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。
2
Step 4: Evaluate the regular expression.
第4步：评估正则表达式。
3
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.