Microsoft/TypeScript

TS1508: Unexpected '?'. Did you mean to escape it with backslash? shouldn't report even with `v` flag

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#62707 opened on Nov 2, 2025

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BugDomain: ParserHelp Wanted

Description

🔎 Search Terms

TS1508, "Did you mean to escape it with backslash?"

🕗 Version & Regression Information

⏯ Playground Link

https://www.typescriptlang.org/play/?target=11#code/PQbQ-AZAusBuBQQ

💻 Code

/[?&]/v

🙁 Actual behavior

The error "TS1508: Unexpected '?'. Did you mean to escape it with backslash?" is given.

🙂 Expected behavior

No error because ? doesn't need to be escaped in a character class even with a v flag.

Additional information about the issue

Other characters like { do need escaping with the v flag, but ? gets no runtime errors, so I don't see why this has to err now.

Contributor guide

Tech stack
typescript
Domain
toolingdeveloper experience
Issue type
bug
DifficultyEstimated implementation difficulty for a new contributor, from 1 for very small changes to 5 for expert-level work.
3
Estimated timeA rough time range for an experienced contributor to investigate, implement, test, and prepare a pull request.
1-3 hours
Activity statusHow available the issue appears right now: fresh, active, stale, blocked, or waiting on maintainer input.
fresh
ClarityHow clearly the issue explains the expected change, acceptance criteria, and next step.
clear
Prerequisites
Understanding of regular expression syntaxFamiliarity with TypeScript compiler codebase
Newbie friendlinessA 1-100 score estimating how approachable this issue is for first-time contributors.
40
Research direction
Start by examining the regular expression syntax checking logic introduced in PR #55600. Look at the checker code that reports TS1508, likely in src/compiler/checker.ts or a dedicated regex validation file. Investigate how the `v` flag affects character class parsing; specifically, the `?` character should be allowed unescaped inside character classes. The fix would involve modifying the regex validation to exempt `?` from requiring escaping when inside character classes with the `v` flag. Reference the playground link and ensure the fix aligns with ECMAScript specification for `v` flag.