Template literal type ${string}${number}${string} errors on valid values

[joslarson]

🔎 Search Terms

"${string}${number}${string}", "template literal types number between strings"

🕗 Version & Regression Information

This is the behavior in every version I tried, and I reviewed the FAQ for entries about template literal types

⏯ Playground Link

https://www.typescriptlang.org/play?#code/MYewdgzgLgBAtgTwMpQE4EswHMBcADAEgG9oNsBfYsAVzgCMBTVSktTLcvGAXhgHIoDaAEYATAGZB0PgG4AUAHoFMFTAB6GzRpgAVBAAcG-AERSoYyUKjG+MdBBhgQsAIYQI6LGBd0ANkagQGCgDIz5CVjIOKlpGZmJSdk4+ADooCAAKCVFRAEo5UEhYRBQo0XwEtgoY+iZOHn4zC1lFZVUtLV1QkyaJGzsHJ1d3T28-AKCQw34IxOqiGlrmPFT0rPEc-MLoeGQqrHEKhdi6yqj63j4LM1kYJRh9NwghGAAzEFQYCBA4I1QGNzgORAA

💻 Code

const myString:`${string}${number}${string}` = 'test123test';
//    ^^^^^^^^ Type '"test123test"' is not assignable to type '`${string}${number}${string}`'.ts(2322)
const myString2:`${string}${number}` = 'test123';
//    ^^^^^^^^ Type '"test123"' is not assignable to type '`${string}${number}`'.ts(2322)
const myString3:`${number}${string}` = '123test'; // passes for some reason

🙁 Actual behavior

Received error "Type '"test123test"' is not assignable to type '${string}${number}${string}'.ts(2322)" when pattern matches

🙂 Expected behavior

I expect it to match any literal string type that has a number within it

Additional information about the issue

My real use case is that I would like to be able to exclude/omit patterns like ${string}${number}${string}, but that's not possible since it doesn't match

[jcalz]

See #57126 and essentially intentional as described in #40336:

A placeholder immediately followed by another placeholder is matched by inferring a single character from the source.

So `${string}${number}${string}` matches the first character against string, the second against number, and then the rest of the string against string. Yes, it's weird.

[RyanCavanaugh]

As a general principle, template string literals aren't regexes and don't have regex capabilities. Anything that requires backtracking (or extensive lookahead) isn't going to happen.