Regular expression matching any subset of a given set?

Question

Is it possible to write a regular expression which will match any subset of a given set of characters
a1 ... an ?
I.e. it should match any string whe

Answer 1

Can't think how to do it with a single regex, but this is one way to do it with n regexes: (I will usr 1 2 ... m n etc for your as)

^[23..n]*1?[23..n]*$
^[13..n]*2?[13..n]*$
...
^[12..m]*n?[12..m]*$

If all the above match, your string is a strict subset of 12..mn.

How this works: each line requires the string to consist exactly of:

• any number of charactersm drawn fromthe set, except a particular one
• perhaps a particular one
• any number of charactersm drawn fromthe set, except a particular one

If this passes when every element in turn is considered as a particular one, we know:

• there is nothing else in the string except the allowed elements
• there is at most one of each of the allowed elements

as required.

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for completeness I should say that I would only do this if I was under orders to "use regex"; if not, I'd track which allowed elements have been seen, and iterate over the characters of the string doing the obvious thing.