Comments by Kyle A.B. (Top 2 by date)

I believe I might have solved this myself in a manner similar to the one you suggested. (Also of note, I figured out the third character can actually take a,b,y & z. So the new regex might look like this one:

[0-6]*[\d]\s*[A-HJ-NP-Xc-hj-np-z]\s*[A-HJ-NP-Za-hj-np-z]{2}\s*([\d]+\s+[\d]+|(\d{2}|\d{4}|\d{6}|\d{8}|\d{10}|\d{12}|\d{14}))

Break down:
[0-6]*
The first digit - can be 0 through 6 or non-existent

[\d]\s*
The second digit - any digit and an optional space

[A-HJ-NP-Xc-hj-np-z]\s*
The third character - A through Z excluding I & O and an optional space

[A-HJ-NP-Za-hj-np-z]{2}\s*
Fourth and Fifth characters: The third character - 2x A through Z excluding I & O and an optional space

([\d]+\s+[\d]+|(\d{2}|\d{4}|\d{6}|\d{8}|\d{10}|\d{12}|\d{14}))
Here's where it gets tricky, most of the time this matches an MGRS string fine but it will also match '13TEF 100100 5' as an MGRS so I think the below would be a better solution:
(([\d]{1}\s+[\d]{1}|[\d]{2}\s+[\d]{2}|[\d]{3}\s+[\d]{3}|[\d]{4}\s+[\d]{4}|[\d]{5}\s+[\d]{5}|[\d]{6}\s+[\d]{6}|[\d]{7}\s+[\d]{7}|)|(\d{2}|\d{4}|\d{6}|\d{8}|\d{10}|\d{12}|\d{14}))

What are your thoughts on performance with this expression? Or am I missing anything?