For example

import mainargs.{main, ParserForMethods}

object Main {
  @main def `opt-for-18+`(): Unit = {}

  def main(args: Array[String]): Unit = ParserForMethods(this).runOrExit(args)
}

when run Main.scala opt-for-18+ it fails

Unknown argument: "opt-for-18+"
Expected Signature: opt$minusfor$minus18$plus

those opt$minusfor$minus18$plus are to be unescaped, at least $minus and $plus

It was fixed in PR #6 which seems to be deleted by the author
(archived at http://archive.today/2021.02.21-032145/https://github.com/lihaoyi/mainargs/pull/6/files)