Your regex is using alternation with incorrect grouping. Moreover you are not matching mont number 10
and you are also not matching a letter in the middle.
You may use this regex:
^(?:0[1-9]|1[0-2])[A-Z][0-9]{2}$
RegEx Demo
RegEx Demo
^
: Start
(?:0[1-9]|1[0-2])
: Match a 2 digit number for month number, of the form 01, 02, 03, ... 10, 11, 12
[A-Z]
: Match an uppercase letter
[0-9]{2}
: Match 2 digits
$
: End
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