I recently enabled the ESLint rule which encourages always using the u flag. When doing so, I found out that my custom regexp escaper, which was

function escapeRegExp(str) {
  return str.replace(/[-[\]/{}()*+?.\\^$|]/ug, "\\$&");
}

was overzealous, and would cause new RegExp(escapeRegExp(input), "ug") to fail when the input string contained a -.

This probably has some intersection with discussions in other threads, e.g. if some delegates require that the result escape - so that it can work in situations like new RegExp("[" + RegExp.escape(input) + "]", "ug"), such a requirement prohibits the result from working in situations like new RegExp(RegExp.escape(input), "ug").