Showing content from https://github.com/tc39/proposal-regexp-set-notation/issues/32 below:
subtract a not-proper-subset · Issue #32 · tc39/proposal-regexp-v-flag · GitHub
In the TC39 meeting this week (2021-may-26) someone asked what happens in subtraction A--B when B is not a proper subset of A.
- My response was that this would be fine, just like in math, and that this is how people implement sets in general in computers. {ab}--{bc}={a}
- The person asking felt like this is somehow different from math sets and might mask a bug in the pattern.
- Mathias responded that it's important to support such a case, so that a regex author can make sure that a character class definitely does not match a certain set of things, without jumping through hoops to make sure that that is a proper subset. (This could require a nested intersection with the left-side set: [A--[B&&A]] rather than just [A--B].)
In my mind, character classes define sets in the mathematical sense, and set operations should behave as usual. From the current spec: “A CharSet is a mathematical set of characters.” and the semantics evaluate a CharacterClass and a CharacterClassEscape to a CharSet, using union operations as appropriate (“Return the union of ...”).
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