代码如下：

replacement_field ::=  "{" [field_name] ["!" conversion] [":" format_spec] "}"
field_name        ::=  arg_name ("." attribute_name | "[" element_index "]")*
arg_name          ::=  [identifier | integer]
attribute_name    ::=  identifier
element_index     ::=  integer | index_string
index_string      ::=   +
conversion        ::=  "r" | "s" | "a"
format_spec       ::=

原文链接

请问上面代码中的{},[],()分别代表了什么意思？这是什么语法？谢谢！