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

问题

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?

回答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.

来源：https://stackoverflow.com/questions/60575231/is-there-a-way-regex-will-match-all-the-combinations-of-the-tokens-in-operat