可以通过将类括在方括号内来表明这是一个类；在括号中，可以有一组字符或者用破折号指定的字符范围。

You can indicate a class by surrounding it in square braces; within the braces, you can have both a set of characters and character ranges indicated with a dash.

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注意方括号(不是圆括号)中需要组成一个匿名列表引用。

Note the brackets (not parentheses), which are required to compose an anonymous list reference.

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注意：尖括号中的参数为可选参数；方括号()中的参数或选项为必需参数，必须提供。

Note: parameters shown in Angle brackets are optional; parameters or options shown in normal brackets () are required and must be provided.

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