1:93:AD:3A

What defines it as such is the 1D (second pair).

I have 4 input fields MAC1, MAC2, MAC3, MAC4.  The above address could be entered into any of the 4 MAC fields.  Currently I enter the MAC address and then choose which entry relates via a choice field "DANTE SELECT" with 4 choices MAC1 through to MAC4.

What I am trying to do is use trigger after update in each MAC input field and check if the 'D' (5th character) is present in either of the 4 MAC fields and choose the corresponding 'Choice'.  So if MAC1 is the 'D' entry then DANTE SELECT shows MAC1

Here you can see a screenshot