This saves you the trouble of having to re-type the regular expression every time.
这样您就不必每次都要重新键入正则表达式。
One noteworthy point about creating a join expression is that every time you join from an expression, it returns a new join expression, as shown in Listing 9.
创建连接表达式需要注意的一个地方是，每次连接一个表达式时，都会返回一个新的表达式，如清单9所示。
Welcome to English in a minute. If you smell smoke, you know there is fire. Should we call the fire department every time we hear this expression?
欢迎来到一分钟英语。如果你闻到烟味，意味着那有火。当我们每次听到这个表达都要打电话给火警吗？
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