给出适当数据结构的引用，第二个子例程将它转换为XML文档，根据参数的不同，产生的XML文档采用字符串格式或文件形式。

Given a reference to an appropriate data structure, the second converts it to an XML document, either in string format or as a file, depending upon its parameters.

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替换字符串可能包含对在之前的匹配中捕获的子序列的引用。

The replacement string may contain references to subsequences captured during the previous match.

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记录一个已匹配的子串用作后面的参考引用。

To mark a matched substring for future reference.

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