I am trying to write a reqular expression to match a word with my input
string.

Input string can be ( inpstr = ABCDD )

I should be able to match all the below words with 0 or 1 A, 0 or 1 B, 0
or 1 C and 0 or 1 or 2 D

A
AC
CA
DAD
BAC
ADD

but not words which have like 2B or 2A
AAB
ABCC
ADDD

I am using an expression like ^[ABCDD]*$,but this matches words like ABB
or ABCC which have like 2B or 2A.

Any suggestion on how I can fix this?

Thanks

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