Is there a way regex will match all the combinations of the tokens in `|` operator

Question

I am doing a parser for nand2tetris project. I want to check if the destination field is either M|D|MD|A|AM|AD|AMD and their different ways of combinations like MA not only AM.

^(M|D|MD|A|AM|AD|AMD)\s*=$

This regex correctly matches AM=, but not MA=.

I don't want to list out all the possible combinations of those tokens, is there a way to do it simply?

Rollmop

I think we can list the regex matches in orders, ^(M|D|DM|A|AM|AD|ADM)\s*=7$ and also the input token in order, that will match – Rollmop

This question is likely a duplicate, but this is the regex you need ^([ADM])(?:(?!\1)([ADM])(?:(?!\1|\2)[ADM])?)?\s*=$ regex101.com/r/SNhJpf/2 – Consternation

Thanks @Nick. Can you show the link for the duplicate and mark this question as duplicate? – Rollmop

I've been searching for it but can't find it. I seem to recall seeing something like this in the last couple of days (it was a more generic (.)(?!\1)(.)(?!\1|\2).) but I can't find it again. – Consternation

Nick, post the answer so at least we can upvote you m8. – Melissiamelita

I believe this is a duplicate post? If you guys are searching for one =). I believe it would boild down to ^(?!(?:.\B)*(.)(?:\B.)*\1)[AMD]+\b=$ then – Doorway

You must have been confident I would fix my answer as it was wrong at the time. :-) – Regimentals

Answer 1

This should do it:

^(?=[MDA]+$)(?!.?(.).?\1).{1,3}$

Demo

The negative lookahead attempts to match two "M"'s, two "D"'s or two "A"'s.

The positive lookahead merely restricts the characters in the string to "M", "D" and "A". That was not necessary but it allows me to use .'s in the rest, rather than (?![MDA]?([MDA])[MDA]?...), which may make it easier to read.

Regimentals