If you forget to mention a prefix in any step, your expression will return nothing-an error for which it's difficult to track the cause.
如果有任何一个步骤忘记使用这个前缀，那么表达式不会返回任何内容，并且很难搞清楚究竟是哪一步出的错。
Step 4: Evaluate the regular expression.
第4步：评估正则表达式。
Each step in a path expression can also contain a predicate that filters the elements that are returned by that step, retaining only elements that satisfy some condition.
路径表达式中的每一步还可以包含一个谓词，用于过滤该步返回的元素，只保留满足某种条件的元素。
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