Is there a way to cast regex groups into Typescript types?

Question

Suppose I have the following string:

"A D"

And I apply the following RegExp:

/([AB]) *([CD])/

This will either return null, or return an array of strings representing the groups. The first group will always be either "A" or "B", in my case "A". The second group will always be "C" or "D".

As such I would like the following:

const str = 'A    D'
const groups = new RegExp('([AB]) *([CD])').exec(str);
return groups[1] === 'Q'

This should give me a typescript error, but it doesn't. I want it to give me ts(2367), this condition will always return 'false' since types groups[1]: "A" | "B" and "Q" have no overlap.

Is there nice way I can have typescript pull the group types out of my regexp?

Answer 1

Not really - TypeScript doesn't try to interpret patterns, and the shape of exec is:

exec(string: string): RegExpExecArray | null;

where RegExpExecArray is:

interface RegExpExecArray extends Array<string> {
    index: number;
    input: string;
}

.match isn't any better.

If your pattern, when executed, produces an output of certain types, you'll have to specify them explicitly in addition to the pattern, eg:

const groups = new RegExp('([AB]) *([CD])').exec(str);
if (groups) {
    const typedGroups = [...groups] as [string, 'A' | 'B', 'C' | 'D'];

It's not very pretty, but there aren't really any good options here.