Alternate regex with -SDR?

Question

I have the following regex in my c#:

(?<!\w)M20A\w+

Actual code:

string regex = $@"(?<!\w){prefix}\w+";

Notice the prefix var matches strings such as M20A and X50G.

It perfectly matches the following cases:

M20A0820
M20A1234
M20A7U8V

But now I got a new requirement from the business to match, for example:

M20A-SDR

It will be the prefix followed by the exact string "-SDR". Not just a dash followed by 3 alphanumerics, but literally "-SDR". The existing matches need to still work, but prefix + "-SDR" must also be matched.

What would be the regex that would match the following:

M20A0820
M20A1234
M20A7U8V
M20A-SDR