8055870: iteration fails if index var is not used
Reviewed-by: hannesw, lagergren
/(?:[^[#\s\\]+|\\(?:[\S\s]|$)|\[\^?]?(?:[^\\\]]+|\\(?:[\S\s]|$))*]?)+|(\s*#[^\n\r\u2028\u2029]*\s*|\s+)([?*+]|{[0-9]+(?:,[0-9]*)?})?/g
/{[0-9]+}?/g
{123}
{a}
f{a}
f
SyntaxError
SyntaxError