/*/book[(.//image)[2]]

这将选择所有book元素,这些元素是XML文档的top元素的子元素,并且具有第二个图像后代.

此表达式的评估可能比以//开头的任何表达式更快,因为以//开头的表达式通常会导致遍历整个文档.

它也比以下更有效：

//book[count(.//image)>1]

即使这个表达式被重写不是以//开头.

这是如此,因为在上面的表达式计数(.// image)会导致所有图像后代被计数,而在我们的解决方案中：

(.//image)[2]

仅验证第二个图像后代是否存在.

最后,这是一个基于XSLT的验证：

<xsl:stylesheet version="1.0"
 xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
 <xsl:output omit-xml-declaration="yes" indent="yes"/>
 <xsl:strip-space elements="*"/>

 <xsl:template match="node()|@*">
     <xsl:copy-of select="/*/book[(.//image)[2]]"/>
 </xsl:template>
</xsl:stylesheet>

当此转换应用于提供的XML文档时：

<inventory>
        <book num="b1">
            <title>Snow Crash</title>
            <author>Neal Stephenson</author>
            <publisher>Spectra</publisher>
            <price>14.95</price>
            <chapter>
                <title>Snow Crash - Chapter A</title>
                <paragraph>
                    This is the <emph>first</emph> paragraph.
                    <image file="firstParagraphImage.gif"/>
                    afetr image...
                </paragraph>
                <paragraph>
                    This is the <emph>second</emph> paragraph.
                    <image file="secondParagraphImage.gif"/>
                    afetr image...
                </paragraph>
            </chapter>
            <chapter>
                <title>Snow Crash - Chapter B</title>
                <section>
                    <title>Chapter B - section 1</title>
                    <paragraph>
                        This is the <emph>first</emph> paragraph of section 1 in chapter B.
                        <image file="Chapter_B_firstParagraphImage.gif"/>
                        afetr image...
                    </paragraph>
                    <paragraph>
                        This is the <emph>second</emph> paragraph of section 1 in chapter B.
                        <image file="Chapter_B_secondParagraphImage.gif"/>
                        afetr image...
                    </paragraph>
                </section>
            </chapter>
            <chapter>
                <title>Chapter C</title>
                <paragraph>
                    This chapter has no images and only one paragraph
                </paragraph>
            </chapter>
        </book>
        <book num="b2">
            <title>Burning Tower</title>
            <author>Larry Niven</author>
            <author>Jerry Pournelle</author>
            <publisher>Pocket</publisher>
            <price>5.99</price>
            <chapter>
                <title>Burning Tower - Chapter A</title>
            </chapter>
            <chapter>
                <title>Burning Tower - Chapter B</title>
                <paragraph>
                    This is the <emph>second</emph> paragraph of chapter B in the 2nd book.
                    <image file="Burning_Tower_Chapter_B_secondParagraphImage.gif"/>
                    afetr image...
                </paragraph>
            </chapter>
        </book>
        <book num="b3">
            <title>Zodiac</title>
            <author>Neal Stephenson</author>
            <publisher>Spectra</publisher>
            <price>7.50</price>
            <chapter>
                <title>Zodiac - Chapter A</title>
            </chapter>
        </book>
        <!-- more books... -->
</inventory>

评估XPath表达式,并将选定的节点(在本例中只是一个)复制到输出：

<book num="b1">
   <title>Snow Crash</title>
   <author>Neal Stephenson</author>
   <publisher>Spectra</publisher>
   <price>14.95</price>
   <chapter>
      <title>Snow Crash - Chapter A</title>
      <paragraph>
                    This is the <emph>first</emph> paragraph.
                    <image file="firstParagraphImage.gif"/>
                    afetr image...
                </paragraph>
      <paragraph>
                    This is the <emph>second</emph> paragraph.
                    <image file="secondParagraphImage.gif"/>
                    afetr image...
                </paragraph>
   </chapter>
   <chapter>
      <title>Snow Crash - Chapter B</title>
      <section>
         <title>Chapter B - section 1</title>
         <paragraph>
                        This is the <emph>first</emph> paragraph of section 1 in chapter B.
                        <image file="Chapter_B_firstParagraphImage.gif"/>
                        afetr image...
                    </paragraph>
         <paragraph>
                        This is the <emph>second</emph> paragraph of section 1 in chapter B.
                        <image file="Chapter_B_secondParagraphImage.gif"/>
                        afetr image...
                    </paragraph>
      </section>
   </chapter>
   <chapter>
      <title>Chapter C</title>
      <paragraph>
                    This chapter has no images and only one paragraph
                </paragraph>
   </chapter>
</book>