Using a Regular Expression for validation of a single line textbook

Hello, I am starting to use Nintex Forms. I want to use a RegExp to validate a single line textbook. Valid inputs should look like:

• MX ??? ??? ??? S 20##

or

• MX ??? ??? S 20##

Where:

• ? is a capital letter [A-Z] 
• S is a any string of at least 3 characters
• # is a digit [0-9] 

I tried with these 2 RegEx unsuccessfully:

/MX.[A-Z]{3}.[A-Z]{3}.[A-Za-z]{3,}.{1,}20{1}[0-9]{1}[0-9]{1}/

/MX[^ ][A-Z]{3}[^ ][A-Z]{3}[^ ][A-Za-z]{3,}[^ ]{1,}20{1}[0-9]{1}[0-9]{1}/

Any expert who  can help?

Thank you very much in advance.