1 /(?:[^[#\s\\]+|\\(?:[\S\s]|$)|\[\^?]?(?:[^\\\]]+|\\(?:[\S\s]|$))*]?)+|(\s*#[^\n\r\u2028\u2029]*\s*|\s+)([?*+]|{[0-9]+(?:,[0-9]*)?})?/g
   2 /{[0-9]+}?/g
   3 {123}
   4 {a}
   5 f{a}
   6 f
   7 SyntaxError
   8 SyntaxError