ts-r
  • Error
  • Reported by the lexer and parser

TS1499

Unknown regular expression flag.

A regular expression literal has a flag that doesn't exist. Use only `d`, `g`, `i`, `m`, `s`, `u`, `v` and `y`.

What it means

The letters after the closing / of a regular expression are its flags. Only d, g, i, m, s, u, v and y exist; any other letter is reported.

Example

index.ts
export const re = /abc/x;
ts-r-check -p tsconfig.json --pretty false ts-r output
index.ts(1,24): error TS1499: Unknown regular expression flag.

Recorded by node scripts/verify-examples.mjs with ts-r built from commit 6deaf01. The example uses the default tsconfig: strict, target: es2022, module: esnext, moduleResolution: bundler.

How to fix

  • Remove the unknown flag. JavaScript has no x (extended) flag; write the pattern without whitespace and comments.