在这个特别的规则表达式中，出现了' [[：space：]] '字符类。

In this particular regular expression, we encounter the '[[: space:]]' character class.

youdao

可以将字符类看作是规则表达式的“原子”，通常会将那些原子组合成“分子”。

You can think of character classes as the "atoms" of regular expressions, and you will usually want to group those atoms into "molecules."

youdao

要记住 AST 类在包外是不可见的， 还有就是客户机（最终）只会传入它们想求值的表达式的一个字符串表示。

Remember that the AST classes aren't visible outside of this package at all and that clients will (eventually) simply pass in a String representation of the expression they want evaluated.

youdao