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JS Type Checker Wish List: Regex validated strings type HexColor = /#[0-9a-f]+/i; const color: HexColor = '#eee'; // fine const color: HexColor = 'red'; // error!

I don't really like using regexes here but it's the most intuitive API for defining a validation rule. Super trivial to implement in a type checker too (if it's written in JS, just delegate to the runtime engine). Could have some way to specify the actual message for errors too.

cc @drosenwasser for @typescriptlang

We have it implemented in a branch but apart from the "make sure you pass in only these strings" case which is really compelling, thinking about how you narrow other strings for validation feels off. It means you have to write the same regex in both expression & type positions.