Hi Jack,
I just played quick with some regex I had it. You probably need to play a little bit more, but this might be give you an idea. Also this probably no the "best/good practice" for regex, so be careful because it can cause an expensive parsing behavior in QRadar, so I would suggest to test it first (may you can use QRadar CE) and monitoring the behavior.
On the DSM Editor:
Log Source Type: Linux OS
Source MAC
Expression: MAC=(([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}))
Format String: $1
Destination MAC
Expression: (((([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}):([A-Fa-f0-9]{2}))))\s
Format String:$1
And it looks like:

Once again, hope this helps!
Cheers!
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