RegExpIndicesArray - Named group indices can be undefined

[TomStrepsil]

🔎 Search Terms

RegExpIndicesArray, Regex indices

🕗 Version & Regression Information

• This is the behaviour in every version I tried, and I reviewed the FAQ for entries about Regular Expressions

⏯ Playground Link

https://www.typescriptlang.org/play/?target=9&ts=6.0.0-dev.20260322#code/PTAEGcCcGNgWwIYEsB2A6ALuAUN6B7FcDUAc0nwFcAHUAXlGAQAoB+AHjiQBMA+AIwCUrWNzQBTAB7jozAOQJocwQEI0qbkmjjwa8lWq6A2gCIu3EwF0A3LgJESGqfTIUaRgAw3sQA

💻 Code

const group = /a(?<mid>b)?c/d.exec('ac')!.indices!.groups!["mid"];
const index = group[0];

The typing assumes that group indices are of type [number, number], but these can also be undefined

🙁 Actual behavior

typeof group === "undefined"

hence, the type system should have complained when attempting to access via index.

🙂 Expected behavior

'group' is possibly 'undefined'

...should be reported by the type system.

Additional information about the issue

#61079 fixed the same issue for the outer RegExpIndicesArray itself, but not the indices array on the matched groups

[RyanCavanaugh]

I think this falls under #32098

[TomStrepsil]

I think this falls under #32098

It'd be great if this could be fixed as part of that, but that feels like a much bigger change - i.e. type safety around the keys of named capture groups themselves.

The example above doesn't really change the reality of keyed groups, the example:

/a(?<mid>b)?c/d.exec('ac').indices.groups;

returns:

{ mid: undefined }

Having strongly-typed keys seems unrelated to the fact the value is undefined rather than [number, number], if the group failed to match.

[TomStrepsil]

@RyanCavanaugh - since the Contribution guide indicates:

NOTE: Code changes in this repo are now limited to a small category of fixes:

• Crashes that were introduced in 5.9 or 6.0 that also repro in 7.0 and have a portable fix and don't incur other behavioral changes
• Security issues
• Language service crashes that substantially impact mainline usage
• Serious regressions from 5.9 (these must seriously impact a large proportion of users)

...It doesn't feel like this kind of small fix would qualify.

Would you recommend me re-raising this issue at https://github.com/microsoft/typescript-go/ ?

Feels like adding | undefined here would be a quick win.

[RyanCavanaugh]

PR rules are about code changes, issues are about behavior changes. This isn't an approved behavior change yet (it'd need to be justified in terms of the breaks it would cause) so PR rules are not relevant.